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University of Amsterdam
Dept. of Social
Science Informatics
(SWI)
Herengracht 196, 1016
BS Amsterdam
The Netherlands
Tel. (+31) 20 5252073
SWI-Prolog 1.6
Reference Manual
Jan Wielemaker
jan@swi.psy.uva.nl
SWI-Prolog is a WAM (Warren Abstract Machine, ) based
implementation of Prolog. SWI-Prolog has been designed and
implemented such that it can easily be modified for experiments
with logic programming and the relation between logic
programming and other programming paradigms (such as the object
oriented PCE environment, ). SWI-Prolog has a rich set of
built-in predicates and reasonable performance, which makes it
possible to develop substantial applications in it. The current
version offers a module system, garbage collection and an
interface to the C language.
This document gives an overview of the features, system limits
and built-in predicates.
Copyright (C) 1990 Jan Wielemaker
Chapter 1
Introduction
SWI-Prolog has been designed and implemented to get a Prolog
implementation which can be used for experiments with logic programming
and the relation to other programming paradigms. The intention was to
build a Prolog environment which offers enough power and flexibility to
write substantial applications, but is straightforward enough to be
modified for experiments with debugging, optimisation or the introduction
of non-standard data types. Performance optimisation is limited due to
the main objectives: portability (SWI-Prolog is entirely written in C and
Prolog) and modifiability.
SWI-Prolog is based on a very restricted form of the WAM (Warren Abstract
Machine) described in which defines only 7 instructions. Prolog can
easily be compiled into this language and the abstract machine code is
easily decompiled back into Prolog. As it is also possible to wire a
standard 4-port debugger in the WAM interpreter there is no need for a
distinction between compiled and interpreted code. Besides simplifying
the design of the Prolog system itself this approach has advantages for
program development: the compiler is simple and fast, the user does not
have to decide in advance whether debugging is required and the system
only runs slightly slower when in debug mode. The price we have to pay is
some performance degradation (taking out the debugger from the WAM
interpreter improves performance by about 20%) and somewhat additional
memory usage to help the decompiler and debugger.
SWI-Prolog extends the minimal set of instructions described in to
improve performance. While extending this set care has been taken to
maintain the advantages of decompilation and tracing of compiled code.
The extensions include specialised instructions for unification, predicate
invocation, some frequently used built-in predicates, arithmetic, or (;/2,
|/2), if-then (->/2) and not (\+/1).
This manual does not describe the full syntax and semantics of SWI-Prolog,
nor how one should write a program in Prolog. These subjects have been
described extensively in the literature. See , and . For more advanced
Prolog material see . Syntax and standard operator declarations confirm
to the `Edinburgh standard'. Most built in predicates are compatible with
those described in . SWI-Prolog also offers a number of primitive
predicates compatible with Quintus Prolog, and BIM_Prolog .
1
1.1 Status
This manual describes version 1.6.0 of SWI-Prolog. SWI-Prolog has been
used now for some years. The application range includes Prolog course
material, meta-interpreters, simulation of parallel Prolog, learning
systems and a large workbench for knowledge engineering. Although we
experienced rather obvious and critical bugs can remain unnoticed for a
remarkable long period, we can assume the basic Prolog system to be fairly
stable. Bugs can be expected in unfrequently used builtin predicates.
Some bugs are known to the author. They are described as footnotes in
this manual.
1.2 Should you be Using SWI-Prolog?
There are a number of reasons why you better choose a commercial Prolog
system, or another academic product:
SWI-Prolog is not supported
Although I usually fix bugs shortly after a bug report arrives, I
cannot promise anything. Now that the sources are provided, you can
always dig into them yourself.
Memory requirements and performance are your first concerns
A number of commercial compilers are more keen on memory and
performance than SWI-Prolog. I do not wish to offer some of the nice
features of the system, nor its portability to compete on raw
performance.
You need features not offered by SWI-Prolog
In this case you may wish to give me suggestions for extensions. If
you have great plans, please contact me (you might have to implement
them yourself however).
On the other hand, SWI-Prolog offers some nice facilities:
Nice environment
This includes `Do What I Mean', automatic completion of atom names,
history mechanism and a tracer that operates on single key-strokes.
Interfaces to standard Unix editors are provided, as well as a
facility to maintain programs (see make/0).
Very fast compiler
The compiler handles about 5K bytes per second per MIPS (i.e. 35K
bytes per second on SUN-4/110).
Transparent compiled code
SWI-Prolog compiled code can be treated just as interpreted code:
you can list it, trace it, assert from or retract to it, etc. This
implies you do not have to decide beforehand whether a module should
be loaded for debugging or not. Also, performance is much better
than the performance of most interpreters.
Profiling
SWI-Prolog offers tools for performance analysis, which can be very
useful to optimise programs. Unless you are very familiar with
Prolog and Prolog performance considerations this might be more
helpful than a better compiler without these facilities.
Flexibility
SWI-Prolog allows for easy and flexible integration with C, both
Prolog calling C functions as C calling Prolog predicates.
SWI-Prolog is provided in source form, which implies SWI-Prolog can
be linked in with another package. Command line options and
predicates to obtain information from the system and feedback into
the system are provided.
Integration with PCE
SWI-Prolog offers a tight integration to the Object Oriented Package
for User Interface Development, called PCE (). PCE is now also
available for X-windows.
1.3 Version 1.5 Release Notes
There are not many changes between version 1.4 and 1.5. The C-sources
have been cleaned and comments have been updated. The stack memory
management based on using the MMU has been changed to run on a number of
system-V Unix systems offering shared memory. Handling dates has been
changed. All functions handling dates now return a floating point number,
expressing the time in seconds since january 1, 1970. A predicate
convert_time/8 is available to get the year, month, etc. The predicate
time/6 has been deleted. get_time/1 and convert_time/8 together do the
same.
From version 1.5, the system is distributed in source form, rather than in
object form as used with previous releases. This allows users to port
SWI-Prolog to new machines, extend and improve the system. If you want
your changes to be incorporated in the next release, please indicate all
changes using a C-preprocessor flag and send complete source files back to
me. Difference listings are of no use, as I generally won't have exactly
the same version around.
1.4 Version 1.6 Release Notes
Version 1.6 is completely compatible with version 1.5. Some new features
have been added, the system has been ported to various new platforms and
there is a provisional interface to GNU-Emacs. This interface will be
improved and documented later.
The WAM virtual-machine interpreter has been modified to use GCC-2's
support for threated code.
From version 1.6, the sources are now versioned using the CVS version
control system.
1.5 Portability
The table below shows which machine dependent features have been ported to
which architectures. Some of the minus signs could be taken away by
spending more effort to the port, others are due to fundamental omisions
or bugs in the operating system. Note that the column `C-interface'
implies object code can be linked dynamically from a running Prolog
environment. For systems that have a minus here C-code can still be added
by relinking SWI-Prolog itself together with the C-extensions to form an
extended version of SWI-Prolog.
Machine OS Dynamic C Saved Profile
Stacks Interface States
SUN-3 and 4 SunOs 4 + + + +
HP 9000s300 HP-UX 8.0 -- + + +
DEC MIPS Ultrix 3.1 -- -- + +
Gould PN UTX-2.1 + -- -- +
IBM PS2 AIX 2.0 -- -- + +
IBM RISC-6000 AIX 3.1 -- + + +
VAX Ultrix -- + + +
ATARI TOS 1.3 -- -- -- --
PC LINUX -- -- -- --
1.6 Acknowledgements
Some small parts of the Prolog code of SWI-Prolog are modified versions of
the corresponding Edinburgh C-Prolog code: grammar rule compilation and
writef/2. Also some of the C-code originates from C-Prolog: finding the
path of the currently running executable and the code underlying
absolute_file_name/2. Ideas on programming style and techniques originate
from C-Prolog and Richard O'Keefe's thief editor. An important source of
inspiration are the programming techniques introduced by Anjo Anjewierden
in PCE version 1 and 2.
I also would like to thank those who had the fade of using the early
versions of this system, suggested extensions or reported bugs. Among
them are Anjo Anjewierden, Huub Knops, Bob Wielinga, Wouter Jansweijer,
Luc Peerdeman, Eric Nombden, Frank van Harmelen, Bert Rengel.
Chapter 2
Overview
8
2.1 Starting SWI-Prolog from the Unix Shell
It is advised to install SWI-Prolog as `pl' in the local binary directory.
SWI-Prolog can then be started from the Unix shell by typing `pl'. The
system will boot from the system's default boot file, perform the
necessary initialisations and then enter the interactive top level.
After the necessary system initialisation the system consults (see
consult/1) the user's initialisation file. This initialisation file
should be named `.plrc' and reside either in the current directory or in
the user's home directory. If both exist the initialisation file from the
current directory is loaded. The name of the initialisation file can be
changed with the `-f file' option. After loading the initialisation file
SWI-Prolog executes a user initialisation goal. The default goal is a
system predicate that prints the banner message. The default can be
modified with the `-g goal' option. Next the toplevel goal is started.
Default is the interactive Prolog loop (see prolog/0). The user can
overwrite this default with the `-t toplevel' option.
2.1.1 Command Line Options
The full set of command line options is given below:
-Lsize
Give local stack size in K bytes (200 K default). Note that
there is no space between the size option and its argument.
For machines with dynamic stack allocation this flag sets
the maximum value to which the stack is allowed to grow (2
Mbytes default). A maximum is useful to stop buggy programs
from claiming all memory resources. -L0 sets the limit to
the highest possible value.
-Gsize
Give global stack size in K bytes (100 K default). For
machines with dynamic stack allocation the default is 4
Mbytes. See -L for more details.
-Tsize
Give trail stack size in K bytes (50 K default). For
machines with dynamic stack allocation the default is 4
Mbytes. See -L for more details.
-Asize
Give argument stack size in K bytes (5 K default). For
machines with dynamic stack allocation the default is 1
Mbytes. See -L for more details.
-c file ...
Compile files into an `intermediate code file'. See
section 2.6.
-o output
Used in combination with -c or -b to determine output file
for compilation.
-O
Optimised compilation. See please/3.
-f file
Use file as initialisation file instead of `.plrc'.
`-f none' stops SWI-Prolog from searching for an
initialisation file.
-g goal
Goal is executed just before entering the top level.
Default is a predicate which prints the welcome message.
The welcome message can thus be suppressed by giving
-g true. goal can be a complex term. In this case quotes
are normally needed to protect it from being expanded by the
Unix shell.
-t goal
Use goal as interactive toplevel instead of the default goal
prolog/0. goal can be a complex term. If the toplevel goal
succeeds SWI-Prolog exits with status 0. If it fails the
exit status is 1. This flag also determines the goal
started by break/0 and abort/0. If you want to stop the
user from entering interactive mode start the application
with `-g goal' and give `halt' as toplevel.
+/-tty
Switches tty control (using ioctl(2)) on (+tty) or off
(-tty). Normally tty control is switched on. This default
depends on the installation. You may wish to switch tty
control off if Prolog is used from an editor such as GNU
EMACS. If switched off get_single_char/1 and the tracer will
wait for a return.
-x bootfile
Boot from bootfile instead of the system's default boot
file. A bootfile is a file resulting from a Prolog
compilation using the -b or -c option.
The following options are for system maintenance. They are given for
reference only.
-b initfile ... -c file ...
Boot compilation. initfile ...are compiled by the
C-written bootstrap compiler, file ...by the normal Prolog
compiler. System maintenance only.
-d level
Set debug level to level. System maintenance only.
2.2 GNU Emacs Interface
A provisional interface to GNU-Emacs is delivered with version 1.6 of
SWI-Prolog. The interface is based on the freely distributed interface
delivered with Quintus Prolog. When running Prolog as an inferior process
under GNU-Emacs, there is support for finding predicate definitions,
completing atoms, finding the locations of compilation-warnings and many
more. For details, see the files pl/lisp/README and
pl/lisp/swi-prolog.el.
2.3 Online Help
Online help provides a fast lookup and browsing facility to this manual.
The online manual can show predicate definitions as well as entire
sections of the manual.
help
Equivalent to help(help/1).
help(+What)
Show specified part of the manual. What is one of:
Name/Arity give help on specified predicate
Name give help on named predicate with any arity
or C interface function with that name
Section display specified section. section numbers
are dash-separated numbers: 2-3 refers to
section 2.3 of the manual. Section numbers
are obtained using apropos/1.
Examples
?- help(assert). give help on predicate assert
?- help(3-4). display section 3.4 of the manual
?- help('PL_retry'). give help on interface function PL_retry()
apropos(+Pattern)
Display all predicates, functions and sections that have Pattern in
their name or summary description. Lowercase letters in Pattern also
match a corresponding uppercase letter. Example:
?- apropos(file). Display predicates, functions and sections
that have `file' (or `File', etc.) in their
summary description.
2.4 Query Substitutions
SWI-Prolog offers a query substitution mechanism similar to that of Unix
csh (csh(1)), called `history'. It allows the user to compose new queries
from those typed before and remembered by the system. It also allows to
correct queries and syntax errors. SWI-Prolog does not offer the Unix csh
capabilities to include arguments. This is omitted as it is unclear how
the first, second, etc. argument should be defined.
The available history commands are shown in table 2.1. Figure 2.1 gives
some examples.
!!. Repeat last query
!nr. Repeat query numbered <nr>
!str. Repeat last query starting with <str>
!?str. Repeat last query holding <str>
^old^new. Substitute <old> into <new> of last query
!nr^old^new. Substitute in query numbered <nr>
!str^old^new. Substitute in query starting with <str>
!?str^old^new. Substitute in query holding <str>
h. Show history list
!h. Show this list
Table 2.1: History commands
/u4/staff/jan/.plrc consulted, 0.066667 seconds, 591 bytes
Welcome to SWI-Prolog (version 1.5.0, August 1990)
Copyright (c) 1990, University of Amsterdam. All rights reserved.
1 ?- append("Hello ", "World", L).
L = [72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100]
Yes
2 ?- !!, writef('L = %s\n', [L]).
append("Hello ", "World", L), writef('L = %s\n', [L]).
L = Hello World
L = [72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100]
Yes
3 ?- sublist(integer, [3, f, 3.4], L).
L = [3]
Yes
4 ?- ^integer^number.
sublist(number, [3, f, 3.4], L).
L = [3, 3.400000]
Yes
5 ?- h.
1 append("Hello ", "World", L).
2 append("Hello ", "World", L), writef('L = %s\n', [L]).
3 sublist(integer, [3, f, 3.4], L).
4 sublist(number, [3, f, 3.4], L).
5 ?- !2^World^Universe.
append("Hello ", "Universe", L), writef('L = %s\n', [L]).
L = Hello Universe
L = [72, 101, 108, 108, 111, 32, 85, 110, 105, 118, 101, 114, 115,
101]
Yes
6 ?- halt.
Figure 2.1: Some examples of the history facility
2.4.1 Limitations of the History System
When in top level SWI-Prolog reads the user's queries using history_read/6
rather than read/1. This predicate first reads the current input stream
up to a full stop. While doing so it maps all contiguous blank space onto
a single space and deletes /* ... */ and % ... <cr> comments. Parts
between double quotes (") or single quotes (') are left unaltered. Note
that a Prolog full stop consists of a `non-symbol' character, followed by
a period (.), followed by a blank character. `Symbol' characters are:
#$&*+-./:<=>?@^`~. A single quote immediately preceded by a digit (0-9)
considered part of the <digit>'<digit>... (e.g. 2'101; binary number 101)
sequence.
After this initial parsing the result is first checked for the special
^old^new. construction. If this fails the string is checked for all
occurrences of the !, followed by a !, ?, a digit, a letter or an
underscore. These special sequences are analysed and the appropriate
substitution from the history list is made.
From the above it follows that it is hard or impossible to correct
quotation with single or double quotes, comment delimiters and spacing.
2.5 Overview of the Debugger
SWI-Prolog has a standard 4-port tracer with an optional fifth port.
This fifth port, called unify allows the user to inspect the result after
unification of the head. The ports are called call, exit, redo, fail and
unify. The tracer is started by the trace/0 command, when a spy point is
reached and the system is in debugging mode (see spy/1 and debug/1) or
when an error is detected at run time. Note that in the interactive
toplevel goal trace/0 means ``trace the next query''. The tracer shows
the port, displaying the port name, the current depth of the recursion and
the goal. The goal is printed using the Prolog predicate print/1
(default), write/1 or display/1. An example using all five ports is shown
in figure 2.2.
Yes
2 ?- visible(+all), leash(-exit).
Yes
3 ?- trace, min([3, 2], X).
Call: ( 3) min([3, 2], G235) ? creep
Unify: ( 3) min([3, 2], G235)
Call: ( 4) min([2], G244) ? creep
Unify: ( 4) min([2], 2)
Exit: ( 4) min([2], 2)
Call: ( 4) min(3, 2, G235) ? creep
Unify: ( 4) min(3, 2, G235)
Call: ( 5) 3 < 2 ? creep
Fail: ( 5) 3 < 2 ? creep
Redo: ( 4) min(3, 2, G235) ? creep
Exit: ( 4) min(3, 2, 2)
Exit: ( 3) min([3, 2], 2)
Figure 2.2: Example trace
On leashed ports (set with the predicate leash/1, default are call, exit,
redo and fail) the user is prompted for an action. All actions are single
character commands which are executed without waiting for a return (Unix
`cbreak' mode), unless the command line option -tty is active. Tracer
options:
+ Spy all
Set a spy point (see spy/1) on the current predicate.
No spy all
Remove the spy point (see nospy/1) from the current predicate.
/ Find all
Search for a port. After the `/', the user can enter a line to
specify the port to search for. This line consists of a set of
letters indicating the port type, followed by an optional term, that
should unify with the goal run by the port. If no term is specified
it is taken as a variable, searching for any port of the specified
type. If an atom is given, any goal whose functor has a name equal
to that atom matches. Examples:
/f Search for any fail port
/fe solve Search for a fail or exit port of any
goal with name solve
/c solve(a, _) Search for a call to solve/2 whose
first argument is a variable or the
atom a
/a member(_, _) Search for any port on member/2.
This is equivalent to setting a spy
point on member/2.
. Repeat find all
Repeat the last find command (see `/')
A Alternatives all
Show all goals that have alternatives.
C Context all
Toggle `Show Context'. If on the context module of the goal is
displayed between square brackets (see section 4). Default is off.
L Listing all
List the current predicate with listing/1.
a Abort all
Abort Prolog execution (see abort/0).
b Break all
Enter a Prolog break environment (see break/0).
c Creep all
Continue execution, stop at next port. (Also return, space).
d Display all
Write goals using the Prolog predicate display/1.
e Exit all
Terminate Prolog (see halt/0).
f Fail call, redo, exit
Force failure of the current goal
g Goals all
Show the list of parent goals (the execution stack). Note that due
to tail recursion optimization a number of parent goals might not
exist any more.
h Help all
Show available options (also `?').
i Ignore call, redo, fail
Ignore the current goal, pretending it succeeded.
l Leap all
Continue execution, stop at next spy point.
n No debug all
Continue execution in `no debug' mode.
p Print all
Write goals using the Prolog predicate print/1 (default).
r Retry redo, exit, fail
Undo all actions (except for database and i/o actions) back to the
call port of the current goal and resume execution at the call port.
s Skip call, redo
Continue execution, stop at the next port of this goal (thus skipping
all calls to children of this goal).
u Up all
Continue execution, stop at the next port of the parent goal (thus
skipping this goal and all calls to children of this goal). This
option is useful to stop tracing a failure driven loop.
w Write all
Write goals using the Prolog predicate write/1.
The ideal 4 port model as described in many Prolog books is not visible
in many Prolog implementations because code optimisation removes part of
the choice- and exit points. Backtrack points are not shown if either the
goal succeeded deterministically or its alternatives were removed using
the cut. When running in debug mode (debug/0) choice points are only
destroyed when removed by the cut. In debug mode tail recursion
optimisation is switched off.
2.6 Compilation
Collections of SWI-Prolog source files can be compiled into an
intermediate code file. An intermediate code file is a data file from
which SWI-Prolog can be started. The command to compile a bundle of
source files is:
pl [options] [-o output] -c file ...
The individual source files may include other files using the standard
list notation, consult/1, ensure_loaded/1 and use_module/[1,2]. When the
-o fileoption is omitted a file named a.out is created that holds the
intermediate code file.
Intermediate code files start with the BSD Unix magic code #! and are
executable. This implies they can be started as a command:
sun% pl -o my_program -c ...
...
sun% my_program [options]
Alternatively, my_program can be started with
sun% pl -x my_program [options]
The following restrictions apply to source files that are to be compiled
with `-c':
term_expansion/2 should not use assert/1 and or retract/1 other than
for local computational purposes.
Files can only be included by the standard include directives:
[...], consult/1, ensure_loaded/1 and use_module/[1,2]. User defined
loading predicate invocations will not be compiled.
Directives are executed both when compiling the program and when loading
the intermediate code file.
2.7 Environment Control
please(+Key, -Old, +New)
The predicate please/3 is a solution to avoid large numbers of
environment control predicates. Later versions will support other
environment control as now provided via the predicates style_check/2,
leash/1, unknown/2, the tracer predicates, etc. These predicates are
then moved into a library for backwards compatibility. The currently
available options are:
optimise on/off (default: off)
Switch optimise mode for the compiler on or off (see also the
command line option -O). Currently optimised compilation only
implies compilation of arithmetic, making it fast, but invisible
to the tracer. Later versions might imply various other
optimisations such as incorporating a number of basic predicates
in the virtual machine (var/1, fail/0, =/2, etc.) to gain speed
at the cost of crippling the debugger. Also source level
optimisations such as integrating small predicates into their
callers, eliminating constant expressions and other predictable
constructs. Source code optimisation is never applied to
predicates that are declared dynamic (see dynamic/1).
autoload on/off (default: on)
If on autoloading of library functions is enabled. If off
autoloading is disabled. See section 2.9.
verbose_autoload on/off (default: off)
If on the normal consult message will be printed if a library is
autoloaded. By default this message is suppressed. Intended to
be used for debugging purposes (e.g. where does this predicate
come from?).
2.8 Stand Alone Executables (Saved States)
The introduction of a foreign language interface raised the problem of
incorporating the compiled foreign code into SWI-Prolog to create a new
stand alone executable. The current also allows one to create
applications that do not need any of the SWI-Prolog system files.
save_program(+NewProgram, +ListOfOptions)
Create a new executable wich will be named NewProgram. ListOfOptions
is a list of Key = Valuepairs that specify the default command line
options that will be saved into the new program. If a default is not
specified the default compiled into the currently running Prolog
executable is used. The available keys are given in table 2.2
Key Option Type Description
local -L K-bytes Size (Limit) of local stack
global -G K-bytes Size (Limit) of global stack
trail -T K-bytes Size (Limit) of trail stack
argument -A K-bytes Size (Limit) of argument stack
goal -g atom Initialisation goal
toplevel -t atom Prolog toplevel goal
init_file -f atom Personal initialisation file
tty +/--tty on/off Use ioctl(2) calls
Table 2.2: Key = Value pairs for save_program/2
As the entire data image of the current process will be saved on the
new executable it is desirable to keep this small. Notably the
Prolog machine stacks should be kept small. The best way to do this
is first to compile the program using the -c option. If this is not
possible try to find the smallest possible stack sizes to compile the
program. On machines with dynamic stack allocation the stacks are
not written to file and so their size does not matter. Figure 2.3
shows a possible session. Note the use of `initialise', which is
supposed to be a predicate of the application doing time consuming
initialisation.
sun% pl -c load
foreign file dbase loaded 0.066667 seconds, 1578 bytes.
setup consulted, 0.500000 seconds, 5091 bytes.
main consulted, 0.333333 seconds, 3352 bytes.
load consulted, 1.000000 seconds, 9867 bytes.
sun% a.out -f none -L10 -G10 -T5
foreign file dbase loaded 0.066667 seconds, 1578 bytes.
Welcome to SWI-Prolog (version 1.5.0, August 1990)
Copyright (c) 1990, University of Amsterdam. All rights reserved.
1 ?- initialise.
Yes
2 ?- save_program(my_program,
[ local = 500
, goal = go
, init_file = none
]).
Running executable: /usr/local/bin/pl
Saving to my_program; text: 204800 ... data: 357000 ... symbols ...
done.
Yes
2 ?- halt.
sun%
Figure 2.3: Create a stand-alone executable
The resulting program can be used for incremental compilation using
-c or another save_program/2.
save_program(+NewProgram)
Equivalent to `save_program(NewProgram, [])'.
2.9 Automatic loading of libraries
If ---at runtime--- an undefined predicate is trapped the system will
first try to import the predicate from the module's default module. If
this fails the auto loader is activated. On first activation an index to
all library files in all library directories is loaded in core (see
library_directory/1). If the undefined predicate can be located in the
one of the libraries that library file is automatically loaded and the
call to the (previously undefined) predicate is resumed. By default this
mechanism loads the file silently. The please/3 option verbose_autoload
is provided to get verbose loading. The please option autoloadcan be
used to enable/disable the entire auto load system.
Autoloading only handles (library) source files that use the module
mechanism described in chapter 4. The files are loaded with use_module/2
and only the trapped undefined predicate will be imported to the module
where the undefined predicate was called. Each library directory must
hold a file INDEX.plthat contains an index to all library files in the
directory. This file consists of lines of the following format:
index(Name, Arity, Module, File).
The predicate make/0 scans the autoload libraries and updates the index if
it exists, is writable and out-of-date. It is adviced to create an empty
file called INDEX.plin a library directory meant for auto loading before
doing anything else. This index file can then be updated by running the
prolog predicate make/0 (`%' is the Unix prompt):
% mkdir ~/lib/prolog
% cd !$
% touch INDEX.pl
% <create library files>
% pl -g true -t make
index for library . ... ok.
%
If there are more than one library files containing the desired predicate
the following search schema is followed:
1. If a there is a library file that defines the module in which the
undefined predicate is trapped, this file is used.
2. Otherwise library files are considered in the order they appear in
the library_directory/1 predicate and within the directory
alphabetically.
2.9.1 Notes on Automatic Loading
The autoloader is a new feature to SWI-Prolog. Its aim is to simplify
program development and program management. Common lisp has a similar
feature, but here the user has to specify which library is to be loaded if
a specific function is called which is not defined. The advantage of the
SWI-Prolog schema is that the user does not have to specify this. The
disadvantage however is that the user might be wondering ``where the hell
this predicate comes from''. Only experience can learn whether the
functionality of the autoloader is appropriate. Comments are welcome.
The autoloader only works if the unknown flag (see unknown/2) is set to
trace(default). A more appropriate interaction with this flag will be
considered.
2.10 Garbage Collection
SWI-Prolog version 1.4 was the first release to support garbage
collection. Together with tail-recursion optimisation this guaranties
forward chaining programs do not waste indefinite amounts of memory.
Previous releases of this manual stressed on using failure-driven loops in
those cases that no information needed to be passed to the next iteration
via arguments. This to avoid large amounts of garbage. This is no longer
stricktly necessary, but it should be noticed that garbage collection is a
time consuming activity. Failure driven loops tend to be faster for this
reason.
BUG: The garbage collector is deactivated when Prolog is called back from
a foreign language predicate. This implies there is no garbage collection
within a break environment. More seriously, there is no garbage
collection when handling call-backs from ---for example--- the PCE
package.
2.11 Syntax Notes
SWI-Prolog uses standard `Edinburgh' syntax. A description of this syntax
can be found in the Prolog books referenced in the introduction. Below
are some non-standard or non-common constructs that are accepted by
SWI-Prolog:
0'<char>
This construct is not accepted by all Prolog systems that claim to
have Edinburgh compatible syntax. It describes the ASCII value of
<char>. To test whether C is a lower case character one can use
`between(0'a, 0'z, C)'.
/* ... /* .... */ ... */
The /* ... */ comment statement can be nested. This is useful if
some code with /* ... */ comment statements in it should be commented
out.
2.12 System Limits
2.12.1 Limits on Memory Areas
SWI-Prolog has a number of memory areas which are not enlarged at run
time, unless you have a version with dynamic stack allocation. The
default sizes for these areas should suffice for small applications, but
most serious application require larger ones. They all can be modified by
command line options. The table below shows these areas. The first
column gives the option name to modify the size of the area. This option
character should be followed immediately by a number and expresses the
number of kilo bytes to use for the area. There are no other limits than
the available memory of the machine to the sizes of the areas. The areas
are described in table 2.3.
The heap is a memory area to store atoms, clauses, records, flags, etc.
This area is dynamically enlarged at runtime on all versions of
SWI-Prolog.
Option Default Area name Description
-L 200K (2M) local stack The local stack is used to store the
execution environments of procedure
invocations. The space for an
environment is reclaimed when it
fails, exits without leaving choice
points, the alternatives are cut of
with the ! predicate or no choice
points have been created since the
invocation and the last subclause is
started (tail recursion
optimisation).
-G 100K (4M) global stack The global stack is used to store
terms created during Prolog's
execution. Terms on this stack will
be reclaimed by backtracking to a
point before the term was created or
by garbage collection (provided the
term is no longer referenced).
-T 50K (4M) trail stack The trail stack is used to store
assignments during execution.
Entries on this stack remain alive
until backtracking before the point
of creation or the garbage collector
determines they are nor needed any
longer.
-A 5K (1M) argument stack The argument stack is used to store
one of the intermediate code
interpreter's registers. The amount
of space needed on this stack is
determined entirely by the depth in
which terms are nested in the clauses
that constitute the program.
Overflow is most likely when using
long strings in a clause.
Table 2.3: Memory areas
2.12.2 Other Limits
ClausesCurrently the following limitations apply to clauses. The arity
may not be more than 128, the number of links to the `outside world'
(predicates, atoms, (large) integers, etc) may not exceed 512 and the
number of variables should be less than 256.
Atoms and StringsSWI-Prolog has no limits on the sizes of atoms and
strings. read/1 and its derivates however normally limit the number
of newlines in an atom or string to 5 to improve error detection and
recovery. This can be switched off with style_check/1.
Address spaceSWI-Prolog uses tagged pointers internally. This limits the
number of available bits for addressing memory to 29 (512 Mb).
IntegersIntegers are tagged values. Their value is limited between 226
and 226 1.
FloatsFloating point numbers are C-doubles with a 12 bit reduction of the
mantisse. For machines using IEEE floating point format, this
implies the range is about 10308and the accurracy about 10 digits.
2.12.3 Reserved Names
The boot compiler (see -b option) does not support the module system
(yet). As large parts of the system are written in Prolog itself we need
some way to avoid name clashes with the user's predicates, database keys,
etc. Like Edinburgh C-Prolog all predicates, database keys, etc. that
should be hidden from the user start with a dollar ($) sign (see
style_check/2).
The compiler uses the special functor $VAR$/1 while analysing the clause
to compile. Using this functor in a program causes unpredictable
behaviour of the compiler and resulting program.
Chapter 3
Built-In Predicates
33
3.1 Notation of Predicate Descriptions
We have tried to keep the predicate descriptions clear and concise. First
the predicate name is printed in bold face, followed by the arguments in
italics. Arguments are preceded by a `+', `--' or `?' sign. `+'
indicates the argument is input to the predicate, `--' denotes output and
`?' denotes `either input or output'. Constructs like `op/3' refer to
the predicate `op' with arity `3'.
3.2 Consulting Prolog Source files
SWI-Prolog source files normally have a suffix `.pl'. Specifying the
suffix is optional. All predicates that handle source files first check
whether a file with suffix `.pl' exists. If not the plain file name is
checked for existence. Library files are specified by embedding the file
name using the functor library/1. Thus `foo' refers to `foo.pl' or `foo'
in the current directory, `library(foo)' refers to `foo.pl' or `foo' in
one of the library directories specified by the dynamic predicate
library_directory/1.
SWI-Prolog recognises grammar rules as defined in . The user may define
additional compilation of the source file by defining the dynamic
predicate term_expansion/2. Transformations by this predicate overrule
the systems grammar rule transformations. It is not allowed to use
assert/1, retract/1 or any other database predicate in term_expansion/2
other than for local computational purposes.
Directives may be placed anywhere in a source file, invoking any
predicate. They are executed when encountered. If the directive fails, a
warning is printed. Directives are specified by :-/1 or ?-/1. There is
no difference between the two.
SWI-Prolog does not have a separate reconsult/1 predicate. Reconsulting
is implied automatically by the fact that a file is consulted which is
already loaded.
consult(+File)
Read File as a Prolog source file. File may be a list of files, in
which case all members are consulted in turn. File may start with
the csh(1) special sequences ~, ~<user> and $<var>. File may also be
library(Name), in which case the libraries are searched for a file
with the specified name. See also library_directory/1. consult/1
may be abbreviated by just typing a number of file names in a list.
Examples:
?- consult(load). % consult `load' or `load.pl'
?- [library(quintus)]. % load Quintus compatibility library
ensure_loaded(+File)
Equivalent to consult/1, but the file is consulted only if this was
not done before. This is the recommended way to load files from
other files.
make
Consult all source files that have been changed since they were
consulted. It checks all loaded source files: files loaded into a
compiled state using pl -c ... and files loaded using consult or one
of its derivates. make/0 is normally invoked by the edit/[0,1] and
ed/[0,1] predicates. make/0 can be combined with the compiler to
speed up the development of large packages. In this case compile the
package using
sun% pl -g make -o my_program -c file ...
If `my_program' is started it will first reconsult all source files
that have changed since the compilation.
library_directory(-Atom)
Dynamic predicate used to specify library directories. Default .,
./lib, ~/lib/prolog and the system's library (in this order) are
defined. The user may add library directories using assert/1 or
remove system defaults using retract/1.
source_file(-File)
Succeeds if File was loaded using consult/1 or ensure_loaded/1. File
refers to the full path name of the file (see expand_file_name/2).
Source_file/1 backtracks over all loaded source files.
source_file(?Pred, ?File)
Is true if the predicate specified by Pred was loaded from file File,
where File is an absolute path name (see expand_file_name/2). Can be
used with any instantiation pattern, but the database only maintains
the source file for each predicate. Predicates declared multifile
(see multifile/1) cannot be found this way.
term_expansion(+Term1, -Term2)
Dynamic predicate, normally not defined. When defined by the user
all terms read during consulting that are given to this predicate.
If the predicate succeeds Prolog will assert Term2 in the database
rather then the read term (Term1). Term2 may be a term of a the form
`?- Goal' or `:- Goal'. Goal is then treated as a directive. Term2
may also be a list, in which case all terms of the list are stored in
the database or called (for directives).
compiling
Succeeds if the system is compiling source files with the -c option
into an intermediate code file. Can be used to perform code
optimisations in expand_term/2 under this condition.
preprocessor(-Old, +New)
Read the input file via a Unix process that acts as preprocessor. A
preprocessor is specified as an atom. The first occurrence of the
string `%f' is replaced by the name of the file to be loaded. The
resulting atom is called as a Unix command and the standard output of
this command is loaded. To use the Unix C preprocessor one should
define:
?- preprocessor(Old, '/lib/cpp -C -P %f'), consult(...).
Old = none
3.3 Listing Predicates and Editor Interface
SWI-Prolog offers an interface to the Unix vi editor (vi(1)), Richard
O'Keefe's top editor and the GNU-EMACS invocations emacs and emacsclient.
Which editor is used is determined by the Unix environment variable
EDITOR, which should hold the full pathname of the editor. If this
variable is not defined, vi(1) is used.
After the user quits the editor make/0 is invoked to reload all modified
source files using consult/1. If the editor can be quit such that an exit
status non-equal to 0 is returned make/0 will not be invoked. top can do
this by typing control-C, vi cannot do this.
A predicate specification is either a term with the same functor and arity
as the predicate wanted, a term of the form Functor/Arityor a single
atom. In the latter case the database is searched for a predicate of this
name and arbitrary arity (see current_predicate/2). When more than one
such predicate exists the system will prompt for confirmation on each of
the matched predicates. Predicates specifications are given to the `Do
What I Mean' system (see dwim_predicate/2) if the requested predicate does
not exist.
ed(+Pred)
Invoke the user's preferred editor on the source file of Pred,
providing a search specification which searches for the predicate at
the start of a line.
ed
Invoke ed/1 on the predicate last edited using ed/1. Asks the user
to confirm before starting the editor.
edit(+File)
Invoke the user's preferred editor on File. File is a file
specification as for consult/1 (but not a list). Note that the file
should exist.
edit
Invoke edit/1 on the file last edited using edit/1. Asks the user to
confirm before starting the editor.
listing(+Pred)
List specified predicates (when an atom is given all predicates with
this name will be listed). The listing is produced on the basis of
the internal representation, thus loosing user's layout and variable
name information. See also portray_clause/1.
listing
List all predicates of the database using listing/1.
portray_clause(+Clause)
Pretty print a clause as good as we can. A clause should be
specified as a term `Head :- Body' (put brackets around it to avoid
operator precedence problems). Facts are represented as
`Head :- true'.
3.4 Verify Type of a Term
var(+Term)
Succeeds if Term currently is a free variable.
nonvar(+Term)
Succeeds if Term currently is not a free variable.
integer(+Term)
Succeeds if Term is bound to an integer.
float(+Term)
Succeeds if Term is bound to a floating point number.
number(+Term)
Succeeds if Term is bound to an integer or a floating point number.
atom(+Term)
Succeeds if Term is bound to an atom.
string(+Term)
Succeeds if Term is bound to a string.
atomic(+Term)
Succeeds if Term is bound to an atom, string, integer or floating
point number.
ground(+Term)
Succeeds if Term holds no free variables.
3.5 Comparison and Unification or Terms
3.5.1 Standard Order of Terms
Comparison and unification of arbitrary terms. Terms are ordered in the
so called ``standard order''. This order is defined as follows:
1. Variables< Atoms< Strings< Numbers< Terms
2. Old Variable < New Variable
3. Atoms are compared alphabetically.
4. Strings are compared alphabetically.
5. Numbers are compared by value. Integers and floats are treated
identically.
6. Terms are first checked on their functor (alphabetically), then on
their arity and finally recursively on their arguments, left most
argument first.
+Term1 == +Term2
Succeeds if Term1 is equivalent to Term2. A variable is only
identical to a sharing variable.
+Term1 \== +Term2
Equivalent to `\+ Term1 == Term2'.
+Term1 = +Term2
Unify Term1 with Term2. Succeeds if the unification succeeds.
+Term1 \= +Term2
Equivalent to `\+ Term1 = Term2'.
+Term1 =@= +Term2
Succeeds if Term1 is `structurally equal' to Term2. Structural
equivalence is weaker than equivalence (==/2), but stronger than
unification (=/2). Two terms are structurally equal if their tree
representation is identical and they have the same `pattern' of
variables. Examples:
a =@=A false
A =@=B true
x(A,A) =@=x(B,C) false
x(A,A) =@=x(B,B) true
x(A,B) =@=x(C,D) true
+Term1 \=@= +Term2
Equivalent to `\+ Term1 =@= Term2'.
+Term1 @< +Term2
Succeeds if Term1 is before Term2 in the standard order of terms.
+Term1 @=< +Term2
Succeeds if both terms are equal (==) or Term1 is before Term2 in the
standard order of terms.
+Term1 @> +Term2
Succeeds if Term1 is after Term2 in the standard order of terms.
+Term1 @>= +Term2
Succeeds if both terms are equal (==) or Term1 is after Term2 in the
standard order of terms.
3.6 Control Predicates
The predicates of this section implement control structures. Normally
these constructs are translated into virtual machine instructions by the
compiler. It is still necessary to implement these constructs as true
predicates to support meta-calls, as demonstrated in the example below.
The predicate finds all currently defined atoms of 1 character long. Note
that the cut has no effect when called via one of these predicates (see
!/0).
one_character_atoms(As) :-
findall(A, (current_atom(A), atom_length(A, 1)), As).
fail
Always fail.
true
Always succeed.
repeat
Always succeed, provide an infinite number of choice points.
!
Cut. Discard choice points of parent frame and frames created after
the parent frame. Note that the control structures ;/2, |/2 ->/2 and
\+/1 are normally handled by the compiler and do not create a frame,
which implies the cut operates through these predicates. Some
examples are given below. Note the difference between t3/1 and t4/1.
Also note the effect of call/1 in t5/0. As the argument of call/1 is
evaluated by predicates rather than the compiler the cut has no
effect.
t1 :- (a, !, fail ; b). % cuts a/0 and t1/0
t2 :- (a -> b, ! ; c). % cuts b/0 and t2/0
t3(G) :- a, G, fail. % if `G = !' cuts a/0 and t1/1
t4(G) :- a, call(G), fail. % if `G = !' cut has no effect
t5 :- call((a, !, fail ; b)).% Cut has no effect
t6 :- \+ (a, !, fail ; b). % cuts a/0 and t6/0
+Goal1 , +Goal2
Conjunction. Succeeds if both `Goal1' and `Goal2' can be proved. It
is defined as (this definition does not lead to a loop as the second
comma is handled by the compiler):
Goal1, Goal2 :- Goal1, Goal2.
+Goal1 ; +Goal2
The `or' predicate is defined as:
Goal1 ; _Goal2 :- Goal1.
_Goal1 ; Goal2 :- Goal2.
+Goal1 | +Goal2
Equivalent to ;/2. Retained for compatibility only. New code should
use ;/2.
+Condition -> +Action
If-then and If-Then-Else. Implemented as:
If -> Then; _Else :- If, !, Then.
If -> _Then; Else :- !, Else.
If -> Then :- If, !, Then.
\+ +Goal
Succeeds if `Goal' cannot be proven (mnemnonic: + refers to provable
and the backslash is normally used to indicate negation).
3.7 Meta-Call Predicates
Meta call predicates are used to call terms constructed at run time. The
basic meta-call mechanism offered by SWI-Prolog is to use variables as a
subclause (which should of course be bound to a valid goal at runtime). A
meta-call is slower than a normal call as it involves actually searching
the database at runtime for the predicate, while for normal calls this
search is done at compile time.
call(+Goal)
Invoke Goal as a goal. Note that clauses may have variables as
subclauses, which is identical to call/1, except when the argument is
bound to the cut. See !/0.
apply(+Term, +List)
Append the members of List to the arguments of Term and call the
resulting term. For example: `apply(plus(1), [2, X])' will call
`plus(1, 2, X)'. Apply/2 is incorporated in the virtual machine of
SWI-Prolog. This implies that the overhead can be compared to the
overhead of call/1.
not +Goal
Succeeds when Goal cannot be proven. Retained for compatibility
only. New code should use \+/1.
once(+Goal)
Defined as:
once(Goal) :-
Goal, !.
Once/1 can in many cases be replaced with ->/2. The only difference
is how the cut behaves (see !/0). The following two clauses are
identical:
1) a :- once((b, c)), d.
2) a :- b, c -> d.
ignore(+Goal)
Calls Goal as once/1, but succeeds, regardless of whether Goal
succeeded or not. Defined as:
ignore(Goal) :-
Goal, !.
ignore(_).
3.8 Database
SWI-Prolog offers three different database mechanisms. The first one is
the common assert/retract mechanism for manipulating the clause database.
As facts and clauses asserted using assert/1 or one of it's derivates
become part of the program these predicates compile the term given to
them. Retract/1 and retractall/1 have to unify a term and therefore have
to decompile the program. For these reasons the assert/retract mechanism
is expensive. On the other hand, once compiled, queries to the database
are faster than querying the recorded database discussed below. See also
dynamic/1.
The second way of storing arbitrary terms in the database is using the
``recorded database''. In this database terms are associated with a key.
A key can be an atom, integer or term. In the last case only the functor
and arity determine the key. Each key has a chain of terms associated
with it. New terms can be added either at the head or at the tail of this
chain. This mechanism is considerably faster than the assert/retract
mechanism as terms are not compiled, but just copied into the heap.
The third mechanism is a special purpose one. It associates an integer or
atom with a key, which is an atom, integer or term. Each key can only
have one atom or integer associated with it. It again is considerably
faster than the mechanisms described above, but can only be used to store
simple status information like counters, etc.
abolish(+Functor, +Arity)
Removes all clauses of a predicate with functor Functor and arity
Arity from the database. Unlike version 1.2, all predicate
attributes (dynamic, multifile, index, etc.) are reset to their
defaults. Abolishing an imported predicate only removes the import
link; the predicate will keep its old definition in its definition
module. For `cleanup' of the dynamic database, one should use
retractall/1 rather than abolish/2.
retract(+Term)
When Term is an atom or a term it is unified with the first unifying
fact or clause in the database. The fact or clause is removed from
the database.
retractall(+Term)
All facts or clauses in the database that unify with Term are
removed.
assert(+Term)
Assert a fact or clause in the database. Term is asserted as the
last fact or clause of the corresponding predicate.
asserta(+Term)
Equivalent to assert/1, but Term is asserted as first clause or fact
of the predicate.
assertz(+Term)
Equivalent to assert/1.
assert(+Term, -Reference)
Equivalent to assert/1, but Reference is unified with a unique
reference to the asserted clause. This key can later be used with
clause/3 or erase/1.
asserta(+Term, -Reference)
Equivalent to assert/2, but Term is asserted as first clause or fact
of the predicate.
assertz(+Term, -Reference)
Equivalent to assert/2.
recorda(+Key, +Term, -Reference)
Assert Term in the recorded database under key Key. Key is an
integer, atom or term. Reference is unified with a unique reference
to the record (see erase/1).
recorda(+Key, +Term)
Equivalent to recorda(Key, Value, _).
recordz(+Key, +Term, -Reference)
Equivalent to recorda/3, but puts the Term at the tail of the terms
recorded under Key.
recordz(+Key, +Term)
Equivalent to recordz(Key, Value, _).
recorded(+Key, -Value, -Reference)
Unify Value with the first term recorded under Key which does unify.
Reference is unified with the memory location of the record.
recorded(+Key, -Value)
Equivalent to recorded(Key, Value, _).
erase(+Reference)
Erase a record or clause from the database. Reference is an integer
returned by recorda/3 or recorded/3, clause/3, assert/2, asserta/2 or
assertz/2. Other integers might conflict with the internal
consistency of the system. Erase can only be called once on a record
or clause. A second call also might conflict with the internal
consistency of the system.
BUG: The system should have a special type for pointers, thus
avoiding the Prolog user having to worry about consistency matters.
Currently some simple heuristics are used to determine whether a
reference is valid.
flag(+Key, -Old, +New)
Key is an atom, integer or term. Unify Old with the old value
associated with Key. If the key is used for the first time Old is
unified with the integer 0. Then store the value of New, which
should be an integer, atom or arithmetic integer expression, under
Key. flag/3 is a very fast mechanism for storing simple facts in the
database. Example:
:- module_transparent succeeds_n_times/2.
succeeds_n_times(Goal, Times) :-
flag(succeeds_n_times, _, 0),
Goal,
flag(succeeds_n_times, N, N+1),
fail ; flag(succeeds_n_times, Times, Times).
3.9 Declaring Properties of Predicates
This section describes directives which manipulate attributes of predicate
definitions. The functors dynamic/1, multifile/1 and discontiguous/1 are
operators of priority 1150 (see op/3), which implies the list of
predicates they involve can just be a comma separated list:
:- dynamic
foo/0,
baz/2.
On SWI-Prolog all these directives are just predicates. This implies they
can also be called by a program. Do not rely on this feature if you want
to maintain portability to other Prolog implementations.
dynamic +Functor/+Arity, ...
Informs the interpreter that the definition of the predicate(s) may
change during execution (using assert/1 and/or retract/1). Currently
dynamic/1 only stops the interpreter from complaining about undefined
predicates (see unknown/2). Future releases might prohibit assert/1
and retract/1 for not-dynamic declared procedures.
multifile +Functor/+Arity, ...
Informs the system that the specified predicate(s) may be defined
over more than one file. This stops consult/1 from redefining a
predicate when a new definition is found.
discontiguous +Functor/+Arity, ...
Informs the system that the clauses of the specified predicate(s)
might not be together in the source file. See also style_check/1.
index(+Head)
Index the clauses of the predicate with the same name and arity as
Head on the specified arguments. Head is a term of which all
arguments are either `1' (denoting `index this argument') or `0'
(denoting `do not index this argument'). Indexing has no
implications for the semantics of a predicate, only on its
performance. If indexing is enabled on a predicate a special purpose
algorithm is used to select candidate clauses based on the actual
arguments of the goal. This algorithm checks whether indexed
arguments might unify in the clause head. Only atoms, integers and
functors (e.g. name and arity of a term) are considered. Indexing
is very useful for predicates with many clauses representing facts.
Due to the representation technique used at most 4 arguments can be
indexed. All indexed arguments should be in the first 32 arguments
of the predicate. If more than 4 arguments are specified for
indexing only the first 4 will be accepted. Arguments above 32 are
ignored for indexing.
By default all predicates with arity 1 are indexed on their first
argument. It is possible to redefine indexing on predicates that
already have clauses attached to them. This will initiate a scan
through the predicate's clause list to update the index summary
information stored with each clause.
If --for example-- one wants to represents sub-types using a fact
list `sub_type(Sub, Super)'that should be used both to determine
sub- and super types one should declare sub_type/2 as follows:
:- index(sub_type(1, 1)).
sub_type(horse, animal).
...
...
3.10 Examining the Program
current_atom(-Atom)
Successively unifies Atom with all atoms known to the system. Note
that current_atom/1 always succeeds if Atom is intantiated to an
atom.
current_functor(?Name, ?Arity)
Successively unifies Name with the name and Arity with the arity of
functors known to the system.
current_flag(-FlagKey)
Successively unifies FlagKey with all keys used for flags (see
flag/3).
current_key(-Key)
Successively unifies Key with all keys used for records (see
recorda/3, etc.).
current_predicate(?Name, ?Head)
Successively unifies Name with the name of predicates currently
defined and Head with the most general term built from Name and the
arity of the predicate. This predicate succeeds for all predicates
defined in the specified module, imported to it, or in one of the
modules from which the predicate will be imported if it is called.
predicate_property(?Head, ?Property)
Succeeds if Head refers to a predicate that has property Property.
Can be used to test whether a predicate has a certain property,
obtain all properties known for Head, find all predicates having
property or even obtaining all information available about the
current program. Property is one of:
interpreted
Is true if the predicate is defined in Prolog. We return true on
this because, although the code is actually compiled, it is
completely transparent, just like interpreted code.
built_in
Is true if the predicate is locked as a built-in predicate. This
implies it cannot be redefined in it's definition module and it
can normally not be seen in the tracer.
foreign
Is true if the predicate is defined in the C language.
dynamic
Is true if the predicate is declared dynamic using the dynamic/1
declaration.
multifile
Is true if the predicate is declared multifile using the
multifile/1 declaration.
undefined
Is true if a procedure definition block for the predicate exists,
but there are no clauses in it and it is not declared dynamic.
This is true if the predicate occurs in the body of a loaded
predicate, an attempt to call it has been made via one of the
meta-call predicates or the predicate had a definition in the
past. See the library package check for example usage.
transparent
Is true if the predicate is declared transparent using the
module_transparent/1 declaration.
exported
Is true if the predicate is in the public list of the context
module.
imported_from(Module)
Is true if the predicate is imported into the context module from
module Module.
indexed(Head)
Predicate is indexed (see index/1) according to Head. Head is a
term whose name and arity are identical to the predicate. The
arguments are unified with `1' for indexed arguments, `0'
otherwise.
dwim_predicate(+Term, -Dwim)
`Do What I Mean' (`dwim') support predicate. Term is a term, which
name and arity are used as a predicate specification. Dwim is
instantiated with the most general term built from Name and the arity
of a defined predicate that matches the predicate specified by Term
in the `Do What I Mean' sence. See dwim_match/2 for `Do What I Mean'
string matching. Internal system predicates are not generated,
unless style_check(+dollar)is active. Backtracking provides all
alternative matches.
clause(?Head, ?Body)
Succeeds when Head can be unified with a clause head and Body with
the corresponding clause body. Gives alternative clauses on
backtracking. For facts Body is unified with the atom true.
Normally clause/2 is used to find clause definitions for a predicate,
but it can also be used to find clause heads for some body template.
clause(?Head, ?Body, ?Reference)
Equivalent to clause/2, but unifies Reference with a unique reference
to the clause (see also assert/2, erase/1). If Reference is
instantiated to a reference the clause's head and body will be
unified with Head and Body.
3.11 Input and Output
SWI-Prolog provides two different packages for input and output. One
confirms to the Edinburgh standard. This package has a notion of
`current-input' and `current-output'. The reading and writing predicates
implicitely refer to these streams. In the second package, streams are
opened explicitely and the resulting handle is used as an argument to the
reading and writing predicate to specify the source or destination. Both
packages are fully integrated; the user may switch freely between them.
3.11.1 Input and Output Using Implicit Source and Destination
The package for implicit input and output destination is upwards
compatible to DEC-10 and C-Prolog. The reading and writing predicates
refer to resp. the current input- and output stream. Initially these
streams are connected to the terminal. The current output stream is
changed using tell/1 or append/1. The current input stream is changed
using see/1. The stream's current value can be obtained using telling/1
for output- and seeing/1 for input streams. The table below shows the
valid stream specifications. The reserved names user_input, user_output
and user_errorare for neat integration with the explicit streams.
user This reserved name refers to the
terminal
user_input Input from the terminal
user_output Output to the terminal
stderr or user_error Unix error stream (output only)
Atom Name of a Unix file
pipe(Atom) Name of a Unix command
Source and destination are either a file, one of the reserved words above,
or a term `pipe(Command)'. In the predicate descriptions below we will
call the source/destination argument `SrcDest'. Below are some examples
of source/destination specifications.
?- see(data). % Start reading from file `data'.
?- tell(stderr). % Start writing on the error stream.
?- tell(pipe(lpr)). % Start writing to the printer.
Another example of using the pipe/1 construct is shown on in figure 3.1.
Note that the pipe/1 construct is not part of Prolog's standard I/O
reportoire.
getwd(Wd) :-
seeing(Old), see(pipe(pwd)),
collect_wd(String),
seen, see(Old),
name(Wd, String).
collect_wd([C|R]) :-
get0(C), C \== -1, !,
collect_wd(R).
collect_wd([]).
Figure 3.1: Get the working directory
see(+SrcDest)
Make SrcDest the current input stream. If SrcDest was already opened
for reading with see/1 and has not been closed since, reading will be
resumed. Otherwise SrcDest will be opened and the file pointer is
positioned at the start of the file.
tell(+SrcDest)
Make SrcDest the current output stream. If SrcDest was already
opened for writing with tell/1 or append/1 and has not been closed
since, writing will be resumed. Otherwise the file is created or
--when existing-- truncated. See also append/1.
append(+File)
Similar to tell/1, but positions the file pointer at the end of File
rather than truncating an existing file. The pipe construct is not
accepted by this predicate.
seeing(-SrcDest)
Unify the name of the current input stream with SrcDest.
telling(-SrcDest)
Unify the name of the current output stream with SrcDest.
seen
Close the current input stream. The new input stream becomes user.
told
Close the current output stream. The new output stream becomes user.
3.11.2 Explicit Input and Output Streams
The predicates below are part of the Quintus compatible stream-based I/O
package. In this package streams are explicitely created using the
predicate open/3. The resulting stream identifier is then passed as a
parameter to the reading and writing predicates to specify the source or
destination of the data.
open(+SrcDest, +Mode, ?Stream)
SrcDest is either an atom, specifying a Unix file, or a term
`pipe(Command)', just like see/1 and tell/1. Mode is one of read,
writeor append. Stream is either a variable, in which case it is
bound to a small integer identifying the stream, or an atom, in which
case this atom will be the stream indentifier. In the latter case
the atom cannot be an already existing stream identifier. Examples:
?- open(data, read, Stream). % Open `data' for reading.
?- open(pipe(lpr), write, printer).% `printer' is a stream to `lpr'.
open_null_stream(?Stream)
On Unix systems, this is equivalent to
open('/dev/null', write, Stream). Characters written to this stream
are lost, but the stream information (see character_count/2, etc.)
is maintained.
close(+Stream)
Close the specified stream. If Stream is not open an error message
is displayed. If the closed stream is the current input or output
stream the terminal is made the current input or output.
current_stream(?File, ?Mode, ?Stream)
Is true if a stream with file specification File, mode Mode and
stream identifier Stream is open. The reserved streams user and
user_error are not generated by this predicate. If a stream has been
opened with mode appendthis predicate will generate mode write.
stream_position(+Stream, -Old, +New)
Unify the position parameters of Stream with Old and set them to New.
A position is represented by the following term:
'$stream_position'(CharNo, LineNo, LinePos).
It is only possible to change the position parameters if the stream
is connected to a disk file.
3.11.3 Switching Between Implicit and Explicit I/O
The predicates below can be used for switching between the implicit- and
the explicit stream based I/O predicates.
set_input(+Stream)
Set the current input stream to become Stream. Thus, open(file,
read, Stream), set_input(Stream) is equivalent to see(file).
set_output(+Stream)
Set the current output stream to become Stream.
current_input(-Stream)
Get the current input stream. Useful to get access to the status
predicates associated with streams.
current_output(-Stream)
Get the current output stream.
3.12 Status of Input and Output Streams
wait_for_input(+ListOfStreams, -ReadyList, +TimeOut)
Wait for input on one of the streams in ListOfStreams and return a
list of streams on which input is available in ReadyList.
wait_for_input/3 waits for at most TimeOut seconds. Timeout may be
specified as a floating point number to specify fractions of a
second. If Timeout equals 0, wait_for_input/3 waits indefinetely.
This predicate can be used to implement timeout while reading and to
handle input from multiple sources. The following example will wait
for input from the user and an explicitely opened second terminal.
On return, Inputs may hold user or P4 or both.
?- open('/dev/ttyp4', read, P4),
wait_for_input([user, P4], Inputs, 0).
character_count(+Stream, -Count)
Unify Count with the current character index. For input streams this
is the number of characters read since the open, for output streams
this is the number of characters written. Counting starts at 0.
line_count(+Stream, -Count)
Unify Count with the number of lines read or written. Counting
starts at 1.
line_position(+Stream, -Count)
Unify Count with the position on the current line. Note that this
assumes the position is 0 after the open. Tabs are assumed to be
defined on each 8-th character and backspaces are assumed to reduce
the count by one, provided it is positive.
fileerrors(-Old, +New)
Define error behaviour on errors when opening a file for reading or
writing. Valid values are the atoms on (default) and off. First Old
is unified with the current value. Then the new value is set to New.
tty_fold(-OldColumn, +NewColumn)
Fold Prolog output to stream user on column NewColumn. If Column is
0 or less no folding is performed (default). OldColumn is first
unified with the current folding column. To be used on terminals
that do not support line folding.
3.13 Primitive Character Input and Output
nl
Write a newline character to the current output stream. On Unix
systems nl/0 is equivalent to put(10).
nl(+Stream)
Write a newline to Stream.
put(+Char)
Write Char to the current output stream, Char is either an
integer-expression evaluating to an ASCII value (0 Char 255) or an
atom of one character.
put(+Stream, +Char)
Write Char to Stream.
tab(+Amount)
Writes Amount spaces on the current output stream. Amount should be
an expression that evaluates to a positive integer (see
section 3.19).
tab(+Stream, +Amount)
Writes Amount spaces to Stream.
flush
Flush pending output on current output stream. flush/0 is
automatically generated by read/1 and derivates if the current input
stream is user and the cursor is not at the left margin.
flush_output(+Stream)
Flush output on the specified stream. The stream must be open for
writing.
ttyflush
Flush pending output on stream user. See also flush/0.
get0(-Char)
Read the current input stream and unify the next character with Char.
Char is unified with -1 on end of file.
get0(+Stream, -Char)
Read the next character from Stream.
get(-Char)
Read the current input stream and unify the next non-blank character
with Char. Char is unified with -1 on end of file.
get(+Stream, -Char)
Read the next non-blank character from Stream.
get_single_char(-Char)
Get a single character from input stream `user' (regardless of the
current input stream). Unlike get0/1 this predicate does not wait
for a return. The character is not echoed to the user's terminal.
This predicate is meant for keyboard menu selection etc.. If
SWI-Prolog was started with the -tty flag this predicate reads an
entire line of input and returns the first non-blank character on
this line, or the ASCII code of the newline (10) if the entire line
consisted of blank characters.
3.14 Term Reading and Writing
display(+Term)
Write Term on the current output stream using standard parenthesised
prefix notation (i.e. ignoring operator declarations). Display is
normally used to examine the internal representation for terms
holding operators.
display(+Stream, +Term)
Display Term on Stream.
displayq(+Term)
Write Term on the current output stream using standard parenthesised
prefix notation (i.e. ignoring operator declarations). Atoms that
need quotes are quoted. Terms written with this predicate can always
be read back, regardless of current operator declarations.
displayq(+Stream, +Term)
Display Term on Stream. Equivalent to Quintus write_canonical/2.
write(+Term)
Write Term to the current output, using brackets and operators where
appropriate.
write(+Stream, +Term)
Write Term to Stream.
writeq(+Term)
Write Term to the current output, using brackets and operators where
appropriate. Atoms that need quotes are quoted. Terms written with
this predicate can be read back with read/1 provided the currently
active operator declarations are identical.
writeq(+Stream, +Term)
Write Term to Stream, inserting quotes.
print(+Term)
Prints Term on the current output stream similar to write/1, but for
each (sub)term of Term first the dynamic predicate portray/1 is
called. If this predicate succeeds print assumes the (sub)term has
been written. This allows for user defined term writing.
print(+Stream, +Term)
Print Term to Stream.
portray(+Term)
A dynamic predicate, which can be defined by the user to change the
behaviour of print/1 on (sub)terms. For each subterm encountered
that is not a variable print/1 first calls portray/1 using the term
as argument. For lists only the list as a whole is given to
portray/1. If portray succeeds print/1 assumes the term has been
written.
read(-Term)
Read the next Prolog term from the current input stream and unify it
with Term. On a syntax error read/1 displays an error message,
attempts to skip the erroneous term and fails. On reaching
end-of-file Term is unified with the atom end_of_file.
read(+Stream, -Term)
Read Term from Stream.
read_clause(-Term)
Equivalent to read/1, but warns the user for variables only occurring
once in a term (singleton variables) which do not start with an
underscore if style_check(singleton)is active (default). Used to
read Prolog source files (see consult/1).
read_clause(+Stream, -Term)
Read a clause from Stream.
read_variables(-Term, -Bindings)
Similar to read/1, but Bindings is unified with a list of
`Name= Var' tuples, thus providing access to the actual variable
names.
read_variables(+Stream, -Term, -Bindings)
Read, returning term and bindings from Stream.
read_history(+Show, +Help, +Special, +Prompt, -Term, -Bindings)
Similar to read_variables/2, but allows for history substitutions.
history_read/6 is used by the top level to read the user's actions.
Show is the command the user should type to show the saved events.
Help is the command to get an overview of the capabilities. Special
is a list of commands that are not saved in the history. Prompt is
the first prompt given. Continuation prompts for more lines are
determined by prompt/2. A %w in the prompt is substituted by the
event number. See section 2.4 for available substitutions.
SWI-Prolog calls history_read/6 as follows:
read_history(h, '!h', [trace], '%w ?- ', Goal, Bindings)
history_depth(-Int)
Dynamic predicate, normally not defined. The user can define this
predicate to set the history depth. It should unify the argument
with a positive integer. When not defined 15 is used as the default.
prompt(-Old, +New)
Set prompt associated with read/1 and its derivates. Old is first
unified with the current prompt. On success the prompt will be set
to New if this is an atom. Otherwise an error message is displayed.
A prompt is printed if one of the read predicates is called and the
cursor is at the left margin. It is also printed whenever a newline
is given and the term has not been terminated. Prompts are only
printed when the current input stream is user.
3.15 Analysing and Constructing Terms
functor(?Term, ?Functor, ?Arity)
Succeeds if Term is a term with functor Functor and arity Arity. If
Term is a variable it is unified with a new term holding only
variables. functor/3 silently fails on instantiation faults
arg(+Arg, +Term, ?Value)
Term should be instantiated to a term, Arg to an integer between 1
and the arity of Term. Value is unified with the Arg-th argument of
Term.
?Term =.. ?List
List is a list which head is the functor of Term and the remaining
arguments are the arguments of the term. Each of the arguments may
be a variable, but not both. This predicate is called `Univ'.
Examples:
?- foo(hello, X) =.. List.
List = [foo, hello, X]
?- Term =.. [baz, foo(1)]
Term = baz(foo(1))
numbervars(+Term, +Functor, +Start, -End)
Unify the free variables of Term with a term constructed from the
atom Functor with one argument. The argument is the number of the
variable. Counting starts at Start. End is unified with the number
that should be given to the next variable. Example:
?- numbervars(foo(A, B, A), this_is_a_variable, 0, End).
A = this_is_a_variable(0)
B = this_is_a_variable(1)
End = 2
In Edinburgh Prolog the second argument is missing. It is fixed to
be '$VAR'.
free_variables(+Term, -List)
Unify List with a list of variables, each sharing with a unique
variable of Term. For example:
?- free_variables(a(X, b(Y, X), Z), L).
L = [G367, G366, G371]
X = G367
Y = G366
Z = G371
copy_term(+In, -Out)
Make a copy of term In and unify the result with Out. Ground parts
of In are shared by Out. Provided In and Out have no sharing
variables before this call they will have no sharing variables
afterwards. copy_term/2 is semantically equivalent to:
copy_term(In, Out) :-
recorda(copy_key, In, Ref),
recorded(copy_key, Out, Ref),
erase(Ref).
3.16 Analysing and Constructing Atoms
name(?Atom, ?String)
String is a list of ASCII values describing Atom. Each of the
arguments may be a variable, but not both. When String is bound to
an ASCII value list describing an integer and Atom is a variable Atom
will be unified with the integer value described by String (e.g.
`name(N, "300"), 400 is N + 100' succeeds).
int_to_atom(+Int, +Base, -Atom)
Convert Int to an ascii representation using base Base and unify the
result with Atom. If Base6= 10 the base will be prepended to Atom.
Base= 0 will try to interpret Int as an ASCII value and return 0'c.
Otherwise 2 Base 36. Some examples are given below.
int_to_atom(45, 2, A) ! A = 20101101
int_to_atom(97, 0, A) ! A = 00a
int_to_atom(56, 10, A) ! A = 56
int_to_atom(+Int, -Atom)
Equivalent to int_to_atom(Int, 10, Atom).
term_to_atom(?Term, ?Atom)
Succeeds if Atom describes a term that unifies with Term. When Atom
is instantiated Atom is converted and then unified with Term.
Otherwise Term is ``written'' on Atom using write/1.
atom_to_term(+Atom, -Term, -Bindings)
Use Atom as input to read_variables/2 and return the read term in
Term and the variable bindings in Bindings. Bindings is a list of
Name= Varcouples, thus providing access to the actual variable
names. See also read_variables/2.
concat(?Atom1, ?Atom2, ?Atom3)
Atom3 forms the concatenation of Atom1 and Atom2. At least two of
the arguments must be instantiated to atoms, intergers or floating
point numbers.
concat_atom(+List, -Atom)
List is a list of atoms, integers or floating point numbers.
Succeeds if Atom can be unified with the concatenated elements of
List. If List has exactly 2 elements it is equivalent to concat/3,
allowing for variables in the list.
atom_length(+Atom, -Length)
Succeeds if Atom is an atom of Length characters long. This
predicate also works for integers and floats, expressing the number
of characters output when given to write/1.
3.17 Representing Text in Strings
SWI-Prolog supports the data type string. Strings are a time and space
efficient mechanism to handle text in Prolog. Atoms are under some
circumstances not suitable because garbage collection on them is next to
impossible (Although it is possible: BIM_prolog does it). Representing
text as a list of ASCII values is, from the logical point of view, the
cleanest solution. It however has two drawbacks: 1) they cannot be
distinguished from a list of (small) integers; and 2) they consume (in
SWI-Prolog) 12 bytes for each character stored.
Within strings each character only requires 1 byte storage. Strings live
on the global stack and their storage is thus reclaimed on backtracking.
Garbage collection can easily deal with strings.
The ISO standard proposes "..." is transformed into a string object by
read/1 and derivates. This poses problems as in the old convention "..."
is transformed into a list of ascii characters. For this reason the style
check option `string' is available (see style_check/2).
The set of predicates associated with strings is incomplete and tentative.
Names and definitions might change in the future to confirm to the
emerging standard.
string_to_atom(?String, ?Atom)
Logical conversion between a string and an atom. At least one of the
two arguments must be instantiated. Atom can also be an integer or
floating point number.
string_to_list(?String, ?List)
Logical conversion between a string and a list of ASCII characters.
At least one of the two arguments must be instantiated.
string_length(+String, -Length)
Unify Length with the number of characters in String. This predicate
is functonally equivalent to atom_length/2 and also accepts atoms,
integers and floats as its first argument.
substring(+String, +Start, +Length, -Sub)
Create a substring of String that starts at character Start (1 base)
and has Length characters. Unify this substring with Sub.
3.18 Operators
op(+Precedence, +Type, +Name)
Declare Name to be an operator of type Type with precedence
Precedence. Name can also be a list of names, in which case all
elements of the list are declared to be identical operators.
Precedence is an integer between 0 and 1200. Precedence 0 removes
the declaration. Type is one of: xf, yf, xfx, xfy, yfx, yfy, fy or
fx. The `f' indicates the position of the functor, while x and y
indicate the position of the arguments. `y' should be interpreted as
``on this position a term with precedence lower or equal to the
precedence of the functor should occur''. For `x' the precedence of
the argument must be strictly lower. The precedence of a term is 0,
unless its principal functor is an operator, in which case the
precedence is the precedence of this operator. A term enclosed in
brackets ((...)) has precedence 0.
The predefined operators are shown in table 3.1. Note that all
operators can be redefined by the user.
1200 xfx -->, :-
1200 fx :-, ?-
1150 fx dynamic, multifile,
module_transparent, discontiguous
1100 xfy ;, |
1050 xfy ->
1000 xfy ,
954 xfy \\
900 fy \+, not
700 xfx <, =, =.., =@=, =:=, =<, ==, =\=, >,
>=, @<, @=<, @>, @>=, \=, \==, is
600 xfy :
500 yfx +, -, /\, \/, xor
500 fx +, -, ?, \
400 yfx *, /, //, <<, >>
300 xfx mod
200 xfy ^
Table 3.1: System operators
current_op(?Precedence, ?Type, ?Name)
Succeeds when Name is currently defined as an operator of type Type
with precedence Precedence. See also op/3.
3.19 Arithmetic
Arithmetic can be divided into some special purpose integer predicates and
a series of general predicates for floating point and integer arithmetic
as appropriate. The integer predicates are as ``logical'' as possible.
Their usage is recommended whenever applicable, resulting in faster and
more ``logical'' programs.
The general arithmic predicates are optionaly compiled now (see please/3
and the -O command line option). Compiled arithmetic reduces global stack
requirements and improves performance. Unfortunately compiled arithmetic
cannot be traced, which is why it is optional.
The general arithmetic predicates all handle expressions. An expression
is either a simple number or a function. The arguments of a function are
expressions. The functions are described in section 3.20.
between(+Low, +High, ?Value)
Low and High are integers, High Low. If Value is an integer,
Low Value High. When Value is a variable it is successively bound
to all integers between Low and High.
succ(?Int1, ?Int2)
Succeeds if Int2= Int1+ 1. At least one of the arguments must be
instantiated to an integer.
plus(?Int1, ?Int2, ?Int3)
Succeeds if Int3= Int1+ Int2. At least two of the three arguments
must be instantiated to integers.
+Expr1 > +Expr2
Succeeds when expression Expr1 evaluates to a larger number than
Expr2.
+Expr1 < +Expr2
Succeeds when expression Expr1 evaluates to a smaller number than
Expr2.
+Expr1 =< +Expr2
Succeeds when expression Expr1 evaluates to a smaller or equal number
to Expr2.
+Expr1 >= +Expr2
Succeeds when expression Expr1 evaluates to a larger or equal number
to Expr2.
+Expr1 =\= +Expr2
Succeeds when expression Expr1 evaluates to a number non-equal to
Expr2.
+Expr1 =:= +Expr2
Succeeds when expression Expr1 evaluates to a number equal to Expr2.
-Number is +Expr
Succeeds when Number has successfully been unified with the number
Expr evaluates to.
3.20 Arithmetic Functions
Arithmetic functions are terms which are evaluated by the arithmetic
predicates described above. SWI-Prolog tries to hide the difference
between integer arithmetic and floating point arithmetic from the Prolog
user. Arithmetic is done as integer arithmetic as long as possible and
converted to floating point arithmetic whenever one of the arguments or
the combination of them requires it. If a function returns a floating
point value which is whole it is automatically transformed into an
integer. There are three types of arguments to functions:
Expr Arbitrary expression, returning either a floating
point value or an integer.
IntExpr Arbitrary expression that should evaluate into an
integer.
Int An integer.
In case integer addition, subtraction and multiplication would lead to an
integer overflow the operands are automatically converted to floating
point numbers. The floating point functions (sin/1, exp/1, etc.) form a
direct interface to the corresponding C library functions used to compile
SWI-Prolog. Please refer to the C library documentation for details on
percision, error handling, etc.
- +Expr
Result= Expr
+Expr1 + +Expr2
Result= Expr1+ Expr2
+Expr1 - +Expr2
Result= Expr1 Expr2
+Expr1 * +Expr2
Result= Expr1Expr2times
+Expr1 / +Expr2
Result= Expr1=Expr2
+IntExpr1 mod +IntExpr2
Result= Expr1mod Expr2 (remainder of division).
+IntExpr1 // +IntExpr2
Result= Expr1div Expr2 (integer division).
abs(+Expr)
Evaluate Expr and return the absolute value of it.
max(+Expr1, +Expr2)
Evaluates to the largest of both Expr1 and Expr2.
min(+Expr1, +Expr2)
Evaluates to the smallest of both Expr1 and Expr2.
.(+Int, [])
A list of one element evaluates to the element. This implies "a"
evaluates to the ASCII value of the letter a (97). This option is
available for compatibility only. It will not work if
`style_check(+string)' is active as "a" will then be tranformed into
a string object. The recommended way to specify the ASCII value of
the letter `a' is 0'a.
random(+Int)
Evaluates to a random integer i for which 0 i < Int. The seed of
this random generator is determined by the system clock when
SWI-Prolog was started.
integer(+Expr)
Evaluates Expr and rounds the result to the nearest integer.
floor(+Expr)
Evaluates Expr and returns the largest integer smaller or equal to
the result of the evaluation.
ceil(+Expr)
Evaluates Expr and returns the smallest integer larger or equal to
the result of the evaluation.
+IntExpr >> +IntExpr
Bitwise shift IntExpr1 by IntExpr2 bits to the right. Note that
integers are only 27 bits.
+IntExpr << +IntExpr
Bitwise shift IntExpr1 by IntExpr2 bits to the left.
+IntExpr \/ +IntExpr
Bitwise `or' IntExpr1 and IntExpr2.
+IntExpr /\ +IntExpr
Bitwise `and' IntExpr1 and IntExpr2.
+IntExpr xor +IntExpr
Bitwise `exclusive or' IntExpr1 and IntExpr2.
\ +IntExpr
Bitwise negation.
sqrt(+Expr)
Result= square root of Expr
sin(+Expr)
Result= sine of Expr. Expr is the angle in radials.
cos(+Expr)
Result= cosine of Expr. Expr is the angle in radials.
tan(+Expr)
Result= tangus of Expr. Expr is the angle in radials.
asin(+Expr)
Result= inverse sine of Expr. Result is the angle in radials.
acos(+Expr)
Result= inverse cosine of Expr. Result is the angle in radials.
atan(+Expr)
Result= inverse tangus of Expr. Result is the angle in radials.
atan(+YExpr, +XExpr)
Result= inverse tangus of YExpr / XExpr. Result is the angle in
radials. The return value is in the range pi:::pi. Used to convert
between rectangular and polar coordinate system.
log(+Expr)
Result= natural logarithm of Expr
log10(+Expr)
Result= 10 base logarithm of Expr
exp(+Expr)
Result= e to the power Expr
+Expr1 ^ +Expr2
Result= Expr1 to the power Expr2
pi
Evaluates to the mathematical constant pi (3.141593...).
e
Evaluates to the mathematical constant e (2.718282...).
cputime
Evaluates to a floating point number expressing the cpu time (in
seconds) used by Prolog up till now. See also statistics/2 and
time/1.
3.21 Adding Arithmetic Functions
Prolog predicates can be given the role of arithmetic function. The last
argument is used to return the result, the arguments before the last are
the inputs. Arithmetic functions are added using the predicate
arithmetic_function/1, which takes the head as its argument. Arithmetic
functions are module sensitive, that is they are only visible from the
module in which the function is defined and delared. Global arithmetic
functions should be defined and registered from module user. Global
definitions can be overruled locally in modules. The builtin functions
described above can be redefined as well.
arithmetic_function(+Head)
Register a Prolog predicate as an arithmetic function (see is/2, >/2,
etc.). The Prolog predicate should have one more argument than
specified by Head, which it either a term Name/Arity, an atom or a
complex term. This last argument is an unbound variable at call time
and should be instantiated to an integer or floating point number.
The other arguments are the parameters. This predicate is module
sensitive and will declare the arithmetic function only for the
context module, unless declared from module user. Example:
1 ?- [user].
:- arithmetic_function(mean/2).
mean(A, B, C) :-
C is (A+B)/2.
user compiled, 0.07 sec, 440 bytes.
Yes
2 ?- A is mean(4, 5).
A = 4.500000
current_arithmetic_function(?Head)
Succesively unifies all arithmetic functions that are visible from
the context module with Head.
3.22 List Manipulation
is_list(+Term)
Succeeds if Term is bound to the empty list ([]) or a term with
functor `.' and arity 2.
proper_list(+Term)
Equivalent to is_list/1, but also requires the tail of the list to be
a list (recursively). Examples:
is_list([x|A]) % true
proper_list([x|A]) % false
append(?List1, ?List2, ?List3)
Succeeds when List3 unifies with the concatenation of List1 and
List2. The predicate can be used with any instantiation pattern
(even three variables).
member(?Elem, ?List)
Succeeds when Elem can be unified with one of the members of List.
The predicate can be used with any instantiation pattern.
memberchk(?Elem, +List)
Equivalent to member/2, but leaves no choice point.
delete(+List1, ?Elem, ?List2)
Delete all members of List1 that simultaneously unify with Elem and
unify the result with List2.
select(?List1, ?Elem, ?List2)
Select an element of List1 that unifies with Elem. List2 is unified
with the list remaining from List1 after deleting the selected
element. Normally used with the instantiation pattern +List1, -Elem,
-List2, but can also be used to insert an element in a list using
-List1, +Elem, +List2.
nth0(?Index, ?List, ?Elem)
Succeeds when the Index-th element of List unifies with Elem.
Counting starts at 0.
nth1(?Index, ?List, ?Elem)
Succeeds when the Index-th element of List unifies with Elem.
Counting starts at 1.
last(?Elem, ?List)
Succeeds if Elem unifies with the last element of List.
reverse(+List1, -List2)
Reverse the order of the elements in List1 and unify the result with
the elements of List2.
flatten(+List1, -List2)
Transform List1, possibly holding lists as elements into a `flat'
list by replacing each list with its elements (recursively). Unify
the resulting flat list with List2. Example:
?- flatten([a, [b, [c, d], e]], X).
X = [a, b, c, d, e]
length(?List, ?Int)
Succeeds if Int represents the number of elements of list List. Can
be used to create a list holding only variables.
merge(+List1, +List2, -List3)
List1 and List2 are lists, sorted to the standard order of terms (see
section 3.5). List3 will be unified with an ordered list holding the
both the elements of List1 and List2. Duplicates are not removed.
3.23 Set Manipulation
is_set(+Set)
Succeeds if set is a proper list (see proper_list/1) without
duplicates.
list_to_set(+List, -Set)
Succeeds if Set holds the same elements as List in the same order,
but has no duplicates. See also sort/1.
intersection(+Set1, +Set2, -Set3)
Succeeds if Set3 unifies with the intersection of Set1 and Set2.
Set1 and Set2 are lists without duplicates. They need not be
ordered.
subtract(+Set, +Delete, -Result)
Delete all elements of set `Delete' from `Set' and unify the
resulting set with `Result'.
union(+Set1, +Set2, -Set3)
Succeeds if Set3 unifies with the union of Set1 and Set2. Set1 and
Set2 are lists without duplicates. They need not be ordered.
subset(+Subset, +Set)
Succeeds if all elements of Subset are elements of Set as well.
merge_set(+Set1, +Set2, -Set3)
Set1 and Set2 are lists without duplicates, sorted to the standard
order of terms. Set3 is unified with an ordered list without
duplicates holding the union of the elements of Set1 and Set2.
3.24 Sorting Lists
sort(+List, -Sorted)
Succeeds if Sorted can be unified with a list holding the elements of
List, sorted to the standard order of terms (see section 3.5).
Duplicates are removed.
msort(+List, -Sorted)
Equivalent to sort/2, but does not remove duplicates.
keysort(+List, -Sorted)
List is a list of Key-Valuepairs (e.g. terms of the functor `-'
with arity 2). keysort/2 sorts List like msort/2, but only compares
the keys. Can be used to sort terms not on standard order, but on
any criterion that can be expressed on a multi-dimensional scale.
Sorting on more than one criterion can be done using terms as keys,
putting the first criterion as argument 1, the second as argument 2,
etc.
predsort(+Pred, +List, -Sorted)
Sorts similar to msort/2, but determines the order of two terms by
applying Pred to pairs of elements from List (see apply/2). The
predicate should succeed if the first element should be before the
second.
3.25 Finding all Solutions to a Goal
findall(+Var, +Goal, -Bag)
Creates a list of the instantiations Var gets successively on
backtracking over Goal and unifies the result with Bag. Succeeds
with an empty list if Goal has no solutions. findall/3 is equivalent
to bagof/3 with all free variables bound with the existence operator
(^), except that bagof/3 fails when goal has no solutions.
bagof(+Var, +Goal, -Bag)
Unify Bag with the alternatives of Var, if Goal has free variables
besides the one sharing with Var bagof will backtrack over the
alternatives of these free variables, unifying Bag with the
corresponding alternatives of Var. The construct Var^Goal tells
bagof not to bind Var in Goal. Bagof/3 fails if Goal has no
solutions.
The example below illustrates bagof/3 and the ^ operator. The
variable bindings are printed together on one line to save paper.
2 ?- listing(foo).
foo(a, b, c).
foo(a, b, d).
foo(b, c, e).
foo(b, c, f).
foo(c, c, g).
Yes
3 ?- bagof(C, foo(A, B, C), Cs).
A = a, B = b, C = G308, Cs = [c, d] ;
A = b, B = c, C = G308, Cs = [e, f] ;
A = c, B = c, C = G308, Cs = [g] ;
No
4 ?- bagof(C, A^foo(A, B, C), Cs).
A = G324, B = b, C = G326, Cs = [c, d] ;
A = G324, B = c, C = G326, Cs = [e, f, g] ;
No
5 ?-
setof(+Var, +Goal, -Set)
Equivalent to bagof/3, but sorts the result using sort/2 to get a
sorted list of alternatives without duplicates.
3.26 Invoking Predicates on all Members of a List
All the predicates in this section call a predicate on all members of a
list or until the predicate called fails. The predicate is called via
apply/2, which implies common arguments can be put in front of the
arguments obtained from the list(s). For example:
?- maplist(plus(1), [0, 1, 2], X).
X = [1, 2, 3]
we will phrase this as ``Predicate is applied on ...''
checklist(+Pred, +List)
Pred is applied successively on each element of List until the end of
the list or Pred fails. In the latter case the checklist/2 fails.
maplist(+Pred, ?List1, ?List2)
Apply Pred on all successive pairs of elements from List1 and List2.
Fails if Pred can not be applied to a pair. See the example above.
sublist(+Pred, +List1, ?List2)
Unify List2 with a list of all elements of List1 to which Pred
applies.
3.27 Forall
forall(+Cond, +Action)
For all alternative bindings of Cond Action can be proven. The
example verifies that all arithmic statements in the list L are
correct. It does not say which is wrong if one proves wrong.
?- forall(member(Result = Formula, [2 = 1 + 1, 4 = 2 * 2]),
Result =:= Formula).
3.28 Formatted Write
The current version of SWI-Prolog provides two formatted write predicates.
The first is writef/[1,2], which is compatible with Edinburgh C-Prolog.
The second is format/[1,2], which is compatible with Quintus Prolog. We
hope the Prolog community will once define a standard formatted write
predicate. If you want performance use format/[1,2] as this predicate is
defined in C. Otherwise compatibility reasons might tell you which
predicate to use.
3.28.1 Writef
write_ln(+Term)
Equivalent to write(Term), nl.
writef(+Atom)
Equivalent to writef(Atom, []).
writef(+Format, +Arguments)
Formatted write. Format is an atom whose characters will be printed.
Format may contain certain special character sequences which specify
certain formatting and substitution actions. Arguments then provides
all the terms required to be output.
Escape sequences to generate a single special character:
\n <NL> is output
\l <LF> is output
\r <CR> is output
\t <TAB> is output
\\ The character `\' is output
\% The character `%' is output
\nnn where nnn is an integer (1-3 digits) the
character with ASCII code nnn is output (NB
: nnn is read as DECIMAL)
Escape sequences to include arguments from Arguments. Each time a %
escape sequence is found in Format the next argument from Arguments
is formatted according to the specification.
%t print/1 the next item (mnemonic: term)
%w write/1 the next item
%q writeq/1 the next item
%d display/1 the next item
%p print/1 the next item (identical to %t)
%n put the next item as a character (i.e. it
is an ASCII value)
%r write the next item N times where N is the
second item (an integer)
%s write the next item as a String (so it must
be a list of characters)
%f perform a ttyflush/0 (no items used)
%Nc write the next item Centered in N columns.
%Nl write the next item Left justified in N
columns.
%Nr write the next item Right justified in N
columns. N is a decimal number with at
least one digit. The item must be an atom,
integer, float or string.
swritef(-String, +Format, +Arguments)
Equivalent to writef/3, but ``writes'' the result on String instead
of the current output stream. Example:
?- swritef(S, '%15L%w', ['Hello', 'World']).
S = "Hello World"
swritef(-String, +Format)
Equivalent to swritef(String, Format, []).
3.28.2 Format
format(+Format)
Defined as `format(Format) :- format(Format, []).'
format(+Format, +Arguments)
Format is an atom, list of ASCII values, or a Prolog string.
Arguments provides the arguments required by the format
specification. If only one argument is required and this is not a
list of ASCII values the argument need not be put in a list.
Otherwise the arguments are put in a list.
Special sequences start with the tilde (~), followed by an optional
numeric argument, followed by a character describing the action to be
undertaken. A numeric argument is either a sequence of digits,
representing a positive decimal number, a sequence `<character>,
representing the ASCII value of the character (only useful for ~t) or
a asterisk (*), in when the numeric argument is taken from the next
argument of the argument list, which should be a positive integer.
Actions are:
~ Output the tilde itself.
a Output the next argument, which should be an atom. This option
is equivalent to w. Compatibility reasons only.
c Output the next argument as an ASCII value. This argument should
be an integer in the range [0, ..., 255] (including 0 and 255).
d Output next argument as a decimal number. It should be an
integer. If a numeric argument is specified a dot is inserted
argument positions from the right (useful for doing fixed point
arithmetic with integers, such as handling amounts of money).
D Same as d, but makes large values easier to read by inserting a
comma every three digits left to the dot or right.
e Output next argument as a floating point number in exponentional
notation. The numeric argument specifies the precission.
Default is 6 digits. Exact representation depends on the C
library function printf(). This function is invoked with the
format %.<percission>e.
E Equivalent to e, but outputs a capital E to indicate the
exponent.
f Floating point in non-exponentional notation. See C library
function printf().
g Floating point in e or f notation, whichever is shorter.
G Floating point in E or f notation, whichever is shorter.
i Ignore next argument of the argument list. Produces no output.
k Give the next argument to displayq/1 (canonical write).
n Output a newline character.
N Only output a newline if the last character output on this stream
was not a newline. Not properly implemented yet.
p Give the next argument to print/1.
q Give the next argument to writeq/1.
r Print integer in radix the numeric argument notation. Thus ~16r
prints its argument hexadecimal. The argument should be in the
range [2, ... 36]. Lower case letters are used for digits above
9.
R Same as r, but uses upper case letters for digits above 9.
s Output a string of ASCII characters from the next argument.
t All remaining space between 2 tabstops is distributed equaly over
~t statements between the tabstops. This space is padded with
spaces by default. If an argument is supplied this is taken to
be the ASCII value of the character used for padding. This can
be used to do left or right alignment, centering, distributing,
etc. See also ~| and ~+ to set tab stops. A tabstop is assumed
at the start of each line.
__Set a tabstop on the current position. If an argument is
supplied set a tabstop on the position of that argument. This
will cause all ~t's to be distributed between the previous and
this tabstop.
+ Set a tabstop relative to the current position. Further the same
as ~|.
w Give the next argument to write/1.
Example:
simple_statistics :-
<obtain statistics> % left to the user
format('~tStatistics~t~72|~n~n'),
format('Runtime: ~`.t ~2f~34| Inferences: ~`.t ~D~72|~n',
[RunT, Inf]),
....
Will output
Statistics
Runtime: .................. 3.45 Inferences: .......... 60,345
sformat(-String, +Format, +Arguments)
Equivalent to format/3, but ``writes'' the result on String instead
of the current output stream. Example:
?- sformat(S, '~w~t~15|~w', ['Hello', 'World']).
S = "Hello World"
sformat(-String, +Format)
Equivalent to `sformat(String, Format, []).'
3.28.3 Programming Format
format_predicate(+Char, +Head)
If a sequence ~c (tilde, followed by some character) is found, the
format derivates will first check whether the user has defined a
predicate to handle the format. If not, the built in formatting
rules described above are used. Char is either an ascii value, or a
one character atom, specifying the letter to be (re)defined. Head is
a term, whose name and arity are used to determine the predicate to
call for the redefined formatting character. The first argument to
the predicate is the numeric argument of the format command, or the
atom defaultif no argument is specified. The remaining arguments
are filled from the argument list. The example below redefines ~n to
produce Arg times return followed by linefeed (so a (Grr.) DOS
machine is happy with the output).
:- format_predicate(n, dos_newline(_Arg)).
dos_newline(Arg) :-
between(1, Ar, _), put(13), put(10), fail ; true.
3.29 Terminal Control
The following predicates form a simple access mechanism to the Unix
termcap library to provide terminal independant I/O for screen terminals.
The library package tty builds on top of these predicates.
tty_get_capability(+Name, +Type, -Result)
Get the capability named Name from the termcap library. See
termcap(5) for the capability names. Type specifies the type of the
expected result, and is one of string, numberor bool. String
results are returned as an atom, number result as an integer and bool
results as the atom on or off. If an option cannot be found this
predicate fails silently. The results are only computed once.
Succesive queries on the same capability are fast.
tty_goto(+X, +Y)
Goto position (X, Y) on the screen. Note that the predicates
line_count/2 and line_position/2 will not have a well defined
behaviour while using this predicate.
tty_put(+Atom, +Lines)
Put an atom via the termcap library function tputs(). This function
decodes padding information in the strings returned by
tty_get_capability/3 and should be used to output these strings.
Lines is the number of lines affected by the operation, or 1 if not
applicable (as in almost all cases).
set_tty(-OldStream, +NewStream)
Set the output stream, used by tty_put/2 and tty_goto/2 to a specific
stream. Default is user_output.
3.30 Unix Interaction
shell(+Command, -Status)
Execute Command on the operating system. Command is given to the
bourne shell (/bin/sh). Status is unified with the exit status of
the command.
shell(+Command)
Equivalent to `shell(Command, 0)'.
shell
Start an interactive Unix shell. Default is /bin/sh, the environment
variable SHELL overrides this default.
getenv(+Name, -Value)
Get Unix environment variable (see csh(1) and sh(1)). Fails if the
variable does not exist.
setenv(+Name, +Value)
Set Unix environment variable. Name and Value should be instantiated
to atoms or integers. The environment variable will be passed to
shell/[0-2] and can be requested using getenv/2.
unsetenv(+Name)
Remove Unix environment variable from the environment.
get_time(-Time)
Return the number of seconds that elapsed since the epoch of Unix, 1
January 1970, 0 hours. Time is a floating point number. Its
granularity is system dependant. On SUN, this is 1/60 of a second.
convert_time(+Time, -Year, -Month, -Day, -Hour, -Minute, -Second, -MilliSeconds)
Convert a time stamp, provided by get_time/1, file_time/2, etc. Year
is unified with the year, Month with the month number (January is 1),
Day with the day of the month (starting with 1), Hour with the hour
of the day (0--23), Minute with the minute (0--59). Second with the
second (0--59) and MilliSecond with the milli seconds (0--999). Note
that the latter might not be acurate or might always be 0, depending
on the timing capabilities of the system.
3.31 Unix File System Interaction
access_file(+File, +Mode)
Succeeds when File exists and can be accessed by this prolog process
under mode Mode. Mode is one of the atoms read, write or execute.
File may also be the name of a directory. Fails silently otherwise.
exists_file(+File)
Succeeds when File exists. This does not imply the user has read
and/or write permission for the file.
same_file(+File1, +File2)
Succeeds if both filenames refer to the same physical file. That is,
if File1 and File2 are the same string or both names exist and point
to the same file (due to hard or symbolic links and/or relative vs.
absolute paths).
exists_directory(+Directory)
Succeeds if Directory exists. This does not imply the user has read,
search and or write permission for the directory.
delete_file(+File)
Unlink File from the Unix file system.
rename_file(+File1, +File2)
Rename File1 into File2. Currently files cannot be moved across
devices.
size_file(+File, -Size)
Unify Size with the size of File in characters.
time_file(+File, -Time)
Unify the last modification time of File with Time. Time is a
floating point number expressing the seconds ellapsed since Jan 1,
1970.
absolute_file_name(+File, -Absolute)
Expand Unix file specification into an absolute path. User home
directory expansion (~ and ~user) and variable expansion is done.
The absolute path is canonised: references to `.' and `..' are
deleted. SWI-Prolog uses absolute file names to register source
files independant of the current working directory.
expand_file_name(+WildChart, -List)
Unify List with a sorted list of files or directories matching
WildChart. The normal Unix wildchart constructs `?', `*', `[...]'
and `{...}' are recognised. The interpretation of `{...}' is
interpreted slightly different from the C shell (csh(1)). The comma
separated argument can be arbitrary patterns, including `{...}'
patterns. The empty pattern is legal as well: `{.pl,}' matches
either `.pl' or the empty string.
chdir(+Path)
Change working directory to Path.
3.32 User Toplevel Manipulation
break
Recursively start a new Prolog top level. This Prolog top level has
it's own stacks, but shares the heap with all break environments and
the top level. Debugging is switched off on entering a break and
restored on leaving one. The break environment is terminated by
typing the system's end-of-filecharacter (control-D). If the
-t toplevelcommand line option is given this goal is started instead
of entering the default interactive top level (prolog/0).
abort
Abort the Prolog execution and start a new top level. If the
-t toplevelcommand line options is given this goal is started
instead of entering the default interactive top level. Break
environments are aborted as well. All open files except for the
terminal related files are closed. The input- and output stream
again refers to user.
BUG: Erased clauses which could not actually be removed from the
database, because they are active in the interpreter, will never be
garbage collected after an abort.
halt
Terminate Prolog execution. Open files are closed and if the command
line option -tty is not active the terminal status (see Unix stty(1))
is restored.
prolog
This goal starts the default interactive top level. prolog/0 is
terminated (succeeds) by typing control-D.
3.33 Creating a Protocol of the Unser Interaction
SWI-Prolog offers the possibility to log the interaction with the user on
a file. All Prolog interaction, including warnings and tracer output, are
written on the protocol file.
protocol(+File)
Start protocolling on file File. If there is already a protocol file
open then close it first. If File exists it is truncated.
protocola(+File)
Equivalent to protocol/1, but does not truncate the File if it
exists.
noprotocol
Stop making a protocol of the user interaction. Pending output is
flushed on the file.
protocolling(-File)
Succeeds if a protocol was started with protocol/1 or protocola/1 and
unifies File with the current protocol output file.
3.34 Debugging and Tracing Programs
trace
Start the tracer. trace/0 itself cannot be seen in the tracer. Note
that the Prolog toplevel treats trace/0 special; it means `trace the
next goal'.
tracing
Succeeds when the tracer is currently switched on. tracing/0 itself
can not be seen in the tracer.
notrace
Stop the tracer. notrace/0 itself cannot be seen in the tracer.
debug
Start debugger (stop at spy points).
nodebug
Stop debugger (do not trace, nor stop at spy points).
debugging
Print debug status and spy points on current output stream.
spy(+Pred)
Put a spy point on all predicates meeting the predicate specification
Pred. See section 3.3.
nospy(+Pred)
Remove spy point from all predicates meeting the predicate
specification Pred.
nospyall
Remove all spy points from the entire program.
leash(?Ports)
Set/query leashing (ports which allow for user interaction). Ports
is one of +Name, -Name, ?Name or a list of these. +Nameenables
leashing on that port, -Name disables it and ?Name succeeds or fails
according to the current setting. Recognised ports are: call, redo,
exit, fail and unify. The special shorthand all refers to all ports,
fullrefers to all ports except for the unify port (default). half
refers to the call, redo and fail port.
visible(+Ports)
Set the ports shown by the debugger. See leash/1 for a description
of the port specification. Default is full.
unknown(-Old, +New)
Unify Old with the current value of the unknown system flag. On
success New will be used to specify the new value. New should be
instantiated to either fail or trace and determines the interpreter's
action when an undefined predicate which is not declared dynamic is
encountered (see dynamic/1). fail implies the predicate just fails
silently. traceimplies the tracer is started. Default is trace.
The unknown flag is local to each module and unknown/2 is module
transparent. Using it as a directive in a module file will only
change the unknown flag for that module. Using the :/2 construct the
behaviour on trapping an undefined predicate can be changed for any
module. Note that if the unknown flag for a module equals fail the
system will not call exception/3 and will not try to resolve the
predicate via the dynamic library system. The system will still try
to import the predicate from the public module.
style_check(+Spec)
Set style checking options. Spec is either +<option>, -<option>,
?<option> or a list of such options. +<option> sets a style checking
option, -<option> clears it and ?<option> succeeds or fails according
to the current setting. consult/1 and derivates resets the style
checking options to their value before loading the file. If --for
example-- a file containing long atoms should be loaded the user can
start the file with:
:- style_check(-atom).
Currently available options are:
Name Default Description
singleton on read_clause/1 (used by consult/1) warns on
variables only appearing once in a term
(clause) which have a name not starting with
an underscore.
atom on read/1 and derivates will produce an error
message on quoted atoms or strings longer
than 5 lines.
dollar off Accept dollar as a lower case character,
thus avoiding the need for quoting atoms
with dollar signs. System maintenance use
only.
discontiguous on Warn if the clauses for a predicate are not
together in the same source file.
string off Read and derivates transform "..." into a
prolog string instead of a list of ASCII
characters.
3.35 Obtaining Runtime Statistics
statistics(+Key, -Value)
Unify system statistics determined by Key with Value. The possible
keys are given in the table 3.2.
cputime (User) cpu time since Prolog was started in
seconds
inferences Total number of passes via the call and redo ports
since Prolog was started.
heapused Bytes heap in use.
local Allocated size of the local stack in bytes.
localused Number of bytes in use on the local stack.
locallimit Size to which the local stack is allowed to grow
global Allocated size of the global stack in bytes.
globalused Number of bytes in use on the global stack.
globallimit Size to which the global stack is allowed to grow
trail Allocated size of the trail stack in bytes.
trailused Number of bytes in use on the trail stack.
traillimit Size to which the trail stack is allowed to grow
atoms Total number of defined atoms.
functors Total number of defined name/arity pairs.
predicates Total number of predicate definitions.
modules Total number of module definitions.
externals Total number of external references in all
clauses.
codes Total amount of byte codes in all clauses.
Table 3.2: Keys for statistics/2
statistics
Display a table of system statistics on the current output stream.
time(+Goal)
Execute Goal just like once/1 (i.e. leaving no choice points), but
print used time, number of logical inferences and the average number
of lips (logical inferences per second). Note that SWI-Prolog counts
the actual executed number of inferences rather than the number of
passes through the call- and redo ports of the theoretical 4-port
model.
3.36 Finding Performance Bottlenecks
SWI-Prolog offers a statistical program profiler similar to Unix prof(1)
for C and some other languages. A profiler is used as an aid to find
performance pigs in programs. It provides information on the time spent
in the various Prolog predicates.
The profiler is based on the assumption that if we monitor the functions
on the execution stack on time intervals not correlated to the program's
execution the number of times we find a procedure on the environment stack
is a measure of the time spent in this procedure. It is implemented by
calling a procedure each time slice Prolog is active. This procedure
scans the local stack and either just counts the procedure on top of this
stack (plain profiling) or all procedures on the stack (cummulative
profiling). To get accurate results each procedure one is interested in
should have a reasonable number of counts. Typically a minute runtime
will suffice to get a rough overview of the most expensive procedures.
profile(+Goal, +Style, +Number)
Execute Goal just like time/1. Collect profiling statistics
according to style (see profiler/2) and show the top Number
procedures on the current output stream (see show_profile/1). The
results are kept in the database until reset_profiler/0 or profile/3
is called and can be displayed again with show_profile/1. profile/3
is the normal way to invoke the profiler. The predicates below are
low-level predicates that can be used for special cases.
show_profile(+Number)
Show the collected results of the profiler. Stops the profiler first
to avoid interference from show_profile/1. It shows the top Number
predicates according the percentage cpu-time used.
profiler(-Old, +New)
Query or change the status of the profiler. The status is one of
off, plain or cummulative. plainimplies the time used by childs of
a predicate is not added to the time of the predicate. For status
cummulativethe time of childs is added (except for recursive calls).
Cummulative profiling implies the stack is scanned upto the top on
each time slice to find all active predicates. This implies the
overhead grows with the number of active frames on the stack.
Cummulative profiling starts debuggin mode to disable tail recursion
optimisation, which would otherwise remove the necessary parent
environments. Switching status from plain to cummulativeresets the
profiler. Switching to and from status off does not reset the
collected statistics, thus allowing to suspend profiling for certain
parts of the program.
reset_profiler
Swiches the profiler to off and clears all collected statistics.
profile_count(+Head, -Calls, -Promilage)
Obtain profile statistics of the predicate specified by Head. Head
is an atom for predicates with arity 0 or a term with the same name
and arity as the predicate required (see current_predicate/2). Calls
is unified with the number of `calls' and `redos' while the profiler
was active. Promilage is unified with the relative number of counts
the predicate was active (cummulative) or on top of the stack
(plain). Promilage is an integer between 0 and 1000.
3.37 Memory Management
Note: limit_stack/2 and trim_stacks/0 have no effect on machines that do
not offer dynamic stack expansion. On these machines these predicates
simply succeed to improve portability.
garbage_collect
Invoke the global- and trail stack garbage collector. Normally the
garbage collector is invoked automatically if necessary. Explicit
invokation might be useful to reduce the need for garbage collections
in time critical segments of the code. After the garbage collection
trim_stacks/0 is invoked to release the collected memory resources.
limit_stack(+Key, +Kbytes)
Limit one of the stack areas to the specified value. Key is one of
local, globalor trail. The limit is an integer, expressing the
desired stack limit in K bytes. If the desired limit is smaller than
the currently used value, the limit is set to the nearest legal value
above the currently used value. If the desired value is larger than
the maximum, the maximum is taken. Finally, if the desired value is
either 0 or the atom unlimitedthe limit is set to its maximum. The
maximum and initial limit is determined by the command line options
-L, -G and -T.
trim_stacks
Release stack memory resources that are not in use at this moment,
returning them to the operating system. Trim stack is a relatively
cheap call. It can be used to release memory resources in a
backtracking loop, where the iterations require typically seconds of
execution time and very different, potentionally large, amounts of
stack space. Such a loop should be written as follows:
loop :-
generator,
trim_stacks,
potentionally_expensive_operation,
stop_condition, !.
The prolog top level loop is written this way, reclaiming memory
resources after every user query.
3.38 Miscellaneous
dwim_match(+Atom1, +Atom2)
Succeeds if Atom1 matches Atom2 in `Do What I Mean' sence. Both
Atom1 and Atom2 may also be integers or floats. The two atoms match
if:
They are identical
They differ by one character (spy == spu)
One character is insterted/deleted (debug == deug)
Two characters are transposed (trace == tarce)
`Sub-words' are glued differently (existsfile == existsFile ==
exists_file)
Two adjacent sub words are transposed (existsFile == fileExists)
dwim_match(+Atom1, +Atom2, -Difference)
Equivalent to dwim_match/2, but unifies Difference with an atom
identifying the the difference between Atom1 and Atom2. The return
values are (in the same order as above): equal, mismatched_char,
inserted_char, transposed_char, separatedand transposed_word.
wildcard_match(+Pattern, +String)
Succeeds if String matches the wildcart pattern Pattern. Pattern is
very simular the the Unix csh pattern matcher. The patterns are
given below:
? Matches one arbitrary character
* Matches any number of arbitrary characters
[...] Matches one of the characters specified at ... <char>-<char> indi*
*cates a range.
f...g Matches any of the patterns of the comma separated list between t*
*he brackets.
Example:
?- wildcard_match('[a-z]*.{pro,pl}[%~]', 'a_hello.pl%').
Yes.
gensym(+Base, -Unique)
Generate a unique atom from base Base and unify it with Unique. Base
should be an atom. The first call will return <base>1, the next
<base>2, etc. Note that this is no warant that the atom is unique in
the system.
BUG: I plan to supply a real gensym/2 which does give this warrant
for future versions.
sleep(+Time)
Suspend execution Time seconds. Time is either a floating point
number or an integer. Granularity is dependent on the system's timer
granularity (1/60 seconds on most Unix systems). A negative time
causes the timer to return immediately. As Unix is a multi-tasking
environment we can only ensure execution is suspended for at least
Time seconds.
Chapter 4
Using Modules
384
4.1 Why Using Modules?
In traditional Prolog systems the predicate space was flat. This approach
is not very suitable for the development of large applications, certainly
not if these applications are developed by more than one programmer. In
many cases, the definition of a Prolog predicate requires sub-predicates
that are intented only to complete the definition of the main predicate.
With a flat and global predicate space these support predicates will be
visible from the entire program.
For this reason, it is desirable that each source module has it's own
predicate space. A module consists of a declaration for it's name, it's
public predicates and the predicates themselves. This approach allow the
programmer to use short (local) names for support predicates without
worrying about name conflicts with the support predicates of other
modules. The module declaration also makes explicit which predicates are
meant for public usage and which for private purposes. Finally, using the
module information, cross reference programs can indicate possible
problems much better.
4.2 Name-based versus Predicate-based Modules
Two approaches to realise a module system are commonly used in Prolog and
other languages. The first one is the name based module system. In these
systems, each atom read is tagged (normally prefixed) with the module
name, with the exception of those atoms that are defined public. In the
second approach, each module actually implements its own predicate space.
A critical problem with using modules in Prolog is introduced by the
meta-predicates that transform between Prolog data and Prolog predicates.
Consider the case where we write:
:- module(extend, [add_extension/3]).
add_extension(Extension, Plain, Extended) :-
maplist(extend_atom(Extension), Plain, Extended).
extend_atom(Extension, Plain, Extended) :-
concat(Plain, Extension, Extended).
In this case we would like maplist to call extend_atom/3 in the module
extend. A name based module system will do this correctly. It will tag
the atom extend_atom with the module and maplist will use this to
construct the tagged term extend_atom/3. A name based module however,
will not only tag the atoms that will eventually be used to refer to a
predicate, but all atoms that are not declared public. So, with a name
based module system also data is local to the module. This introduces
another serious problem:
:- module(action, [action/3]).
action(Object, sleep, Arg) :- ....
action(Object, awake, Arg) :- ....
:- module(process, [awake_process/2]).
awake_process(Process, Arg) :-
action(Process, awake, Arg).
This code uses a simple object-oriented implementation technique were
atoms are used as method selectors. Using a name based module system,
this code will not work, unless we declare the selectors puclic atoms in
all modules that use them. Predicate based module systems do not require
particular precautions for handling this case.
It appears we have to choose either to have local data, or to have trouble
with meta-predicates. Probably it is best to choose for the predicate
based approach as novice users will not often write generic
meta-predicates that have to be used across multiple modules, but are
likely to write programs that pass data around across modules.
Experienced Prolog programmers should be able to deal with the
complexities of meta-predicates in a predicate based module system.
4.3 Defining a Module
Modules normally are created by loading a module file. A module file is a
file holding a module/2 directive as its first term. The module/2
directive declares the name and the public (i.e. externally visible)
predicates of the module. The rest of the file is loaded into the module.
Below is an example of a module file, defining reverse/2.
:- module(reverse, [reverse/2]).
reverse(List1, List2) :-
rev(List1, [], List2).
rev([], List, List).
rev([Head|List1], List2, List3) :-
rev(List1, [Head|List2], List3).
4.4 Importing Predicates into a Module
As explained before, in the predicate based approach ad<apted by
SWI-Prolog, each module has it's own predicate space. In SWI-Prolog, a
module initially is completely empty. Predicates can be added to a module
by loading a module file as demonstrated in the previous section, using
assert or by importing them from another module.
Two mechanisms for importing predicates explicitely from another module
exist. The use_module/[1,2] predicates load a module file and import
(part of the) public predicates of the file. The import/1 predicate
imports any predicate from any module.
use_module(+File)
Load the file(s) specified with File just like ensure_loaded/1. The
files should all be module files. All exported predicates from the
loaded files are imported into the context module. The difference
between this predicate and ensure_loaded/1 becomes apparent if the
file is already loaded into another module. In this case
ensure_loaded/1 does nothing; use_module will import all public
predicates of the module into the current context module.
use_module(+File, +ImportList)
Load the file specified with File (only one file is accepted). File
should be a module file. ImportList is a list of name/arity pairs
specifying the predicates that should be imported from the loaded
module. If a predicate is specified that is not exported from the
loaded module a warning will be printed. The predicate will
nevertheless be imported to simplify debugging.
import(+Head)
Import predicate Head into the current context module. Head should
specify the source module using the <module>:<term> construct.
Note that predicates are normally imported using one of the
directives use_module/[1,2]. import/1 is meant for handling imports
into dynamically created modules.
It would be rather inconvient to have to import each predicate refered to
by the module, including the system predicates. For this reason each
module is assigned a default module. All predicates in the default module
are available without extra declarations. Their definition however can be
overruled in the local module. This schedule is implemented by the
exception handling mechanism of SWI-Prolog: if an undefined predicate
exception is raised for a predicate in some module, the exception handler
first tries to import the predicate from the module's default module. On
success, normal execution is resumed.
4.4.1 Reserved Modules
SWI-Prolog contains two special modules. The first one is the module
system. This module contains all built-in predicates described in this
manual. Module system has no default module assigned to it. The second
special module is the module user. This module forms the initial working
space of the user. Initially it is empty. The default module of module
user is system, making all built-in predicate definitions available as
defaults. Built-in predicates thus can be overruled by defining them in
module user before they are used.
All other modules default to module user. This implies they can use all
predicates imported into user without explicitely importing them.
4.5 Using the Module System
The current structure of the module system has been designed with some
specific organisations for large programs in mind. Many large programs
define a basic library layer on top of which the actual program itself is
defined. The module user, acting as the default module for all other
modules of the program can be used to distribute these definitions over
all program module without introducing the need to import this common
layer each time explicitely. It can also be used to redefine built-in
predicates if this is required to maintain compatibility to some other
Prolog implementation. Typically, the loadfile of a large application
looks like this:
:- use_module(compatibility). % load XYZ prolog compatibility
:- use_module( % load generic parts
[ error % errors and warnings
, goodies % general goodies (library
extensions)
, debug % application specific debugging
, virtual_machine % virtual machine of application
, ... % more generic stuff
]).
:- ensure_loaded(
[ ... % the application itself
]).
The `use_module' declarations will import the public predicates from the
generic modules into the user module. The `ensure_loaded' directive loads
the modules that constitute the actual application. It is assumed these
modules import predicates from each other using use_module/[1,2] as far as
necessary.
In combination with the object-oriented schema described below it is
possible to define a neat modular architecture. The generic code defines
general utilities and the message passing predicates (invoke/3 in the
example below). The application modules define classes that communicate
using the message passint predicates.
4.5.1 Object Oriented Programming
Another typical way to use the module system is for defining classes
within an object oriented paradigm. The class structure and the methods
of a class can be defined in a module and the explicit module-boundary
overruling describes in section ?? can by used by the message passing
code to invoke the behaviour. An outline of this mechanism is given
below.
% Define class point
:- module(point, []). % class point, no exports
% name type, default access
% value
variable(x, integer, 0, both).
variable(y, integer, 0, both).
% method name predicate name arguments
behaviour(mirror, mirror, []).
mirror(P) :-
fetch(P, x, X),
fetch(P, y, Y),
store(P, y, X),
store(P, x, Y).
The predicates fetch/3 and store/3 are predicates that change instance
variables of instances. The figure below indicates how message passing
can easily be implemented:
% invoke(+Instance, +Selector, ?ArgumentList)
% send a message to an instance
invoke(I, S, Args) :-
class_of_instance(I, Class),
Class:behaviour(S, P, ArgCheck), !,
convert_arguments(ArgCheck, Args, ConvArgs),
Goal =.. [P|ConvArgs],
Class:Goal.
The construct `Module:Goal' explicitely calls Goal in module Module. It
is discussed in more detail in section ??.
4.6 Meta-Predicates in Modules
As indicated in the introduction, the problem with a predicate based
module system lies in the difficulty to find the correct predicate from a
Prolog term. The predicate `solution(Solution)' can exist in more than
one module, but `assert(solution(4))' in some module is supposed to refer
to the correct version of solution/1.
Various approaches are possible to solve this problem. One is to add an
extra argument to all predicates (e.g. `assert(Module, Term)'). Another
is to tag the term somehow to indicate which module is desired (e.g.
`assert(Module:Term)'). Both approaches are not very attractive as they
make the user responsible for chosing the correct module, inviting unclear
programming by asserting in other modules. The predicate assert/1 is
supposed to assert in the module it is called from and should do so
without being told explicitely. For this reason, the notion context
module has been introduced.
4.6.1 Definition and Context Module
Each predicate of the program is assigned a module, called it's definition
module. The definition module of a predicate is always the module in
which the predicate was originally defined. Each active goal in the
Prolog system has a context module assigned to it.
The context module is used to find predicates from a Prolog term. By
default, this module is the definition module of the predicate running the
goal. For meta-predicates however, this is the context module of the goal
that invoked them. We call this module_transparent in SWI-Prolog. In the
`using maplist' example above, the predicate maplist/3 is declared
module_transparent. This implies the context module remains extend, the
context module of add_extension/3. This way maplist/3 can decide to call
extend_atom in module extend rather than in it's own definition module.
All built-in predicates that refer to predicates via a Prolog term are
declared module_transparent. Below is the code defining maplist.
:- module(maplist, maplist/3).
:- module_transparent maplist/3.
% maplist(+Goal, +List1, ?List2)
% True if Goal can succesfully be applied to all succesive
pairs
% of elements of List1 and List2.
maplist(_, [], []).
maplist(Goal, [Elem1|Tail1], [Elem2|Tail2]) :-
apply(Goal, [Elem1, Elem2]),
maplist(Goal, Tail1, Tail2).
4.6.2 Overruling Module Boundaries
The mechanism above is sufficient to create an acceptable module system.
There are however cases in which we would like to be able to overrule this
schema and explicitely call a predicate in some module or assert
explicitly in some module. The first is useful to invoke goals in some
module from the user's toplevel or to implement a object-oriented system
(see above). The latter is useful to create and modify dynamic modules
(see section 4.7).
For this purpose, the reserved term :/2 has been introduced. All built-in
predicates that transform a term into a predicate reference will check
whether this term is of the form `<Module>:<Term>'. If so, the
predicate is searched for in Module instead of the goal's context module.
The :/2 operator may be nested, in which case the inner-most module is
used.
The special calling construct <Module>:<Goal> pretends Goal is called
from Module instead of the context module. Examples:
?- assert(world:done). % asserts done/0 into module world
?- world:assert(done). % the same
?- world:done. % calls done/0 in module world
4.7 Dynamic Modules
Sofar, we discussed modules that were created by loading a module-file.
These modules have been introduced on facilitate the development of large
applications. The modules are fully defined at load-time of the
application and normally will not change during execution. Having the
notion of a set of predicates as a self-contained world can be attractive
for other purposes as well. For example, assume an application that can
reason about multiple worlds. It is attractive to store the data of a
particular world in a module, so we extract information from a world
simply by invoking goals in this world.
Dynamic modules can easily be created. Any built-in predicate that tries
to locate a predicate in a specific module will create this module as a
side-effect if it did not yet exist. Example:
?- assert(world_1, consistent),
world_1:unkown(_, fail).
These calls create a module called world and make the call
`world:consistent' succeed. Undefined predicates will not start the
tracer or autoloader for this module (see unknown/2).
Import and export from dynamically created world is arranged via the
predicates import/1 and export/1:
?- world_5:export(solve(_,_)). % exports solve/2 from
world_5
?- world_3:import(world_5:solve(_,_)). % and import it to world_3
4.8 Module Handling Predicates
This section gives the predicate definitions for the remaining built-in
predicates that handle modules.
:- module(+Module, +PublicList)
This directive can only be used as the first term of a source file.
It declares the file to be a module file, defining Module and
exporting the predicates of PublicList. PublicList is a list of
name/arity pairs.
module_transparent +Name/+Arity, ...
Preds is a comma separated list of name/arity pairs (like dynamic/1).
Each goal associated with a transparent declared predicate will
inherit the context module from its parent goal.
context_module(-Module)
Unify Module with the context module of the current goal.
context_module/1 itself is transparent.
export(+Head)
Add a predicate to the public list of the context module. This
implies the predicate will be imported into another module if this
module is imported with use_module/[1,2]. Note that predicates are
normally exported using the directive module/2. export/1 is meant to
handle export from dynamically created modules.
4.9 Compatibility of the Module System
The principles behind the module system of SWI-Prolog differ in a number
of aspects from the Quintus Prolog module system.
The SWI-Prolog module system allows the user to redefine system
predicates.
All predicates that are available in the system and user modules are
visible in all other modules as well.
Quintus has the `meta_predicate/1' declaration were SWI-Prolog has
the module_transparent/1 declaration.
The meta_predicate/1 declaration causes the compiler to tag arguments that
pass module sensitive information with the module using the :/2 operator.
This approach has some disadvantages:
Changing a meta_predicate declaration implies all predicates calling
the predicate need to be reloaded. This can cause serious
consistency problems.
It does not help for dynamically defined predicates calling module
sensitive predicates.
It slows down the compiler (at least in the SWI-Prolog architecture).
At least within the SWI-Prolog architecture the run-time overhead is
larger than the overhead introduced by the transparent mechanism.
Unfortunately the transparent predicate approach also has some
disadvantages. If a predicate A passes module sensitive information to a
predicate B, passing the same information to a module sensitive system
predicate both A and B should be declared transparent. Using the Quintus
approach only A needs to be treated special (i.e. declared with
meta_predicate/1). A second problem arises if the body of a transparent
predicate uses module sensitive predicates for which it wants to refer to
its own module. Suppose we want to define findall/3 using assert/1 and
retract/1. The example in figure 4.1 gives the solution.
:- module(findall, [findall/3]).
:- dynamic
solution/1.
:- module_transparent
findall/3,
store/2.
findall(Var, Goal, Bag) :-
assert(findall:solution('$mark')),
store(Var, Goal),
collect(Bag).
store(Var, Goal) :-
Goal, % refers to context module of
% caller of findall/3
assert(findall:solution(Var)),
fail.
store(_, _).
collect(Bag) :-
...,
Figure 4.1: Findall using modules
The Quintus meta_predicate/1 directive can in many cases be replaced by
the transparent declaration. Figure 4.2 gives a definition of
meta_predicate/1 as available from the `quintus' library package.
:- op(1150, fx, (meta_predicate)).
meta_predicate((Head, More)) :- !,
meta_predicate1(Head),
meta_predicate(More).
meta_predicate(Head) :-
meta_predicate1(Head).
meta_predicate1(Head) :-
Head =.. [Name|Arguments],
member(Arg, Arguments),
module_expansion_argument(Arg), !,
functor(Head, Name, Arity),
module_transparent(Name/Arity).
meta_predicate1(_). % just a mode declaration
module_expansion_argument(:).
module_expansion_argument(N) :- integer(N).
Figure 4.2: Definition of meta_predicate/1
The discussion above about the problems with the transparent mechanism
show the two cases in which this simple transformation does not work.
Chapter 5
Foreign Language Interface
SWI-Prolog offers a powerful interface to C . The main design objectives
of the foreign language interface are flexibility and performance. Most
Prolog foreign language interfaces allow the user only to pass primitive
data via the interface. The user should normally specify for each
argument whether it is an input or output argument as well as the type of
the argument. Because type checking and conversion to/from C data types
is done by Prolog the actual foreign code is usually short if something
simple is wanted. The SWI-Prolog interface does not offer these
primitives. Instead Prolog terms in their internal representation are
passed via the interface. This allows the user to write `logical'
predicates and pass arbitrary Prolog data over the interface. As a
trade-off the user is responsible for type checking and should be careful
not to violate consistency rules as Prolog provides access to its internal
data structures.
406
5.1 Portability of the Foreign Interface
The foreign language interface is highly system dependent. It can easily
be ported to machines for which the C linker allows you to link an object
file using the symbol table of a (running) executable and use BSD Unix
format a.out files. On many Unix systems such an object file can be
created using the -A option of ld(1). See the introduction section for a
list of systems to which the foreign interface is available.
5.2 Overview of the Interface
A special include file called ``SWI-prolog.h'' should be included with
each C-source file that is to be loaded via the foreign interface. This
C-header file defines various data types, macros and functions that can be
used to communicate with SWI-Prolog. Functions and macros can be divided
into the following categories:
Analysing arbitrary Prolog terms
Constructing new terms or instantiating existing ones
Returning control information to Prolog
Registering foreign predicates with Prolog
Calling Prolog from C
Communicating about modules
Printing standard Prolog warning/error messages
Global actions on Prolog (halt, break, abort, etc.)
Querying the status of Prolog
A C-file to be included normally defines a number of functions that
implement foreign language Prolog predicates, private support functions
and an installation function. The user should compile this file into a
`.o' file using `cc -c file ...', after which Prolog can be asked to load
the file using the simplified load_foreign/2 predicate or the more
flexible load_foreign/5 predicate. Prolog will call the Unix loader ld(1)
to create an executable. It will then determine the actual size of the
executable, allocate memory for it and call the loader again to create an
executable that can be loaded in the allocated memory. After the
executable is loaded the entry point of the new executable is called.
This function should register all defined foreign predicates with Prolog.
5.3 Loading Foreign Modules
load_foreign(+File, +Entry)
Load a foreign file or list of files specified by File. The files
are searched for similar to consult/1. Except that the `.o'
extension is used rather than `.pl'. Thus `test' is a valid
specification for `test.o' in the current directory,
`[test, library(basics)]' specifies `test.o' in the current directory
and `basics.o' in one of the library directories. To simplify
maintenance of packages in heterogeneous networks the system first
tries whether the object file is available from a subdirectory whose
name depends on the system used. The names of the subdirectories is
shown below.
Directory Machine
sun4 Sparc Station 1 and SUN-4
sun3 SUN-3
hpux HP9000 running HPUX
Entry defines the entry point of the resulting executable. The entry
point will be called by Prolog to install the foreign predicates.
load_foreign(+File, +Entry, +Options, +Libraries, +Size)
The first two arguments are identical to those of load_foreign/2.
Options is (a list of) additional option to be given to the loader.
The default command is:
ld -N -A <symbolfile> -T <offset> -e <entry> -o <executable>
<files> -lc
The options are inserted just before the files. Libraries is (a list
of) libraries to be passed to the loader. They are inserted just
after the files.
If Size is specified Prolog first assumes that the resulting
executable will fit in Size bytes and do the loading in one pass. If
the executable turns out to be larger than Size bytes the loading
sequence is started again, using the calculated size. To determine
the size of an executable specify a size that is too small. Prolog
will then print the actual size on the current output stream.
foreign_file(?File)
Is true if File is the absolute path name of a file loaded as foreign
file.
5.4 Interface Data types
The interface functions can be divided into two groups. The first group
are functions to obtain data from Prolog or pass data to Prolog. These
functions use Prolog internal data types. The second group consists of
type conversion functions convert between Prolog internal data and C
primitive types. Below is a description of the Prolog data types used in
the interface.
termTerms represent arbitrary Prolog data (variables, atoms, integers,
floats and compound terms). Terms live either until backtracking
takes us back to a point before the term was created or the garbage
collector has collected the term.
atomicAtomics are Prologs primitive data types (integers, atoms and
floats). They can be transformed to C data types (int, char * resp.
double). The Prolog representation for an integer is a tagged
version of that integer. The mapping between C ints and Prolog
integers can only be different over different releases of SWI-Prolog.
Atoms are represented by a pointer to a data structure on the Prolog
heap. Each such data structure is a unique representation of a
string (e.g. to verify that two atoms represent the same string
comparing the atoms suffices). The mapping between atoms and string
are likely to vary over different sessions of Prolog. Floats are
represented as (tagged) pointers to a float living on the global
stack. For their life time the same rules apply as for terms.
Whether two floats represent the same number can only be discovered
by transforming both to C floats and then comparing them. Strings
are represented as a tagged pointer to a char * on the global stack
or heap. The rules for their lifetime and comparison equal those for
floats and terms.
functorA functor is the internal representation of a name/arity pair.
They are used to find the name and arity of a compound term as well
as to construct new compound terms. Like atoms they live for the
whole Prolog session and are unique.
moduleA module is a unique handle to a Prolog module. Modules are used
only to call predicates in a specific module.
5.5 The Foreign Include File
5.5.1 Argument Passing and Control
If Prolog encounters a foreign predicate at run time it will call a
function specified in the predicate definition of the foreign predicate.
The arguments (1, ..., arity) pass the Prolog arguments to the goal as
Prolog terms. Foreign functions should be declared of type foreign_t.
Deterministic foreign functions have two alternatives to return control
back to Prolog:
void PL_succeed
Succeed deterministically. PL_succeed is defined as ``return TRUE''.
void PL_fail
Fail and start Prolog backtracking. PL_fail is defined as ``return
FALSE''.
5.5.1.1 Non-deterministic Foreign Predicates
By default foreign predicates are deterministic. Using the
PL_FA_NONDETERMINISTIC attribute (see PL_register_foreign()) it is
possible to register a predicate as a non-deterministic predicate.
Writing non-deterministic foreign predicates is slightly more complicated
as the foreign function needs context information for generating the next
solution. Note that the same foreign function should be prepared to be
simultaneously active in more than one goal. Suppose the
natural_number_below_n/2 is a non-deterministic foreign predicate,
backtracking over all natural numbers lower than the first argument. Now
consider the following predicate:
quotient_below_n(Q, N) :-
natural_number_below_n(N, N1),
natural_number_below_n(N, N2),
Q =:= N1 / N2, !.
In this predicate the function natural_number_below_n/2 simultaneously
generates solutions for both its invocations.
Non-deterministic foreign functions should be prepared to handle three
different calls from Prolog:
Initial call (PL_FIRST_CALL)
Prolog has just created a frame for the foreign function and asks it
to produce the first answer.
Redo call (PL_REDO)
The previous invocation of the foreign function associated with the
current goal indicated it was possible to backtrack. The foreign
function should produce the next solution.
Terminate call (PL_CUTTED)
The choice point left by the foreign function has been destroyed by a
cut. The foreign function is given the opportunity to clean the
environment.
Both the context information and the type of call is provided by an
argument of type foreign_t appended to the argument list for deterministic
foreign functions. The macro PL_foreign_control() extracts the type of
call from the control argument. The foreign function can pass a context
handle using the PL_retry() macro and extract the handle from the extra
argument using the PL_foreign_context() macro.
void PL_retry(handle)
The foreign function succeeds while leaving a choice point. On
backtracking over this goal the foreign function will be called
again, but the control argument now indicates it is a `Redo' call and
the macro PL_foreign_context() will return the handle passed via
PL_retry(). This handle is a 30 bits signed value (two bits are used
for status indication).
int PL_foreign_control(control_argument)
Extracts the type of call from the control argument. The return
values are described above. Note that the function should be
prepared to handle the PL_CUTTED case and should be aware that the
other arguments are not valid in this case.
long PL_foreign_context(control_argument)
Extracts the context from the context argument. In the call type is
PL_FIRST_CALL the context value is 0L. Otherwise it is the value
returned by the last PL_retry() associated with this goal (both if
the call type is PL_REDO as PL_CUTTED).
Note: If a non-deterministic foreign function returns using PL_succeed or
PL_fail, Prolog assumes the foreign function has cleaned its environment.
No call with control argument PL_CUTTED will follow.
The code of figure 5.1 shows a skeleton for a non-deterministic foreign
predicate definition.
typedef struct /* define a context structure */
{ ...
} context;
foreign_t
my_function(a0, a1, handle)
term a0, a1;
foreign_t handle;
{ struct context * ctxt;
switch( PL_foreign_control(handle) )
{ case PL_FIRST_CALL:
ctxt = (struct context *) malloc(sizeof(struct context));
...
PL_retry(ctxt);
case PL_REDO:
ctxt = (struct context *) PL_foreign_context(handle);
...
PL_retry(ctxt);
case PL_CUTTED:
free(ctxt);
PL_succeed;
}
}
Figure 5.1: Skeleton for non-deterministic foreign functions
5.5.2 Analysing Terms via the Foreign Interface
Each argument of a foreign function (except for the control argument) is
of type term. To analyse a term one should first obtain the type of the
term. Primitive terms can then be transformed into atomic data in
internal Prolog representation. This atomic data can be transformed into
C-data types. Complex terms are analysed in terms on their functor and
arguments. The arguments themselves are terms, allowing the same
procedure to be repeated recursively.
int PL_type(term)
Obtain the type of term, which should be a term returned by one of
the other interface predicates or passed as an argument. The
function returns the type of the Prolog term. The type identifiers
are listed below.
PL_VARIABLE An unbound variable. The value of term as
such is a unique identifier for the
variable.
PL_ATOM A Prolog atom.
PL_STRING A Prolog string.
PL_INTEGER A Prolog integer.
PL_FLOAT A Prolog floating point number.
PL_TERM A compound term. Note that a list is a
compound term with name `.' and arity 2.
atomic PL_atomic(term)
Return the atomic value of term in Prolog internal representation.
Term should be atomic (e.g. atom, integer, float or string).
long PL_integer_value(atomic)
Transforms an integer from Prolog internal representation into a C
long.
double PL_float_value(atomic)
Transforms a float from Prolog internal representation into a C
double.
char * PL_atom_value(atomic)
Transforms an atom from Prolog internal representation into a
0-terminated C char *. The pointer points directly into the Prolog
heap and can assumed to be static. The contents of the character
string however should under NO circumstances be modified.
char * PL_string_value(string)
Transform a string from Prolog internal representation into a C char
*. The pointer points directly into the Prolog data area. Unlike
the pointer returned by PL_atom_value() the C user should copy the
value to a private data area if its value should survive the current
foreign language call. Like PL_atom_value(), changing the contents
of the character string is NOT allowed.
functor PL_functor(term)
term should be a complex term. The return value is a unique
identifier of the term's name and arity. The following example
demonstrates this:
pl_same_functor(t1, t2)
term t1, t2;
{ if ( PL_type(t1) != PL_TERM || PL_type(t2) != PL_TERM )
PL_fail;
if ( PL_functor(t1) == PL_functor(t2) )
PL_succeed;
PL_fail;
}
atomic PL_functor_name(functor)
Return an atom representing the name of functor. To get the functor
name as char * of a term which is known to be compound:
#define functor_name(term) PL_atom_value(PL_functor_name(PL_functor(term*
*)))
int PL_functor_arity(functor)
Return a C integer representing the arity of functor.
term PL_arg(term, int)
Return the int-th argument of term. Argument counting starts at 1
and is valid up to and including the arity of term. No checks on
these boundaries are performed.
Figure 5.2 shows a definition of display/1 to illustrate the described
functions.
pl_display(t)
term t;
{ functor functor;
int arity, n;
switch( PL_type(t) )
{ case PL_VARIABLE:
printf("_%d", t);
break;
case PL_ATOM:
printf("%s", PL_atom_value(PL_atomic(t)));
break;
case PL_STRING:
printf("\"%s\"", PL_string_value(PL_atomic(t)));
break;
case PL_INTEGER:
printf("%d", PL_integer_value(PL_atomic(t)));
break;
case PL_FLOAT:
printf("%f", PL_float_value(PL_atomic(t)));
break;
case PL_TERM:
functor = PL_functor(t);
arity = PL_functor_arity(functor);
printf("%s", PL_atom_value(PL_functor_name(functor)));
printf("(");
pl_display(PL_arg(t, 1));
for( n = 2; n <= arity; n++)
{ printf(", ");
pl_display(PL_arg(t, n));
}
printf(")");
break;
default:
PL_fatal_error("Illegal type in pl_display(): %d",
PL_type(t));
}
PL_succeed;
}
Figure 5.2: Foreign definition of display/1
5.5.3 Instantiating and Constructing Terms
Terms are instantiated as in Prolog itself by unification. Variables can
be unified with atomic data, with a functor and with other terms. New
terms are first constructed as a single unbound variable.
term PL_new_term()
Create a new term. The term is an unbound variable living on the
global stack. In the current implementation it lives until Prolog
backtracks to before this call. Garbage collection might change this
in the future.
atomic PL_new_atom(char *)
Create a Prolog atom from a C char *. The contents of the char * are
copied to the Prolog heap.
atomic PL_new_string(char *)
Create a Prolog string, living on the global stack. The contents of
the char * are copied into Prolog's data area.
atomic PL_new_integer(long)
Create a Prolog integer from a C long. Note that the integer is
truncated to 28 bits. No checks on this are performed.
atomic PL_new_float(double)
Create a Prolog float living on the global stack from double.
functor PL_new_functor(atomic, int)
Create a Prolog functor identifier form atomic (which should be an
atom) and int, the arity. Arity is valid for any arity 0. Arity = 0
is used internally, but none of the interface functions use it.
bool PL_unify(term, term)
Unify two terms. Return value is TRUE or FALSE.
bool PL_unify_atomic(term, atomic)
Unify a term with an atomic value.
bool PL_unify_functor(term, functor)
Unify a term with a functor. The unification simply succeeds if term
is already instantiated to a term with functor functor. If term is
variable it will be instantiated to the most general term of functor
(e.g. a term with all arguments unbound variables). Otherwise FALSE
is returned.
If this call succeeds the arguments can be further instantiated by
calling PL_arg() and recursively unifying the returned terms.
An example of using these functions is shown in figure 5.3.
5.5.4 Calling Prolog from C
The Prolog system can be called back from C. This is done by constructing
a term with the functions described above and then calling PL_call().
PL_call() executes the goal and returns TRUE or FALSE depending on success
or failure of the called predicate. There are no means to backtrack over
the Prolog predicate. If alternatives are wanted call findall/3 and read
the result from the third argument.
bool PL_call(term, module)
Call term just like the Prolog predicate once/1. Term is called in
the specified module, or in the context module if module = NULL.
Returns TRUE if the call succeeds, FALSE otherwise. Figure 5.3 shows
an example to obtain the number of defined atoms. All checks are
omitted to improve readability.
5.5.5 Discarding Data
The Prolog data created during setting up the call and calling Prolog can
in most cases be discarded right after the call. See figure 5.3 for an
example.
void PL_mark(bktrk_buf)
Mark the global and trail stacks in bktrk_buf.
void PL_bktrk(bktrk_buf)
Undo all changes in the runtime stacks since a snapshot has been made
in buffer using PL_mark(). Changes to the heap are not affected.
It is not necessary to call PL_bktrk() for each PL_mark(). The user
should ensure that PL_bktrk() is never called with a buffer that is
created after a buffer to which PL_bktrk() has been called. Thus
PL_mark(b1) ... PL_mark(b2) ... PL_bktrk(b1) is valid, but it is not
allowed to call PL_bktrk(b2) after this sequence.
int
count_atoms()
{ term t;
int atoms;
bktrk_buf buf;
PL_mark(&buf); /* mark the global stack */
t = PL_new_term(); /* create `statistics(atoms, Var)' */
PL_unify_functor(t, PL_new_functor(PL_new_atom("statistics"), 2));
PL_unify_atomic(PL_arg(t,1), PL_new_atom("atoms"));
PL_call(t); /* call it */
/* extract the value from the 2nd arg */
atoms = PL_integer_value(PL_atomic(PL_arg(t, 2)));
PL_bktrk(&buf); /* discard global stack data created */
return atoms;
}
Figure 5.3: Calling Prolog
5.5.6 Foreign Code and Modules
Modules are identified via a unique handle. The following functions are
available to query and manipulate modules.
module PL_context()
Return the module identifier of the context module of the currently
active foreign predicate.
term PL_strip_module(term, module *)
Utility function. If term is a term, possibly holding the module
construct module:rest this function will return rest and fill module
* with module. For further nested module constructs the inner most
module is returned via module *. If term is not a module construct
term will simply be returned. If module * is NULL it will be set to
the context module. Otherwise it will be left untouched. The
following example shows how to obtain the plain term and module if
the default module is the user module:
{ module m = PL_new_module(PL_new_atom("user"));
if ( (term = PL_strip_module(term, &m)) == NULL )
return PL_warning("Illegal module specification");
...
atomic PL_module_name(module)
Return the name of module as an atom.
module PL_new_module(atomic)
Find an existing or create a new module with name specified by the
atom atomic.
5.5.7 Catching Unix Signals
SWI-Prolog catches the Unix signals SIGINT, SIGFPE and SIGSEGV. To avoid
problems with foreign code attempting to catch these signals foreign code
should call PL_signal() to install signal handlers rather than the Unix
library function signal(). SWI-Prolog will always handle SIGINT itself.
SIGFPE and SIGSEGV are passed to the foreign code handlers if Prolog did
not expect that signal.
void (*PL_signal(sig, func))()
This function should be used to install signal handlers rather than
the Unix library function signal(). It ensures consistent signal
handling between SWI-Prolog and the foreign code and reinstalls
signal handlers if a state created with save_program/1 is restarted.
5.5.8 Errors and warnings
Two standard functions are available to print standard Prolog errors to
the standard error stream.
bool PL_warning(format, a1, ...)
Print an error message starting with `[WARNING:', followed by the
output from format, followed by a `]' and a newline. Then start the
tracer. format and the arguments are the same as for printf(2). No
more than 10 arguments can be provided.
void PL_fatal_error(format, a1, ...)
Print a message like PL_warning(), but starting with `FATAL ERROR:'
and then exits Prolog.
5.5.9 Environment Control from Foreign Code
bool PL_action(int, C_type)
Perform some action on the Prolog system. int describes the action,
C_type provides the argument if necessary. The actions are listed in
table 5.1.
PL_ACTION_TRACE Start Prolog tracer
PL_ACTION_DEBUG Switch on Prolog debug mode
PL_ACTION_BACKTRACE Print backtrace on current output stream
PL_ACTION_HALT Halt Prolog execution. This action should
be called rather than Unix exit() to give
Prolog the opportunity to clean up. This
call does not return.
PL_ACTION_ABORT Generate a Prolog abort. This call does not
return.
PL_ACTION_BREAK Create a standard Prolog break environment.
Returns after the user types control-D.
PL_ACTION_SYMBOLFILE The argument (a char *) is considered to be
hold the symbolfile for further incremental
loading. Should be called by user
applications that perform incremental
loading as well and want to inform Prolog of
the new symbol table.
Table 5.1: PL_action() options
5.5.10 Querying Prolog
C_type PL_query(int)
Obtain status information on the Prolog system. The actual argument
type depends on the information required. int describes what
information is wanted. The options are given in table 5.2.
PL_QUERY_ARGC Return an integer holding the number of
arguments given to Prolog from Unix.
PL_QUERY_ARGV Return a char ** holding the argument vector
given to Prolog from Unix.
PL_QUERY_SYMBOLFILE Return a char * holding the current symbol
file of the running process.
PL_QUERY_ORGSYMBOLFILE Return the initial symbol file (before
loading) of Prolog. By setting the symbol
file to this value no name clashes can occur
with previously loaded foreign files (but no
symbols can be shared with earlier loaded
modules as well).
Table 5.2: PL_query() options
5.5.11 Registering Foreign Predicates
bool PL_register_foreign(name, arity, function, [...option...] 0)
Register a C-function to implement a Prolog predicate. After this
call returns successfully a predicate with name name (a char *) and
arity arity (a C int) is created. When called in Prolog, Prolog will
call function. [...option...] forms a 0-terminated list of options
for the installation. These are:
PL_FA_NOTRACE Predicate cannot be seen in the
tracer
PL_FA_TRANSPARENT Predicate is module transparent
PL_FA_NONDETERMINISTIC Predicate is non-deterministic. This
attribute is currently ignored, but
will probably be used in future
versions.
5.6 Example of Using the Foreign Interface
Below is an example showing all stages of the declaration of a foreign
predicate that transforms atoms possibly holding uppercase letters into an
atom only holding lower case letters. Figure 5.4 shows the C-source file.
5.6.1 C-Source file (lowercase.c)
/* Include file depends on local installation */
#include "/usr/local/lib/pl/library/SWI-prolog.h"
#include <ctype.h>
long
pl_lowercase(u, l)
term u, l;
{ char *copy;
char *s, *q;
atomic la;
if ( PL_type(u) != PL_ATOM )
return PL_warning("lowercase/2: instantiation fault");
s = PL_atom_value(PL_atomic(u));
copy = (char *) malloc(strlen(s)+1);
for( q=copy; *s; q++, s++)
*q = (isupper(*s) ? tolower(*s) : *s);
*q = '\0';
la = PL_new_atom(copy);
free(copy);
return PL_unify_atomic(l, la);
}
init_lowercase()
{ PL_register_foreign("lowercase", 2, pl_lowercase, 0);
}
Figure 5.4: Lowercase source file
5.6.2 Compiling and Loading Foreign Code
sun% cc -O -c lowercase.c
sun% pl
/staff/jan/.plrc consulted, 0.166667 seconds, 2256 bytes.
Welcome to SWI-Prolog (version 1.5.0, August 1990)
Copyright (c) 1990, University of Amsterdam
1 ?- load_foreign(lowercase, init_lowercase).
foreign file(s) lowercase loaded, 0.016667 seconds, 464 bytes.
Yes
2 ?- lowercase('Hello World!', L).
L = 'hello world!'
Yes
5.7 Notes on Using Foreign Code
5.7.1 Garbage Collection and Foreign Code
Currently no interface between foreign code and the garbage collector has
been defined. The garbage collector is disabled during execution of
foreign code. Future versions might define such an interface. This
probably will introduce incompatible changes to the current interface
definition.
5.7.2 Memory Allocation
SWI-Prolog's memory allocation is based on the malloc() library routines.
Foreign applications can savely use malloc(), realloc() and free().
Memory allocation using brk() or sbrk() is not allowed as these calls
conflict with malloc().
5.7.3 Debugging Foreign Code
NOTE: this section is highly machine dependent. The tricks described here
are tested on SUN-3 and SUN-4. They might work on other BSD variants of
Unix.
Debugging incrementally loaded executables is a bit more difficult than
debugging normal executables. The oldest way of debugging (putting print
statements in your code at critical points) of course still works.
`Post-crash' debugging however is not possible. For adb/dbx to work they
need (besides the core) the text segment and the symbol table. The symbol
table lives somewhere on /tmp (called `/tmp/pl_ld_..._.', where `...' is
the process id and `.' is an additional number to make sure the temporary
file is unique. The text segment lives partly in the core (the
incremental loaded bit) and partly in the SWI-Prolog executable.
The only way to debug foreign language code using a debugger is by
starting the debugger on the running core image. Dbx(1) can do this.
First compile the source files to be debugged with the `-g' option to
include dbx debugging information. Then load them into SWI-Prolog. Now
obtain the name of the current symbol table and the process id of Prolog.
Then start dbx (or dbxtool) using
sun% dbx[tool] <symbol file> <pid>
Should this be done often then the following foreign predicate definition
might help:
pl_dbx()
{ char *symbolfile = PL_query(PL_QUERY_SYMBOLFILE);
char cmd[256];
sprintf(cmd, "dbxtool %s %d &", symbolfile, getpid());
if ( system(cmd) == 0 )
PL_succeed;
else
PL_fail;
}
Register this predicate as dbx/0 using the following call in your
initialisation function:
PL_register_foreign("dbx", 0, pl_dbx, 0);
5.7.4 Name Conflicts in C modules
In the current version of the system all public C functions of SWI-Prolog
are in the symbol table. This can lead to name clashes with foreign code.
Someday I should write a program to strip all these symbols from the
symbol table (why does Unix not have that?). For now I can only suggest
to give your function another name. You can do this using the C
preprocessor. If --for example-- your foreign package uses a function
warning(), which happens to exist in SWI-Prolog as well, the following
macro should fix the problem.
#define warning warning_
5.7.5 Compatibility of the Foreign Interface
As far as I' aware of, there is no standard for foreign language
interfaces in Prolog. The SWI-Prolog interface is no attempt to propose
such a standard. It is (in part) tailored to the possibilities of the
SWI-Prolog machinery. BIM-Prolog has a similar interface to analyse and
construct terms. The major difference is that they have garbage
collection and calls are made available to lock and unlock terms for
garbage collection. I built a similar interface to Edinburgh C-Prolog
(although less clean). This at least tells us that the interface can work
for various forms of the WAM as well as a structure sharing Prolog.
As no standard exists nor emerges, users of the foreign language interface
should carefully design the interface if the C-code should be portable to
other Prolog implementation. The best advice to give is to define a small
interface layer around the C-application and interface this to Prolog.
Chapter 6
Hackers corner
This appendix describes a number of predicates which enable the Prolog
user to inspect the Prolog environment and manipulate (or even redefine)
the debugger. They can be used as entry points for experiments with
debugging tools for Prolog. The predicates described here should be
handled with some care as it is easy to corrupt the consistency of the
Prolog system by misusing them.
477
6.1 Examining the Environment Stack
prolog_current_frame(-Frame)
Unify Frame with an integer providing a reference to the parent of
the current local stack frame. A pointer to the current local frame
cannot be provided as the predicate succeeds deterministically and
therefore its frame is destroyed immediately after succeeding.
prolog_frame_attribute(+Frame, +Key, -Value)
Obtain information about the local stack frame Frame. Frame is a
frame reference as obtained through prolog_current_frame/1,
prolog_trace_interception/3 or this predicate. The key values are
described in table 6.1.
Key Value
alternative Value is unified with an integer reference
to the local stack frame in which execution
is resumed if the goal associated with Frame
fails. Fails if the frame has no
alternative frame.
has_alternatives Value is unified with `1' if Frame still is
a candidate for backtracking. `0'
otherwise.
goal Value is unified with the goal associated
with Frame. If the definition module of the
active predicate is not user the goal is
represented as module:goal. Do not
instantiate variables in this goal unless
you know what you are doing!
level Value is unified with the recursion level of
Frame. The top level frame is at level `0'.
parent Value is unified with an integer reference
to the parent local stack frame of Frame.
Fails if Frame is the top frame.
context_module Value is unified with the name of the
context module of the environment.
top Value is unified with `1' if Frame is the
top Prolog goal from a recursive call back
from the foreign language. `0' otherwise.
Table 6.1: Key values of prolog_current_frame/1
6.2 Intercepting the Tracer
prolog_trace_interception(+Port, +Frame, -Action)
Dynamic predicate, normally not defined. This predicate is called
from the SWI-Prolog debugger just before it would show a port. If
this predicate succeeds the debugger assumes the trace action has
been taken care of and continues execution as described by Action.
Otherwise the normal Prolog debugger actions are performed.
Port is one of call, redo, exit, fail or unify. Frame is an integer
reference to the current local stack frame. Action should be unified
with one of the atoms continue(just continue execution), retry
(retry the current goal) or fail (force the current goal to fail).
Leaving it a variable is identical to continue.
Together with the predicates described in section 3.34 and the other
predicates of this chapter this predicate enables the Prolog user to
define a complete new debugger in Prolog. Besides this it enables
the Prolog programmer monitor the execution of a program. The
example shown in figure 6.1 records all goals trapped by the tracer
in the database. To trace the execution of `go' this way the
following query should be given:
?- trace, go, notrace.
prolog_trace_interception(Port, Frame, continue) :-
prolog_frame_attribute(Frame, goal, Goal),
prolog_frame_attribute(Frame, level, Level),
recordz(trace, trace(Port, Level, Goal)).
Figure 6.1: Record a trace in the database
prolog_skip_level(-Old, +New)
Unify Old with the old value of `skip level' and than set this level
according to New. New is an integer, or the special atom very_deep
(meaning don't skip). The `skip level' is a global variable of the
Prolog system that disables the debugger on all recursion levels
deeper than the level of the variable. Used to implement the trace
options `skip' (sets skip level to the level of the frame) and `up'
(sets skip level to the level of the parent frame (i.e. the level of
this frame minus 1).
6.3 Exception Handling
A start has been made to make exception handling available to the Prolog
user. On exceptions a dynamic and multifile defined predicate exception/3
is called. If this user defined predicate succeeds Prolog assumes the
exception has been taken care of. Otherwise the system default exception
handler is called.
exception(+Exception, +Context, -Action)
Dynamic predicate, normally not defined. Called by the Prolog system
on run-time exceptions. Currently exception/3 is only used for
trapping undefined predicates. Future versions might handle signal
handling, floating exceptions and other runtime errors via this
mechanism. The values for Exception are described below.
undefined_predicate
If Exception is undefined_predicateContext is instantiated to a
term Name/Arity. Name refers to the name and Arity to the arity
of the undefined predicate. If the definition module of the
predicate is not user Context will be of the form
Module:Name/Arity. If the predicate fails Prolog will print the
default error warning and start the tracer. If the predicate
succeeds it should instantiate the last argument either to the
atom fail to tell Prolog to fail the predicate or the atom retry
to tell Prolog to retry the predicate. This only makes sense if
the exception handler has defined the predicate. Otherwise it
will lead to a loop.
warning
If prolog wants to give a warning while reading a file, it will
first raise the exception warning. The context argument is a
term of the form warning(Path, LineNo, Message), where Path is
the absolute filename of the file prolog is reading; LineNo is an
extimate of the line number where the error occurred and Message
is a Prolog string indicating the message. The Action argument
is ignored. The error is supposed to be presented to the user if
the exception handler succeeds. Otherwise the standard Prolog
warning message is printed.
This exception is used by the library emacs_interface, that
integrates error handling with GNU-Emacs.
Chapter 7
Predicate Summary
!/0 Cut. Discard choicepoints
*/2 Arithmetic: multiplication
+/2 Arithmetic: addition
,/2 Conjuction of goals
-/1 Arithmetic: unary minus
-/2 Arithmetic: subtraction
->/2 If-then-else
./2 List operator. Also consult
///2 Arithmetic: Integer division
//2 Arithmetic: division
/\/2 Arithmetic: bitwise and
;/2 Disjunction of goals
</2 Arithmetic smaller
<</2 Arithmetic: bitwise left shift
=../2 Univ. Term to list conversion
=/2 Unification
=:=/2 Arithmetic equal
=</2 Arithmetic smaller or equal
==/2 Identical
=@=/2 Structural identical
=\=/2 Arithmetic not equal
>/2 Arithmetic larger
>=/2 Arithmetic larger or equal
>>/2 Arithmetic: bitwise right shift
@</2 Standard order smaller
@=</2 Standard order smaller or equal
@>/2 Standard order larger
@>=/2 Standard order larger or equal
\/1 Bitwise negation
\//2 Arithmetic: bitwise or
\+/1 Negation by failure (not provable)
\=/2 Not unifyable
\==/2 Not identical
\=@=/2 Not structural identical
^/2 Existential quantification (bagof/3, setof/3)
abolish/2 Remove predicate definition from the database
abort/0 Abort execution, return to top level
abs/1 Arithmetic: absolute value
absolute_file_name/2 Get absolute Unix path name
access_file/2 Check access permissions of a file
acos/1 Arithmetic: inverse (arc) cosine
append/1 Append to a file
append/3 Concatenate lists
apply/2 Call goal with additional arguments
apropos/1 Show related predicates and manual sections
arithmetic_function/1 Register an evaluable function
arg/3 Access argument of a term
asin/1 Arithmetic: inverse (arc) sine
assert/1 Add a clause to the database
assert/2 Add a clause to the database, give reference
asserta/1 Add a clause to the database (first)
asserta/2 Add a clause to the database (first)
assertz/1 Add a clause to the database (last)
assertz/2 Add a clause to the database (last)
atan/1 Arithmetic: inverse (arc) tangent
atan/2 Arithmetic: rectangular to polar conversion
atom/1 Type check for an atom
atom_length/2 Determine length of an atom
atom_to_term/3 Convert between atom and term
atomic/1 Type check for primitive
bagof/3 Find all solutions to a goal
between/3 Integer range checking/generating
break/0 Start interactive toplevel
call/1 Call a goal
ceil/1 Arithmetic: smallest integer larger than argument
character_count/2 Get character index on a stream
chdir/1 Change working directory
checklist/2 Invoke goal on all members of a list
clause/2 Get clauses of a predicate
clause/3 Get clauses of a predicate
close/1 Close stream
compiling/0 Is this a compilation run?
concat/3 Append two atoms
concat_atom/2 Append a list of atoms
consult/1 Read (compile) a Prolog source file
context_module/1 Get context module of current goal
convert_time/8 Convert time stamp
copy_term/2 Make a copy of a term
cos/1 Arithmetic: cosine
cputime/0 Arithmetic: get CPU time
current_atom/1 Examine existing atoms
current_arithmetic_function/1 Examine evaluable functions
current_flag/1 Examine existing flags
current_functor/2 Examine existing name/arity pairs
current_input/1 Get current input stream
current_key/1 Examine existing database keys
current_op/3 Examine current operator declaractions
current_output/1 Get the current output stream
current_predicate/2 Examine existing predicates
current_stream/3 Examine open streams
debug/0 Test for debugging mode
debugging/0 Show debugger status
delete/3 Delete all matching members from a list
delete_file/1 Unlink a file from the Unix file system
discontiguous/1 Indicate distributed definition of a predicate
display/1 Write a term, ignore operators
display/2 Write a term, ignore operators on a stream
displayq/1 Write a term with quotes, ignore operators
displayq/2 Write a term with quotes, ignore operators on a str*
*eam
dwim_match/2 Atoms match in ``Do What I Mean'' sense
dwim_match/3 Atoms match in ``Do What I Mean'' sense
dwim_predicate/2 Find predicate in ``Do What I Mean'' sense
dynamic/1 Indicate predicate definition may change
e/0 Arithmetic: mathematical constant
ed/0 Edit last edited predicate
ed/1 Edit a predicate
edit/0 Edit last edited file
edit/1 Edit a file
ensure_loaded/1 Consult a file if that has not yet been done
erase/1 Erase a database record or clause
exception/3 Handle runtime exceptions
exists_directory/1 Check existence of Unix directory
exists_file/1 Check existence of Unix file
exp/1 Arithmetic: exponent (base e)
expand_file_name/2 Wildcard expansion of file names
export/1 Export a predicate from a module
fail/0 Always false
fileerrors/2 Do/Don't warn on file errors
findall/3 Find all solutions to a goal
flag/3 Simple global variable system
flatten/2 Transform nested list into flat list
float/1 Type check for a floating point number
floor/1 Arithmetic: largest integer below argument
flush/0 Output pending characters on current stream
flush_output/1 Output pending characters on specified stream
forall/2 Prove goal for all solutions of another goal
foreign_file/1 Examine loaded foreign files
format/1 Produce formatted output
format/2 Produce formatted output on a stream
format_predicate/2 Program format/[1,2]
free_variables/2 Find unbound variables in a term
functor/3 Get name and arity of a term or construct a term
garbage_collect/0 Invoke the garbage collector
gensym/2 Generate unique atoms from a base
get/1 Read first non-blank character
get/2 Read first non-blank character from a stream
get0/1 Read next character
get0/2 Read next character from a stream
get_single_char/1 Read next character from the terminal
get_time/1 Get current time
getenv/2 Get Unix environment variable
ground/1 Verify term holds no unbound variables
halt/0 Exit from Prolog
help/0 Give help on help
help/1 Give help on predicates and show parts of manual
history_depth/1 Number of remembered queries
read_history/6 Read using history substitution
ignore/1 Call the argument, but always succeed
import/1 Import a predicate from a module
index/1 Change clause indexing
int_to_atom/2 Convert from integer to atom
int_to_atom/3 Convert from integer to atom (non-decimal)
integer/1 Arithmetic: round to nearest integer
integer/1 Type check for integer
intersection/3 Set intersection
is/2 Evaluate arithmetic expression
is_list/1 Type check for a list
is_set/1 Type check for a set
keysort/2 Sort, using a key
last/2 Last element of a list
leash/1 Change ports visited by the tracer
length/2 Length of a list
library_directory/1 Directories holding Prolog libraries
limit_stack/2 Limit stack expansion
line_count/2 Line number on stream
line_position/2 Character position in line on stream
list_to_set/2 Remove duplicates
listing/0 List program in current module
listing/1 List predicate
load_foreign/2 Load foreign (C) module
load_foreign/5 Load foreign (C) module
log/1 Arithmetic: natural logarithm
log10/1 Arithmetic: 10 base logarithm
make/0 Reconsult all changed source files
maplist/3 Transform all elements of a list
max/2 Arithmetic: Maximum of two numbers
member/2 Element is member of a list
memberchk/2 Deterministic member/2
merge/3 Merge two sorted lists
merge_set/3 Merge two sorted sets
min/2 Arithmetic: Minimum of two numbers
mod/2 Arithmetic: remainder of division
module/2 Declare a module
module_transparent/1 Indicate module based meta predicate
msort/2 Sort, do not remove duplicates
multifile/1 Indicate distributed definition of predicate
name/2 Convert between atom and list of ASCII characters
nl/0 Generate a newline
nl/1 Generate a newline on a stream
nodebug/0 Disable debugging
nonvar/1 Type check for bound term
noprotocol/0 Disable logging of user interaction
nospy/1 Remove spy point
nospyall/0 Remove all spy points
not/1 Negation by failure (not provable)
notrace/0 Stop tracing
nth0/3 N-th element of a list (0-based)
nth1/3 N-th element of a list (1-based)
number/1 Type check for integer or float
numbervars/4 Enumerate unbound variables of a term
once/1 Call a goal deterministicaly
op/3 Declare an operator
open/3 Open a file (creating a stream)
open_null_stream/1 Open a stream to discard output
pi/0 Arithmetic: mathematical constant
please/3 Query/change environment parameters
plus/3 Logical integer addition
portray/1 Modify behaviour of print/1
portray_clause/1 Pretty print a clause
predicate_property/2 Query predicate attributes
predsort/3 Sort, using a predicate to determine the order
preprocessor/2 Install a preprocessor before the compiler
print/1 Print a term
print/2 Print a term on a stream
profile/3 Obtain execution statistics
profile_count/3 Obtain profile results on a predicate
profiler/2 Obtain/change status of the profiler
prolog/0 Run interactive toplevel
prolog_current_frame/1 Reference to goal's environment stack
prolog_frame_attribute/3 Obtain information on a goal environment
prolog_skip_level/2 Indicate deepest recursion to trace
prolog_trace_interception/3 Intercept the Prolog tracer
prompt/2 Change the prompt used by read/1
proper_list/1 Type check for list
protocol/1 Make a log of the user interaction
protocola/1 Append log of the user interaction to file
protocolling/1 On what file is user interaction logged
put/1 Write a character
put/2 Write a character on a stream
random/1 Arithmetic: generate random number
read/1 Read Prolog term
read/2 Read Prolog term from stream
read_clause/1 Read clause
read_clause/2 Read clause from stream
read_variables/2 Read clause including variable names
read_variables/3 Read clause including variable names from stream
recorda/2 Record term in the database (first)
recorda/3 Record term in the database (first)
recorded/2 Obtain term from the database
recorded/3 Obtain term from the database
recordz/2 Record term in the database (last)
recordz/3 Record term in the database (last)
rename_file/2 Change name of Unix file
repeat/0 Succeed, leaving infinite backtrackpoints
reset_profiler/0 Clear statistics obtained by the profiler
retract/1 Remove clause from the database
retractall/1 Remove unifying clauses from the database
reverse/2 Inverse the order of the elements in a list
same_file/2 Succeeds if arguments refer to same file
save_program/1 Save the current program on a file
save_program/2 Save the current program on a file
see/1 Change the current input stream
seeing/1 Query the current input stream
seen/0 Close the current input stream
select/3 Select element of a list
set_input/1 Set current input stream from a stream
set_output/1 Set current output stream from a stream
set_tty/2 Set `tty' stream
setenv/2 Set Unix environment variable
setof/3 Find all unique solutions to a goal
sformat/2 Format on a string
sformat/3 Format on a string
shell/0 Execute interactive Unix subshell
shell/1 Execute Unix command
shell/2 Execute Unix command
show_profile/1 Show results of the profiler
sin/1 Arithmetic: sine
size_file/2 Get size of a file in characters
sleep/1 Suspend execution for specified time
sort/2 Sort elements in a list
source_file/1 Examine currently loaded source files
source_file/2 Obtain source file of predicate
spy/1 Force tracer on specified predicate
sqrt/1 Arithmetic: square root
statistics/0 Show execution statistics
statistics/2 Obtain collected statistics
stream_position/3 Get/seek to position in file
string/1 Type check for string
string_length/2 Determine length of a string
string_to_atom/2 Conversion between string and atom
string_to_list/2 Conversion between string and list of ASCII
style_check/1 Change level of warnings
sublist/3 Determine elements that meet condition
subset/2 Generate/check subset relation
substring/4 Get part of a string
subtract/3 Delete elements that do not meet condition
succ/2 Logical integer successor relation
swritef/2 Formatted write on a string
swritef/3 Formatted write on a string
tab/1 Output number of spaces
tab/2 Output number of spaces on a stream
tan/1 Arithmetic: tangent
tell/1 Change current output stream
telling/1 Query current output stream
term_expansion/2 Convert term before compilation
term_to_atom/2 Convert between term and atom
time/1 Determine time needed to execute goal
time_file/2 Get last modification time of file
told/0 Close current output
trace/0 Start the tracer
tracing/0 Query status of the tracer
trim_stacks/0 Release unused memory resources
true/0 Succeed
tty_fold/2 Make terminal fold long lines in output
tty_get_capability/3 Get terminal parameter
tty_goto/2 Goto position on screen
tty_put/2 Write control string to terminal
ttyflush/0 Flush output on terminal
union/3 Union of two sets
unknown/2 Trap undefined predicates
unsetenv/1 Delete Unix environment variable
use_module/1 Import a module
use_module/2 Import predicates from a module
var/1 Type check for unbound variable
visible/1 Set ports that are visible in the tracer
wait_for_input/3 Wait for input with optional timeout
wildcard_match/2 Csh(1) style wildcard match
write/1 Write term
write/2 Write term to stream
write_ln/1 Write term, followed by a newline
writef/1 Formatted write
writef/2 Formatted write
writeq/1 Write term, insert quotes
writeq/2 Write term, insert quotes on stream
xor/2 Arithmetic: exclusive or
|/2 Disjunction of goals
486