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A.D.A PROLOG Documentation Version 1.91
for the Educational and Public Domain Versions
May 28, 1986
Copyright Robert Morein and Automata Design Associates
1570 Arran Way
Dresher, Pa. 19025
(215)-646-4894
News
Release 1.91 of PD PROLOG accesses an editor. The editor
"PROLOGED.COM", by Li Su of Richfield, Minnesota, is a simple,
and compact. Three cheers for Li Su!
Copyright Notice
The public domain PD PROLOG system has been contributed to
the public domain for unrestricted use with one exception: the
object code may not be disassembled or modified. Electronic
bulletin boards and SIG groups are urged to aid in giving this
software the widest possible distribution.
This documentation may be reproduced freely, but it may not
be included in any other documentation without the permission of
the author.
Introduction
We hope that you'll get some fun out of this PROLOG. It will
afford you exposure to THE fifth generation language at the cost
only of some intellectual effort. The motive is perfectly
explicable: We want you to think of Automata Design Associates
for fifth generation software. It also gives us a nice warm
feeling.
The minimum memory requirement is 200 k of transient program
area, plus whatever space is needed to execute programs from
within PROLOG. DOS or MSDOS 2.0 are required. The program does
not require IBM PC compatibility to run, although the screen
access routines do require compatibility.
2
Products by Automata Design Associates
Automata Design Associates specializes in software for
artificial intelligence and robotic applications. A PROLOG
language system is available in various configurations. A LISP
interpreter will be introduced in March of 1985.
Six PROLOG Development Systems available from Automata
Design Associates.
The ADA PROLOG Runtime Systems permit no-royalty
distribution of a complete logic system with your product. Each
system is functionally equivalent to the corresponding
Development System, except that the user command loop is absent
and tracing is not implemented.
The systems run under the MSDOS or PCDOS, and access all
available memory up to 1 megabyte.
.Public Domain PROLOG
This serves to further the general awareness of the public about
PROLOG. It also is an excellent adjunct to anyone learning the
language. Most of the core PROLOG described by Clocksin and
Mellish in the book Programming In PROLOG is implemented. A
complete IBM PC video screen support library is included in this
and all other A.D.A. prologs. Trace predicates are not. This
version is available from us for $10.00 postage paid.
.Educational PROLOG
At extremely modest cost this affords an educational institution
or individual a PROLOG system which provides the maximum
available programming area available within the 8086 small
programming model. Tracing, a debugging aid, allows monitoring
a program as it runs. User settable spy points selectively allow
this. Exhaustive tracing is also available. I/O redirection
gives some file ability.
An "exec" function allows the execution of a program or
editor from within PROLOG, thus encouraging an interactive
environment.
An "interrupt" menu is added, permitting the control of
tracing, toggling the printer, and screen printing.
Definite clause grammar support is now included.
Educational PROLOG Development System: $29.95.
Educational PROLOG Runtime System: $29.095
3
.FS PROLOG
A small increment in price adds full random access file
capability. Character and structure I/O are allowed.
The "asserta and "assertz" predicates are expanded and work
with a clause indexing ability. One can assert clauses anywhere
in the database under precise pattern matching control.
A tree structured lexical scoping system and floating point
arithmetic are other enhancements.
FS PROLOG Development System: $49.95
FS PROLOG Runtime System: $49.95
.VMI PROLOG -- Virtual Memory (Replaces type VMS)
At reasonable cost the addition of virtual memory gives an
expansion of capabilities of an order of magnitude.
The database on disk is treated transparently. No special
provisions need be made by the user. Virtual and resident
databases may be mixed. A unique updating algorithim preserves
the format of the database as typed by the user while making only
those changes necessary to make it equivalent to the database in
central memory.
VMI PROLOG Development System: $99.95
VMI PROLOG Runtime System: $99.95
4
.VML PROLOG Large model Virtual Memory System
A.D.A. PROLOG is a remarkable fifth generation developement
tool for the implementation of intelligent strategies and
optimized control. It is both the kernel for applications of
virtually unlimited scope and a sophisticated developement tool
that multiplies the productivity of the programmer many times.
With a cost/performance ratio exceeding that of any other
product and a compatibility insured by compliance to the
Edinburgh syntax, performance is enhanced by numerous extensions,
many of them invisible to the user.
A quick overview of some of the features discloses:
1) Invisible compilation to a semantic network
preserves the flexibility of the interpreted mode and
the speed of a compiler.
The programmer can compile and recompile any portion
of a module at any time. The edit/compile/test cycle
is short and free of strain. An interface is provided
to an editor of choice.
2) Floating point arithmetic with a full complement
of input and output methods, transcendental and
conversion functions.
3) Virtual memory. Module size and number are
unrestricted, with a total capacity of several
hundred megabytes. Resident and virtual modules may
be co-resident. Compilation is incremental. The cache
algorithim is sophisticated. Changes made in the
database can be updated to disk by a single command.
4) A powerful exec function and acceptance of stream
input make integration into applications practical.
5) Global polymorphic variables retain information
that would otherwise require the "assertion" of
facts.
6) A quoted variable class, borrowed from LISP,
permits referencing variables as objects as well as
by value.
7) Multidimensional arrays, dynamically created and
destroyed, efficiently store numeric and nonnumeric
structures. Arrays are ideal for representing spatial
and ordinal relationships.
5
8) Debugging facilities let you see your program run
without any additional generation steps.
9) Totally invisible and incremental garbage
collection. There is NEVER any wait for this
function.
10) A tree structured, dynamically configurable
lexical scoping system. The work of many programmers
can be coupled together in the form of libraries and
nested domains.
Each lexically scoped domain is a hidden space which
communicates with the parent domain via exports and
imports. Domains can be linked together under program
control to achieve any desired configuration.
11) The Grammar Rule Notation is an integral feature.
12) Keyword redefinition makes porting code easy.
The cost of this system is $200 for the MSDOS version.
The corresponding Runtime System is available for $200.00
.VMA PROLOG Large model Virtual Memory System
This system has additional forms of virtual memory. It was
intended for deep reasoning problems. Contact us for more
information.
VMA PROLOG Development System: $250.00
VMA PROLOG Runtime System: $250.00
6
Upgrade Policy
Half the cost of any A.D.A. PROLOG system may be credited to
the purchase of a higher level version. The full cost of VMS
prolog may be applied to the purchase of VMI or VML PROLOG.
Updates to a particular level product vary from $15.00 to $35.00.
Technical Information
Technical information may be obtained at (215) - 646- 4894
Perhaps we can answer the following questions in advance:
There is no support for: APPLE II, Atari, Commodore, or
CPM 80 . Other machines from these manufactures may be supported
in the future.
The MSDOS products are available on 5" and 8" diskettes.
To Place Your Order:
You may place your order at the following number:
(215)-646-4894 - day and night.
Returns
The software may be returned within 30 days of purchase for
a full refund. This applies whether or not the software has been
used. We do ask that manuals, disks and packaging be returned in
excellent condition.
7
Installation
You will need an IBM PC compatible machine with a minimum of
256 kbytes of memory. ED PROLOG benefits from all available
memory up to the 1 megabyte INTEL limit.
To determine the amount of TPA your machine has, run the
"chkdsk" program which is supplied with DOS. The last line reads:
XXXXXX bytes free
where XXXXXX is a six digit number.
If this number is greater than 200000, ED PROLOG will have
reduced workspace. If it's over 245000, the amount of memory is
optimal. If this is not the case, there are two possibilities:
1) The machine doesn't have enough memory.
2) Something else is removing memory from TPA, such as a co-
resident program, a ramdisk, a large dos driver, or a large
number of file or disk buffers.
If you're short of memory, make sure that no other programs,
ramdisks, or drivers besides DOS are running in the machine. You
may find it helps to eliminate (by renaming) the config.sys file
when you intend to run ED PROLOG.
In order to use the built-in predicate "exec", the system must be
configured for memory, which is detailed further on.
How to run the Demonstration Programs
without Knowing What You're Doing
We strongly advise that you purchase the book Programming in
PROLOG by Clocksin and Mellish, publisher Springer Verlag, 1981.
For the impatient we give some advice. Type the demonstration
program you wish to run. There must be at least one entry point
within the program.
Note: Please understand that these are demonstrations programs.
Regarding user interface, they are poorly written. You will
probably have to read Clocksin and Mellish to appreciate that the
following examples of input are not equivalent: "yes." , "yes" .
The Hematology Diagnosis Program - "hemat"
Although the logical structure is not as sophisticated as
that of "animals", it is interesting for several reasons:
1) The program evaluates numerical data to arrive at a
diagnosis.
8
2) Although inaccurate, it demonstrates that useful question
answering systems are not difficult to write in PROLOG.
3) There are some mistakes in the program, which only
slightly impede its usefulness.
This program uses structure input. Terminate all your
answers with a period, as in "y.<CR>", or "no.<CR>".
The starting point is "signs.<CR>". PROLOG will prompt you
for signs of anemia. The program attempts to diagnose two
varieties of a hemolytic anemia.
The program could use a good working over by a hematologist
and we would be delighted to collaborate.
Prime Number Generator - "sieve"
This program demonstrates that anything can be programed in
PROLOG if one tries hard enough. Asking the question
"primes( 50, L ).<CR>" causes a list of prime numbers less than
50 to be printed out. "Sieve" is heavily recursive and quickly
exhausts the stack space of the small model interpreters.
Grrules
This is an example of the use of the definite clause
grammer notation. PD PROLOG does not have this translation
facility, but ED PROLOG and all of our other versions do. It is
possible to perform the translations by hand if you have
thoroughly read C & M. Then you would have the pleasure of
asking:
?-sentence( X, [every,man,loves,a,woman], [] ).
and having the meaning elucidated as a statment in the predicate
calculus.
9
Special Offer # 1
For some inexplicable reason, demonstration programs are
hard to come by. We are too busy writing PROLOG fill this gap. We
will reward the contribution of "cute" sample programs with the
following:
1) A free copy of type VMI virtual memory PROLOG
2) The sample program will be published as an intact file
together with whatever comments or advertisments the author
may see fit to include, on our distribution disks.
3) Exceptional contributions may merit a copy of type VML
large model virtual memory PROLOG which now incorporates a
UNIX1 style tree structured domain system.
Special Offer # 2
If you are a hardware manufacturer and would like a PROLOG
language for your system, the solution is simple. Just send us
one of your machines! Provided your system implements a "C"
compiler, it will be ported in no time flat.
______
1. Trademark of AT & T.
10
Writing Programs For ED PROLOG
You do not type in programs at the "?-" prompt. There is no
built-in editor. The command "consult( user )" is accepted but
does not cause PROLOG to enter an editing mode. We feel that the
most universally acceptable editing method is for the user to use
a text editor of choice, which can be invoked from within PROLOG
by the "exec" predicate.
Use Wordstar or your customary editor to write a program.
Then run PD PROLOG and use the consult function to load the
program.
In all cases except PD PROLOG, you can run your editor
without leaving PROLOG by use of the "exec" predicate.
11
Running the Interpreter
COMMANDS: Give commands in lower case.
TO RUN:
Invoke PROLOG.EXE. After the "?-" prompt appears,
type "consult( <filename><CR> )", where <filename> is the
desired database.
To exit, type "exitsys.<CR>"
TO ENTER A FACT:
Don't do it except with the "assert" predicates. This is the
most frequently misunderstood aspect of A.D.A. Prolog. If
you want to enter a bunch of facts, put them in a file and
"consult" them using the "consult" predicate.
TO ASK A QUESTION:
At the prompt, type "<expression>.<CR>", where
<expression> is a question as described by Clocksin and
Mellish. Be sure to terminate the question with a period.
The question may be up to 500 characters long.
TO INPUT A STRUCTURE AT THE KEYBOARD:
The structure may be up to 500 characters in length. Be sure
to terminate with a period.
TO ASK FOR ANOTHER SOLUTION:
If a solution has been provided, the PROLOG interpreter will
ask "More? (Y/N):". Only if a "y" is typed will the
interpreter perform a search.
TO ABORT A SEARCH:
Simply type the escape key. The interpreter will
respond with "Interrrupted.", and return to the command
prompt.
TO LOAD ANOTHER DATABASE:
Type "consult(<filename>).<CR>" The file name must have the
extension ".PRO". It is not necessary to include the
extension in the argument of consult. The file name as given
must not be the same as a predicate name in the file or any
file which will be loaded.
TO TRACE:
When the system displays the prompt "?-", type "trace.<CR>".
The display will likely move too rapidly for you to read. To
stop the display, type Control S. To restart the display,
type Control S. To turn the trace display off, type
"notrace<CR>" at the prompt "?-". The interrupt menu
contains additional options, such as sending all trace
output to a file, as well as display at the console.
12
TO INTERRUPT A PROGRAM:
See the section entitled "The Interrupt Menu" for a
description of the flexible options. Basically, one types
ESC to terminate a program, while Control V or Control I
interrupt a program.
TO RECONSULT A FILE:
The predicate "recon" is identical to the Edinburgh
predicate "reconsult."
TO REMOVE A DATABASE:
Type "forget(<filename>).<CR>"
TO EDIT A FILE:
The "exec" function allows you to execute other programs
from within PROLOG. In the type PD, it is restricted to a program
of name "PROLOGED", which is a text editor supplied by you, or
the program "PROLOGED.COM" supplied by us. This is covered
further on.
TO EXIT TO THE OPERATING SYSTEM:
Type "exitsys.<CR>"
The system is totally interactive; any commands the operator
gives are and must be valid program statements. Statements must
terminate with a period. All commands which take a file name
also accept a path name. Any name which is not a valid PROLOG
atom (refer to C & M) must be enclosed in single quotes. Thus one
could say
consult( expert )
but one would need single quotes with
consult( 'b:\samples\subtype\expert' ).
To exit the system, type "exitsys.<CR>"
Atoms may contain MSDOS pathnames if they are enclosed by single
quotes, ie., '\b:\samples\animal' .
You may consult more than one file at a time. However, all names
are public and name conflicts must be avoided. The order in which
modules are loaded may, in cases of poor program design, affect
the behavior.
13
Command Line Arguments
ED and PD PROLOG accept one command line argument, which is
the name of a "stream" which replaces the console for input. The
"stream" in MSDOS is a pipe or file which supplies input until
end-of-file is reached. Control then reverts back to the console.
To avoid noisy parser error messages when end-of-file is reached,
the last command in the file should be "see( user )." See Simon
Blackwell's PIE program for an example of this.
A Reference of Note
With minor exceptions, the syntax is a superset of that
described by Clocksin and Mellish in the book Programming in
Prolog by W.F. Clocksin and C.S. Mellish, published by Springer
Verlag in Berlin, Heidelberg, New York, and Tokyo. We shall refer
to this book as "C & M".
There are very few syntactical differences, mostly
unrecognized and/or minor.
When an operator is declared using the "op" statement, the
operator must be enclosed in single quotes in the "op" statement
itself, if it would not otherwise be a legal Edinburgh functor.
Subsequently, however, the parser will recognize it for what it
is, except in the "unop" statement, where it must again be
enclosed in single quotes.
Variable numbers of functor paramaters are supported.
A goal may be represented by a variable, which is less
restrictive than the C & M requirement that all goals be
functors. The variable must be instantiated to a functor when
that goal is pursued.
Rules which appear inside other expressions must be enclosed
in parenthesis if the "," operator is to be recognized as a
logical connective.
All infix operators described by C & M, and user defined
infix, prefix, and postfix operators with variable associativity
and precedence are supported exactly as in C & M.
14
Editing and Preparing Programs for ADA PROLOG
ADA PROLOG programs are ordinary text files. They may be
prepared using your customary text editor, in which case one then
runs PROLOG.EXE, "consults" a file which must have an extension
of ".PRO", and enters the interactive, goal giving phase.
The alternative to this is to prepare files without leaving
PROLOG, which requires that more memory be available. We supply a
small text editor on disk, "PROLOGED.COM". This, or any other
editor which is renamed as such, can be used provided that one
realizes that whatever memory the editor uses is not available
for use by PROLOG. The built in predicate "exec" is used to load
an editor in PD PROLOG, or any program in ED PROLOG.
In order to use the "exec" function, it is necessary to
reserve enough space using the program "PRCONF.EXE" to configure
the PROLOG system. This procedure is described in the next
section. Once this is done, the syntax is as follows:
exec( <string1>, <string2>, <string3>,... )
Execute a DOS program from within PROLOG. The file
"COMMAND.COM" must be in the DOS search path. A <string> may not
contain embedded blanks. Acceptable forms:
exec( ws, 'test.pro' )
exec( 'ws test.pro' )
exec( prologed )
Interestingly the form
exec( command )
causes an apparent exit to DOS. However, you can resume PROLOG by
typing "exit", <CR>, and all your work will be intact.
In the type PD, the capabilities of the "exec" function are
limited. The only form is:
exec
with no arguments, which causes execution of the program
"PROLOGED.COM", or "PROLOGED.EXE". We supply such a program on
disk, and it is a small editor.
15
Configuring the PROLOG System for Memory
The PRCONF.EXE program, which can be used with versions 1.9
and higher, allows you to configure the memory used by the PROLOG
system. Thus the amount of memory left over for use by the
"exec" function can be controlled. Type PD does not have an
"exec" function, and therefore has no need of this program. It
grabs all of transient program area by default.
To use PRCONF, copy your ??PROLOG.EXE file to a file named
PROLOG.EXE. PRCONF looks for this file, queries the operator for
the amount of workspace desired, and then alters the file. The
total space used by the altered copy is the sum of the size of
PROLOG.EXE itself plus the size of the workspace you specified.
After you've done this, run PROLOG.EXE and look at the
copyright message. It will display the highest address used. Use
the "exec" function to load and execute a program you're
interested in having available without leaving PROLOG. If the
"exec" function fails, you must rerun PRCONF and reduce the
amount of workspace, thereby leaving more room for the "exec"
function.
16
The Built In Predicate Library
Available Operators in PD and ED PROLOG
Column 1 gives the function symbol.
Column 2 gives the precedence. The range of precedence is 1 to 255.
A zero in the precedence column indicates the symbol is parsed as
a functor, and precedence is meaningless in this case.
Column 3 gives the associativity.
A zero in the associativity column indicates the symbol is parsed
as a functor, and associativity is meaningless in this case.
Column 4 indicates which version the function is available in.
Unless otherwise noted, the function is available in all versions.
Nonstandard predicates are indicated by "NS".
op/pred precedence associativity availability
"!" 0 0
"|" 0 0
"=" 40, XFX
"==" 40, XFX
"\\=" 40, XFX
"\\==" 40, XFX
"/" 21, YFX
"@=" 40, XFX
">=" 40, XFX
"=<" 40, XFX
">" 40, XFX
"<" 40, XFX
"-" 31, YFX
"*" 21, YFX
"+" 31, YFX
"=.." 40, XFX
"-->" 255, YFY (not in PD PROLOG)
"?-" 255, FY
"arg" 0, 0,
"asserta" 0, 0,
"assertz" 0, 0,
"atom" 0, 0,
"atomic" 0, 0,
"batch" 0, 0
"clause" 0, 0,
"clearops" 0, 0,
"cls" 0, 0, NS
"concat" 0, 0,
"consult" 8, FX,
"crtgmode" 0, 0, NS
17
"crtset" 0, 0, NS
"curset" 0, 0, NS
"curwh" 0, 0, NS
"debugging 0, 0,
"dir" 0, 0,
"display" 0, 0,
"dotcolor" 0, 0, NS
"drawchar" 0, 0, NS
"drawdot" 0, 0, NS
"drawline" 0, 0, NS
"exec" 0, 0,
"exitsys" 0, 0, NS
"forget" 0, 0, NS
"functor" 0, 0,
"get0" 8, FX,
"integer" 0, 0,
"is" 40, XFX,
"listing" 0, 0,
"memleft" 0, 0, NS
"mod" 11, XFX,
"name" 0, 0,
"nl" 0, 0,
"nodebug" 0, 0, (not in PD PROLOG)
"nonvar" 0, 0,
"nospy" 50, FX, (not in PD PROLOG)
"not" 60 FX
"notrace" 0, 0, (not in PD PROLOG)
"op" 0, 0,
"popoff" 0, 0, NS
"popoffd" 0, 0, NS
"popon" 0, 0, NS
"popond" 0, 0, NS
"print" 0, 0,
"prtscr" 0, 0, NS
"put" 0, 0,
"ratom" 0, 0,
"read" 0, 0,
"recon" 0, 0, (Note: this is "reconsult")
"repeat" 0, 0,
"retract" 0, 0
"rnum" 0, 0,
"see" 0, 0,
"seeing" 0, 0,
"seen" 0, 0,
"skip" 0, 0, (not in PD PROLOG)
"spy" 50, FX,
"tab" 0, 0,
"tell" 0, 0,
"telling" 0, 0,
"told" 0, 0,
"trace" 0, 0, (not in PD PROLOG)
"true" 0, 0,
"unop" 0, 0,
"var" 0, 0,
"write" 0, 0,
18
Description of the Modifications.
I/O Redirection
I/O redirection is a feature described by Clocksin and Mellish.
The predicates "see", "seeing", "seen", "tell", "telling", and
"told" are used to select the streams used for input and output.
The predicates "seen" and "told" require as arguments the
name of the stream that is to be closed. This enables the system
to remember the indices of several streams and switch back and
forth between them.
The predicate "batch", when inserted at the beginning of a
stream file, has the following properties:
1) The normal prompt, "?-", and advisory messages do not
appear at the screen.
2) It is self cancelling if the input stream is reassigned
to the console.
3) It may also be cancelled by the predicate "batch".
call( <goal> )
The predicate as defined in C & M is obsolete. The purpose
was to permit a goal search where the goal name was a variable
instantiated to some functor name. A.D.A. permits writing of
goals with such names, so the mediation of the "call" clause is
no longer necessary.
The "call" predicate may be trivially implemented for
compatibility with the PROLOG definition
call( X ) :- X.
19
debugging
"Debugging" prints a list of the current spypoints. After
each name a sequence of numbers may appear, indicating the number
of arguments that is a condition of the trace. The word "all"
appears if the number of arguments is not a condition of the
trace.
op( <prec>, <assoc>, <functor> )
Defines the user definable grammar of a functor. The
definition conforms to that in C & M. We mention here a minor but
important point. If <functor> is not a valid PROLOG atom it must
be enclosed in single quotes when declared in the "op"
declaration. It is not necessary or legal to do this when the
functor is actually being used as an operator. In version 1.6, a
declared or built-in operator can be used either as an operator
or as a functor. For example,
+(2,3) = 2 + 3.
is a true statement.
Declared operators are annotated in the directory display
with their precedence and associativity.
20
Output predicates
display
write
print
put
These functions have been modified to accept multiple
arguments in the form:
print( <arg1>, <arg2>, <arg3>,... )
Thus, "put( a, b, c )" would result in the display of "abc".
The names of some PROLOG atoms that may occur are not
accepted by the PROLOG scanner unless surrounded by single
quotes. This only applies when such an atom is read in, not when
it is internally generated. Nevertheless, this presents us with a
problem: We would like to be capable of writing valid PROLOG
terms to a file. In some cases, it is necessary to add the single
quotes. In other cases, such as human oriented output, they are
an irritant. The modified definitions of the following predicates
are an attempt at a solution:
display
Operator precedence is ignored, all functors are printed
prefix and single quotes are printed if needed or they were
supplied if and when the atom was originally input.
write
Operator precedence is taken into account and operators are
printed according to precedence. Single quotes are printed
under the same conditions as for "display."
print
Operator precedence is taken into account and operators are
printed according to precedence. Single quotes are never
displayed. This is a human oriented form of output and
should never be used for writing of terms for the PROLOG
scanner.
21
get0
read
The functions "get0" and "read" have been extended to
support input from a stream other than the one currently selected
by "see". To direct output to a file or other stream, an optional
argument is used. For example, "get0( char, <file name> )" or
"get0( char, user )" would cause input to come from <file name>
or the console. If the file has not already been opened, "get0"
will fail.
reconsult( <file> )
The name of this predicate has been changed to "recon". Otherwise
it is identical in function to the C & M version.
see( <stream> )
This predicate does not cause the previous stream to be
rewound or closed. If the argument of "see" is a previously
"seen" stream, reading resumes at the place in the file where it
was interrupted.
seen( <stream> )
According to the C & M standard, "seen" closes the stream
which the system is "seeing". In this system, many different
streams can be open at the same time. "seen" reduces the open
file overhead by closing a stream which is no longer needed.
tell( <stream> )
This predicate does not cause the previous stream to be
rewound or closed. If the argument of "tell" is a previously
"told" stream, writing resumes at the place in the file where it
was interrupted.
told( <stream> )
According to the C & M standard, "told" closes the stream
which the system is "telling". In this system, many different
streams can be open at the same time. "told" reduces the open
file overhead by closing a stream which is no longer needed.
Atoms enclosed by single quotest, eg. '\nthis is a new line'
can contain the escape sequences
'\n', '\r', '\t' and '\''.
If these atoms are printed by "display" or "write" they are
printed just as they are. If they are printed by the "print"
22
clause they are translated as follows:
'\n' results in the printing of a carriage return and a line
feed.
'\r' results in the printing of a carriage return only.
'\t' results in the printing of a tab character.
'\'' allows the printing of a single quote within a quoted atom.
The "portray" feature is not presently implemented.
23
Description of the New Predicates
clearops-
Nullify the operator status of every operator in the
database.
concat( (<variable> | <functor>), <List> )
A list of functors or operators is concatenated into one
string, which becomes the name of a new atom to which <variable>
or <functor> must match or be instantiated.
dir( option )
Provide an alphabetized listing to the console of atoms,
constants, or open files. Without options, simply type
"dir.<CR>". Options are:
dir( p ) - list clause names only.
dir( c ) - list consulted files only.
Consulted files are prefixed by "S:".
exitsys
Exit to the operating system.
forget( <file name> )
Make a database unavailable for use and reclaim the storage
it occupied.
ratom( <arg>, <stream> )-
Read an atom from the input stream, to which <arg> matches
or is instantiated. <stream> is optional. If <stream> is not
given, the input stream defaults to the standar input.
Input is terminated by a CR or LF, which are not included in
the stream.
24
Arithmetic Capabilities
Integer arithmetic is supported. Numbers are 32 bit signed
quantities. The following arithmetic operators are supported:
"+", "-", "*", "/", <, <=, >, >=, mod.
Arithmetic operators must never be used as goals, although they
may be part of structures. It is legal to write:
X = a + b
which results in the instantiation of X to the struture (a + b).
But the following is not legal:
alpha( X, Y ) :- X + Y, beta( Y ).
Evaluation of an arithemtic expression is mediated by the "is"
and inequality predicates. For instance, the following would be
correct:
alpha( X, Y, Z ) :- Z is X + Y.
beta( X, Y ) :- X + 2 < Y + 3.
25
Memory Metric Predicate
The purpose of this predicate is to give the prolog system
a sense of how much memory remains so that expensive search
strategies can be controlled. It is not possible to exactly
quantify how much memory remains. At the lowest level, there are
two types of memory - the stack and the heap. The stack expands
down from high memory, while the heap tends to expand at
unpredictable intervals upwards. If the stack and heap meet, the
prolog system must abort the search and return to the prompt.
Judicious use of the memory metric predicates reduces the
probability of this happening.
The stack is easy to quantify because it expands downwards
in a predictable way with recursion. The symbol space is a
"heap". For those interested, the structure of the heap is
determined by the C compiler under which Prolog was compiled.
There is a function internal to Prolog known as the allocator
searches the heap for enough contiguous memory to create a new
symbol. The heap resembles a piece of Swiss cheese; the holes
represent symbols and already allocated memory while the remained
is searched by the allocator for a piece of contiguous memory
large enough to satisfy a request. If one cannot be found, the
uppermost bound of the heap is expanded upwards, and that bound
is the number which we measure for an estimate of remaining
memory.
The sqace between the top of the heap, and the top of the
stack, which we call "gap", serves as a rough guide to how much
memory remains. The demands of the system are not entirely
predictable, however. For example, the creation of a new symbol
larger than "gap" would cause an abort. The user must use the
numbers supplied by these functions as a heuristic guide,
tempered by experience, to minimize the possibility of an
unexpected abort.
"Gap" is measured in 256 byte pages.
memleft( X )
If the argument is an integer, this is satisfied if the size of
"gap" is greater than "X".
If the argument is a variable, it is instantiated to the amount
of "gap" remaining.
26
IBM PC Video Display Predicates
A high level method is provided for drawing and displaying on the
screen of IBM PC and compatible computers.
cls
Clear the screen and position the cursor at the upper left hand
corner.
crtgmode( X )
Matches the argument to the mode byte of the display which is
defined as follows:
mode meaning
0 40 x 25 BW (default)
1 40 x 25 COLOR
2 80 x 25 BW
3 80 x 25 COLOR
4 320 x 200 COLOR
5 320 x 200 BW
6 640 x 200 BW
7 80 x 25 monochrome display card
crtset( X )
This sets the mode of the display. The argument must be one of
the modes given above.
curset( <row>, <column>, <page> )
Sets the cursor to the given row, column, and page. The arguments
must be integers.
curwh( <row>, <column> )
Reports the current position of the cursor. The argument must be
an integer or variable. The format is:
1) page zero is assumed.
2) The row is in the range 0 to 79, left to right.
3) The column is in the range 0 to 24, bottom to top.
27
dotcolor( <row>, <column>, <color> )
The argument <color> is matched to the color of the specified
dot. The monitor must be in graphics mode.
drawchar( <character>, <attribute> )
Put a character at the current cursor position with the specified
attribute. The arguments <character> and <attribute> must be
integers. Consult the IBM technical reference manual regarding
attributes.
drawdot( <row>, <column>, <color> )
Put a dot at the specified position. The monitor must be in the
graphics mode. The arguments must be integer. The argument
<color> is mapped to integers by default in the following manner:
drawline( <X1>, <Y1>, <X2>, <Y2>, <color> )
Draw a line on the between the coordinate pairs. The monitor must
be in the graphics mode and the arguments are integer.
prtscr
Print the screen as it currently appears. Be sure that the
printer is on line and ready before invoking this predicate,
since otherwise, the system may lock up or abort.
The integer argument <color> referred to in the above predicates
is represented as follows:
COLOR PALETTE 0 PALETTE 1
0 background background
1 green cyan
2 red magenta
3 brown white
To change the palette and the background, see the IBM Technical
Reference Bios listings for more information.
28
Trace Files
(type ED only)
You can now dump your trace to disk, instead of (groan)
wasting reams of printer paper. This option is described in the
next section.
The Interrupt Menu
(type ED only)
This menu has been modified. It was formerly called the
ESCAPE menu, but the meaning of the ESCAPE key has been
redefined. It is no longer necessary to display the menu to
perform one of the menu functions. This reduces the amount of
display which is lost by scrolling off the screen.
Version 1.9 offers the ability to attempt a goal from the
interrupt menu, while not disturbing the ongoing computation.
At any time while searching, printing, or accepting keyboard
input, you can break to this menu. It is generally not possible
to do this during disk access, since control passes to the
operating system at this time. Two keys cause this break to
occur:
^V: The menu is displayed and a command is accepted at the
prompt "INTERRUPT>". After a command, the menu is
redisplayed until the user selects a command which
causes an exit.
^I: The menu is not displayed. Command is accepted at the
prompt "INTERRUPT>" until the user selects a command
which causes an exit.
ESC: Typing this key causes a termination of the PROLOG
search and control returns to the user command level
with a prompt of "?-". Notice that previously, the ESC
key invoked this menu.
29
As the resulting menu indicates, the following functions are
possible:
A: Abort the search and return to the prompt.
O Open a trace file. The user is prompted for the file
name. The file receives all trace output. If a file is
already opened it is closed with all output preserved.
C Close the trace file if one is open. Otherwise there is
no effect.
^C: Immediately exit PROLOG without closing files. This is
not advised.
^P: Typing <Control>P toggles the printer. If the printer is
on, all input and output will also be routed to the
printer.
S: If the machine in use is an IBM PC compatible machine,
the currently displayed screen will be printed. If the
machine is not an IBM PC compatible, do not use this
function.
T: If trace is in use, most of the trace output can be
temporarily turned off by use of this function, which is
a toggle.
G: Satisfy a goal. The system prompt will appear. Do not be
confused into thinking that the system has reverted to
the prompt. After the user has entered the goal in the
usual manner, satisfaction of the goal is immediately
attempted. This need not affect the running computation
in any way, although it is certainly possible to do so.
After the goal is attempted, control returns to the
INTERRUPT menu. The purpose is to facilitate detailed
analysis of a running system.
R: Entering another ESC causes a return to the current
activity (keyboard input or search) with no residual
effect from the interruption.
30
Conserving memory space
Success popping is controlled by the predicates "popond",
"popoffd", "popon", and "popoff". Success popping is means of
reclaiming storage which is used on backtracking to reconstruct
how a particular goal was satisfied. If it is obvious that there
is no alternative solution to a goal this PROLOG system is smart
enough to reclaim that storage.
In this system, succees popping is an expensive operation.
Therefore, there is a tradeoff of memory versus time. On the
other hand, discrete use of success popping can actually speed up
a program by recreating structures in a more accessible form.
The definitions of the control predicates is given in this
manual and their use is totally optional. The modulation of
success popping has no effect on program logic (read solution.)
The "cut" can save substantial time and computational
overhead as well as storage. Although the execution of the cut
costs time, you can design your program to use cuts in critical
places to avoid unnecessary backtracking. Thus the execution
speed of the program can actually increase.
Anyone who has read Clocksin and Mellish is aware, of
course, that the "cut" has a powerful logical impact which is not
always desirable.
popoff
See the below definition.
popon
The inference engine does complete success popping for goals
which appear after "popon". Consider this example:
goal :- a, popon, b, c, popoff, d.
If no alternative solutions exist for b, then success popping
will reclaim storage by removing unnecessary records describing
how "b" was satisfied. If the Prolog system cannot rule out
possible additional solutions, success popping will never occur,
regardless of your use of "popon".
Since goal "d" occurs after "popoff", success popping will
never occur.
31
popoffd
If no "popon" or "popoff" declarations occur in a clause, the
default action is determined by "popoffd" and "popond". If
"popoffd" has been invoked, the default is that success popping
will not occur.
popond
The inverse of "popoffd". Turns on default success popping.
printf( <stream>, <term1>,<term2>,... )
32
Prolog Tutorial
Introduction
Probably you have heard of the language PROLOG within the
last year or so. You probably wondered the following things:
1) What does the name stand for? Names of computer languages are
almost always acronyms.
2) What is it good for?
3) Why now?
4) Can I get a copy to play with?
Congratulations! You obviously know the answer to the fourth
question. We now respond to the other three.
1) The name stands for "programming in logic." This we shall
elaborate on in depth later on.
2) PROLOG is good for writing question answering systems. It is
also good for writing programs that perform complicated
strategies that compute the best or worst way to accomplish a
task, or avoid an undesirable result.
3) PROLOG was virtually unknown in this country until researchers
in Japan announced that it was to be the core language of that
country's fifth generation computer project. This is the project
with which Japan hopes to achieve a domainant position in the
world information industry of the 1990's.
PROLOG is one of the most unusual computer languages ever
invented. It cannot be compared to FORTRAN, PASCAL, "C", or
BASIC. The facilities complement, rather than replace those of
conventional languages. Although it has great potential for
database work, it has nothing in common with the database
languages used on microcomputers.
Perhaps the best point to make is that while conventional
languages are prescriptive, PROLOG is descriptive. A statement in
a conventional language might read:
if( car_wheels = TRUE ) then
begin
(some sort of procedure)
X = X + 1;
end
33
A statment in PROLOG could just be a statment of fact about cars
and wheels. There are many relationships that hold. For instance,
has( car, wheels ).
has( car, quant(wheels, four) ).
round( wheels ).
Each of these statments is an independent fact relating cars,
wheels, and the characteristics of wheels. Because they are
independent, they can be put into a PROLOG program by programmers
working separately. The man who is a specialist on car bodies can
say his thing, the wheel specialist can have his say, and the
participants can work with relative independence. And this brings
to light a major advantage of PROLOG:
PARALLEL PROGRAMMING!!!
With conventional programming languages projects can still be
"chunked", or divided between programmers. But efficiency on a
team project drops drastically below that of the individual
programmer wrapped up in his own trance. As the number of
participants grows the need for communication grows
geometrically. The time spent communicating can exceed that spent
programming!
Although PROLOG does not eliminate the need for
task coordination, the problem is considerably simplified. It
also provides the ability to answer questions in a "ready to eat
form." Consider your favorite BASIC interpreter. Based upon the
statements about cars and wheels previously given, could you ask
it the following question?
has( car, X ), round( X ).
Does a car have anything which is round? The question
instructs the PROLOG interpreter to consider all the objects that
it knows are possessed by a car and find those which are round.
Perhaps you are beginning to guess that PROLOG has the abilities
of a smart database searcher. It can not only find the facts but
selectively find them and interpret them.
34
Consider the problem of a fault tree, as exemplified by this
abbreviated one:
{Car won't start}
|
|
[Engine turns over](No) --> [Battery voltage](no)-\
(Yes) v
| {Check battery}
|
[Smell gasoline](yes) --> {Try full throttle cranking}
| (failure)
/--------/ |
(details omitted)
The fault tree is easily programmed in BASIC. Later we shall
show that PROLOG supplies a superior replacement for the fault
tree. Though the tree is capable of diagnosing only the problem
for which it was designed, PROLOG dynamically constructs the
appropriate tree from facts and rules you have provided. PROLOG
is not limited to answering one specific question. Given enough
information, it will attempt to find all deductive solutions to
any problem.
35
PROLOG PRIMER
I. Rules and Facts
This is where you should start if you know nothing about
PROLOG. Let us consider a simple statment in PROLOG, such as:
1) has( car, wheels ).
This statement is a "fact. The word "has" in this statment is
known either as a functor or predicate. It is a name for the
relationship within the parenthesis. It implies that a car has
wheels. But the order of the words inside the bracket is
arbitrary and established by you. You could just as easily say:
2) has( wheels, car ).
and if you wrote this way consistently, all would be well. The
words has, wheels, and car are all PROLOG atoms. "Wheels" and
"car" are constants.
A database of facts can illustrate the data retrieval
capabilities of PROLOG. For instance:
3) has( car, wheels ).
has( car, frame ).
has( car, windshield ).
has( car, engine ).
You could then ask PROLOG the question:
4) has( car, Part ).
The capital "P" of Part means that Part is a variable. PROLOG
will make Part equal to whatever constant is required to make the
question match one of the facts in the database. Thus PROLOG will
respond:
Part = wheels.
More?(Y/N):
If you type "y" the next answer will appear:
Part = frame.
More?(Y/N):
If you continue, PROLOG will produce the answers Part = windshield
and Part = engine. Finally, you will see:
More?(Y/N):y
36
No.
indicating that PROLOG has exhausted the database. Incidentally,
when a variable is set equal to a constant or other variable,
it is said to be instantiated to that object.
Notice that PROLOG searches the database forwards and in
this case, from the beginning. The forward search path is built
into PROLOG and cannot be changed. An author of a program written
in a prescriptive language is quite conscious of the order of
execution of his program, while in PROLOG it is not directly
under his control.
The other major element is the rule which is a fact which is
conditionally true. In logic this is called a Horn clause:
5) has( X, wheels ) :- iscar( X ).
The fact iscar( car ) and the above rule are equivalent to
6) has( car, wheels).
The symbol :- is the "rule sign." The expression on the left of
:-is the "head" and on the right is the body. The variable X has
scope of the rule, which means that it has meaning only within
the rule. For instance, we could have two rules in the database
using identically named variables.
7) has( X, transportation ) :-
has( X, car ), has( license, X ).
8) has( X, elephant ) :- istrainer( X ), hasjob( X ).
The variables X in the two expressions are completely distinct
and have nothing to do with each other.
The comma between has( X, car ) and has( license, X ) means "and"
or logical conjuction. The rule will not be true unless both the
clauses has(X, car) and has( license, X ) are true.
On the other hand if there is a rule
9) has( license, X ) :- passedexam( X ).
consider what PROLOG will do in response to the question:
10) has( harry, transportation ).
(Notice that harry has not been capitalized because we do not
want it taken as a variable. We could, however, say 'Harry'
enclosed in single quotes.)
37
It will scan the database and use (7), in which X will be
instantiated to harry. The rule generates two new questions:
11) has( harry, car ).
12) has( license, harry ).
Assuming that harry has a car, the first clause of (7) is
satisfied and the database is scanned for a match to (12). PROLOG
picks up rule (9) in which X is instantiated to harry and the
question is now posed:
13) passedexam( harry ).
If there is a fact:
passedexam( harry ).
in the database then all is well and harry has transportation.
If there is not, then PROLOG will succinctly tell you:
No.
But suppose Harry has money and can hire a chauffer as any good
programmer can. That could be made part of the program in the
following way.
The rule which PROLOG tried to use was:
14) has( X, transportation ) :-
has( X, car ), has( license, X ).
At any point following it there could be included another rule:
15) has( X, transportation ) :- has( X, money ).
or simply the bald fact:
16) has( harry, transportation ).
These additional rules or facts would be used in two
circumstances. If at any point a rule does not yield a solution,
PROLOG will scan forward from that rule to find another
applicable one. This process is known as "backtracking search"
and can be quite time consuming.
If in response to the "More?" prompt you answer "y" PROLOG will
search for an additional distinct solution. It will attempt to
find an alternate rule or fact for the last rule or fact used. If
that fails, it will back up to the antecedent rule and try to
find an alternate antecedent. And it will continue to back up
until it arrives at the question you asked, at which point it
will say:
38
No.
"Antecedent" to a rule means that it gave rise to its' use. For
example, (7) is the antecedent of (9) in the context of the
question (16).
II. Grammar
It is a boring subject, but it must be discussed. All PROLOG
statements are composed of valid terms, possibly a rule sign (":-
"), commas representing conjunction ("and"), and a period at the
very end.
A term is a structure, constant, variable, or number.
What is a structure? It is a kind of grouping:
1) Structures consist of a functor, and a set of objects or
structures in parenthesis.
2) Objects are constants, variables, numbers, or lists,
which we have not discussed yet.
3) A constant or functor must be a string of letters and
numbers, beginning with a lower case letter, unless
you choose to enclose it in single quotes ( 'howdy
pardner' ), in which case you are freed from these
restrictions.
4) A variable must be a string of letters and numbers
beginning with a capital letter.
5) A functor may optionally have arguments enclosed in
parenthesis , as in: hascar( X ) or hascar.
An example: "has( X, transportation )." is a structure.
39
III. Input / Output
You now know enough to write simple databases and
interrogate them profitably. But before we examine more
sophisticated examples, it will be necessary to add input and
output to the language. There are built in functions which appear
as rules which are satisfied once. Thus the statment:
write( 'Hello world.' ).
can be included on the right side of a rule:
greetings( X ) :- ishuman( X ), write( 'Hello world.' ). You
can also write "write( X )" where X is some variable. Note that
'Hello world.' is not enclosed in double quotes. Single quotes,
which denote a constant, are required. Double quotes would denote
a list, which is another thing entirely.
Provided that a match to "ishuman" can be found, the builtin
function "write" is executed and the message printed to the
screen.
The builtin read( X ) reads a "structure" that you input
from the keyboard. More formally, we have
read( <variable> or <constant> )
write( <variable> or <constant> )
If you write read( Input ), then the variable "keyboard" will be
assigned to whatever is typed at the keyboard, provided that the
input is a valid PROLOG structure. The builtin "read" will fail
if instead of Keyboard we wrote read( baloney ), where "baloney"
is a constant, and the user at the keyboard did not type exactly
"baloney."
When you input a structure in response to a "read" statement, be
sure to end it with a period and an <ENTER>.
There is a convenient way of putting the cursor on a new
line. This is the builtin "nl". For example:
showme :- write( 'line 1' ), nl, write( 'line 2' ).
would result in:
line 1
line 2
There is also a primitive form of input/output for single
characters. It will be discussed later.
40
IV. A Fault Tree Example
Consider the "won't start" fault tree for an automobile:
{Car won't start}
|
|
[Engine turns over](No) --> [Battery voltage](no)-\
(Yes) v
| {Check battery}
|
[Smell gasoline](yes) --> {Try full throttle cranking}
| (failure)
/--------/ |
| /------------------------/
| |
| |
| [Check for fuel line leaks](yes)-->{Replace fuel line}
| (no)
| |
| |
| [Check for defective carburator](yes)--\
| (no) v
| {Repair carburator}
\----\
|
|
[Is spark present](no)-->[Do points open and close](no)-\
| (yes) v
/----/ | {Adjust points}
| /------------------------/
| |
| [Pull distributor wire, observe spark](blue)--\
| (orange) v
| | {Check plug wires & cap}
| |
| [Measure voltage on coil primary](not 12V)--\
| (12V) v
| | {Check wiring, ballast resistor}
| |
| [Check condenser with ohmmeter](conducts)--\
| (no conduction) v
| | {Replace condenser}
| |
| [Open and close points](voltage not 0 - 12)--\
| (voltage swings 0 - 12) v
| | {Fix primary circuit}
| |
| {Consider hidden fault, swap components]
|
|
\-------{Call a tow truck!!}
41
A PROLOG program to implement this is simple. Each statment
represents a decision point fragment of the tree. The PROLOG
interpreter dynamically assembles the tree as it attempts a
solution.
'car wont start' :- write( 'Is the battery voltage low?' ),
affirm, nl,
write( 'Check battery' ).
'car wont start' :- write( 'Smell gasoline?' ),
affirm, nl,
'fuel system'.
'fuel system' :- write( 'Try full throttle cranking' ).
'fuel system' :- write( 'Are there fuel line leaks?' ),
affirm, nl,
write( 'Replace fuel line.' ).
'fuel system' :- write( 'Check carburator' ).
'car wont start' :- write( 'Is spark present?' ),
not( affirm ), nl,
'no spark'.
'no spark' :- write( 'Do points open and close?' ),
not( affirm ), nl,
write( 'Adjust or replace points.' ).
'no spark' :- write( 'Is the spark off the coil good?' ),
affirm,
write( 'Check plug wires and cap.' ).
'no spark' :- write( 'What is the voltage on the primary
of the coil: ' ),
read( Volts ),
Volts < 10,
nl,
write('Check wiring and ballast resistor.').
'no spark' :- write( 'Does the capacitor leak?' ),
affirm,
write( 'Replace the capacitor.' ).
'no spark' :- not( 'primary circuit' ).
'primary circuit'
:- write( 'Open the points. Voltage across
coil?:'), nl,
read( Openvolts ), Openvolts < 1,
write( 'Close the points. Voltage across
coil?:'),
read( Closevolts ), Closevolts > 10, nl,
write( 'Primary circuit is OK.' ).
42
'no spark' :- write( 'Consider a hidden fault. Swap
cap, rotor,points,capacitor.' ).
'Car wont start' :- write( 'Get a tow truck!!' ).
--End program--
The above is a simple example of an expert system. A
sophisticated system would tell you exactly the method by which
it has reached a conclusion. It would communicate by a "shell"
program written in PROLOG which would accept a wider range of
input than the "valid structure" required by the PROLOG
interpreter directly.
43
V. Lists
Consider a shopping list given you by your wife. It is a
piece of paper with items written on it in an order that probably
symbolizes their importance. At the top it may say EGGS!!!,
followed by carrots, hamburger, and finally a flea collar for the
dog, if you can find one. In PROLOG such a list would be written:
1) [eggs, carrots, hamburger, fleacollar]
The order of a list is important so that eggs and carrots cannot
be reversed and PROLOG be uncaring.
Let us put the list in a structure:
shopping( [eggs, carrots, hamburger, fleacollar] ).
Then if you wished to isolate the head of the list you could ask
the question:
shopping( [ Mostimportant | Rest ] ).
and PROLOG would respond:
Mostimportant = eggs,
Rest = [carrots, hamburger, fleacollar].
The vertical bar "|" is crucial here. It is the string extraction
operator, which performs a combination of the CDR and CAR
functions of LISP. When it appears in the context [X|Y] it can
separate the head of the list from the rest, or tail.
You may have gained the impression that PROLOG is a rather
static language capable of answering simple questions, but it is
far more powerful than that. The string extraction operator is
the key. It permits PROLOG to whittle a complex expression down
to the bare remainder. If the rules you have given it permit it
to whittle the remainder down to nothing, then success is
achieved. An example of this is the definition of "append."
Let us suppose you have not yet done yesterday's shopping,
let alone today's. You pull it out of your wallet and sootch tape
it to the list your wife just gave you. Yesterday's list was:
[tomatoes, onions, ketchup]
Combined with [eggs, carrots, hamburger, fleacollar] we obtain
[eggs,carrots,hamburger,fleacollar,tomatoes,onions,garlic].
To take one list and to attach it to the tail of another list is
to "append" the first to the second. The PROLOG definition of
append is:
44
Rule1: append( [], L, L ).
Rule2: append( [X|List1], List2, [X|List3] ) :-
append( List1, List2, List3 ].
The general scheme is this:
The definition consists of one rule and one fact. The rule will
be used over and over again until what little is left matches the
fact. The [] stands for empty list, which is like a bag without
anything in it. This is an example of a recursive definition.
Suppose we ask:
append( [a,b,c], [d,e,f], Whatgives ).
1. Rule 2 is invoked with arguments ( [a,b,c], [d,e,f], Whatgives ).
2. Rule 2 is invoked again with arguments:
( [b,c], [d,e,f], List3 ).
3. Rule 2 is invoked again with arguments:
( [b], [d,e,f], List3 ).
4. The arguments are now ([], [d,e,f], List3 ). Rule 1 now
matches. End.
How does this cause a list to be constructed? The key is to watch
the third argument. Supplied by the user, it is named
"Whatgives". The inference engine matches it to [X|List3] in rule
2. Now lets trace this as rule two is successivly invoked:
Whatgives
|
|
|
v
Rule2: [X|List3] (List1 = [b,c])
| \
| \
| \
v \
Rule2: a [X'|List3'] (List1' = [c])
| \
| \
| \
v \
Rule2: b [X''|List3''] (List1'' = [], ie., empty set.)
| \
| \
| \
Rule1: c L ( in Rule1 = [d,e,f] )
End.
45
L in rule 1 is [d,e,f] for the following reason: Notice that rule
2 never alters List2. It supplies it to whatever rule it invokes.
So L in rule 1 is the original List2, or [a,b,c].
This example would not have worked if the order of rules one
and two were reversed. The PROLOG inference engine always
attempts to use the the first rule encountered. You could imagine
it as always reading your program from the top down in attempting
to find an appropriate rule. Since rule 2 would always satisfy,
an unpleasant thing would have happened if the order were
reversed. The program would loop forever.
I hope that this tiny introduction to PROLOG whets your
appetite. You should now purchase the book
Programming In Prolog
W.F. Clocksin and C.S. Mellish
Springer - Verlag
Berlin,Heidelberg,New York. 1981,1984
46
