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ADVSYS - An Adventure Writing System
by David Betz
January 11, 1986
Copyright (c) 1986, by David Betz
All Rights Reserved
Permission is hereby granted for unrestricted non-commercial use
INTRODUCTION
ADVSYS is a special purpose programming language that was
specifically designed to be used to write computer text
adventure games. It includes a facility for defining the kinds
of objects that are common in adventures. Some objects
represent locations on the game map, some objects represent
things that the player can find while exploring the adventure
world, and some objects represent other characters that the
adventurer can encounter during his or her journeys. The
adventure language also provides a facility to define actions.
Actions are short sections of code that determine what happens
in response to a command from the player. These two concepts,
"objects" and "actions" form the basis for the adventure
language.
ACKNOWLEDGEMENTS
Although I have written all of the code associated with this
adventure writing system, I must acknowledge the assistance of
one individual without whom this project would probably never
have reached completion. That person is Gary McGath. Gary was
interested in writing a commercial quality adventure game and I
convinced him to write it using my system (which was as yet
almost completely unspecified) instead of using a traditional
programming language. The input that Gary provided during the
development of his game contributed significantly to the overall
design of the system. I would like to thank Gary for that
contribution.
ADVSYS An Adventure Writing System Page 2
USING THE SYSTEM TO WRITE AN ADVENTURE
In order to write an adventure using this system, you need to
write an adventure description. This is an ordinary ASCII text
file containing definitions for all of the objects and actions
in your adventure game. This file is used as input to the
adventure compiler. The compiler takes the adventure
description and compiles it into a set of data structures.
In order to play an adventure written using this system, you
need the data structure file that was produced by the compiler
and the adventure interpreter program. The interpreter uses the
information produced by the adventure compiler to allow a player
to play the adventure game. Notice that it is not necessary for
the player to have access to the original adventure description
file. All of the information that is necessary to play the
adventure game is contained within the data structure file that
is produced by the compiler. This file is a binary file that
cannot be simply "listed" to reveal the internal workings of the
adventure.
The adventure compiler is called ADVCOM and the interpreter is
called ADVINT. These two programs in conjunction with this
documentation are all that is required to write and play
adventure games using this system.
ADVSYS An Adventure Writing System Page 3
RUNNING THE COMPILER
If you have created an adventure definition file called
"MYADV.ADV", you can compile it with the command:
A>advcom myadv
Typing this command will invoke the adventure compiler and cause
it to compile the file named "MYADV.ADV". The ".ADV" extension
is added to the file name by the compiler. During the process
of compiling the file, many messages will be printed telling
about the progress of the compiler. At the end of the
compilation process, the compiler prints a set of statistics
describing the resulting data structure file. This file will be
called "MYADV.DAT". It contains the data structures needed by
the adventure interpreter to allow a player to play the
adventure game.
Note: The "A>" in the line above is the MS-DOS prompt and should
not be typed as part of the command.
RUNNING THE INTERPRETER
Assuming that you have a compiled adventure data file called
"MYADV.DAT", you can play the adventure by typing the command:
A>advint myadv
This command will start the adventure. There will probably be
some text printed at this point describing the adventure and the
initial situation. You will then be prompted to type a command.
The prompt is the colon character. The format for commands is
described under the section about the parser. After typing a
command, you will be told what happened as a result of your
command, your new situation will be described and you will begin
the loop again.
ADVSYS An Adventure Writing System Page 4
ADVENTURE DESCRIPTION FILE FORMAT
All adventure description files contain a collection of
statements. These statements must be formed according to
the following rules:
The adventure definition statement:
All adventure definitions should have an ADVENTURE
statement. This statement gives the name of the adventure
and the version number of the definition file. Each
adventure should have a unique name. This name is used to
identify "saved position" files and insure that only files
that correspond to the current adventure are restored. The
version number allows the author to have many versions of
the same adventure during development and guarantee that
"save" files from one version aren't restored into another
version.
(ADVENTURE name version)
Example:
(ADVENTURE sample 1)
Vocabulary statements:
These statements add words to the adventure vocabulary.
(ADJECTIVE word*)
(PREPOSITION word*)
(CONJUNCTION word*)
(ARTICLE word*)
(SYNONYM word synonym*)
Examples:
(ADJECTIVE red blue)
(CONJUNCTION and)
(SYNONYM big large)
Note:
Words are also added to the vocabulary by the object
and action definitions using the NOUN, ADJECTIVE, VERB
and PREPOSITION statements.
ADVSYS An Adventure Writing System Page 5
Constant definition statement:
(DEFINE name value)
Examples:
(DEFINE what "I don't understand what you're saying!\n")
(DEFINE max-load 100)
Function definition statement:
(DEFINE (function-name [arg-name]* [&aux tmp-name*]) expr*)
Example:
(DEFINE (factorial n)
(IF (< n 2)
1
(* n (factorial (- n 1)))))
Variable definition statement:
(VARIABLE variable-name*)
Example:
(VARIABLE score i j)
Property name definition statement:
(PROPERTY property-name*)
Example:
(PROPERTY weight value)
ADVSYS An Adventure Writing System Page 6
Comments:
Comments begin with a semi-colon and end with the end of the
line.
Example:
; this is a comment
Include files:
Any line that begins with a "@" causes the inclusion of
another file. The file name immediately follows the at-sign
and extends to the end of the line. Only one level of
include is supported.
Example:
@basic.adv
ADVSYS An Adventure Writing System Page 7
Handler definition statements:
(INIT expr*)
(UPDATE expr*)
(BEFORE expr*)
(AFTER expr*)
(ERROR expr*)
Example:
(INIT
(print "Welcome to the sample adventure!\n"))
Handlers:
All activity within an adventure game is controlled by a
built-in handler loop. Each of the handlers in the loop
contains code that is provided by the adventure author. The
sequencing from handler to handler is provided by the
adventure system itself.
The first handler that is called in an adventure game is the
INIT handler. It prints some sort of introductory text and
initializes all global variables in order to start the
adventure game.
After the INIT handler has completed, the normal loop is
entered. It starts with the UPDATE handler. The UPDATE
handler prepares for the player's next turn. It should
describe the player's location if it has changed since the
last turn. After the UPDATE handler completes, the parser
is called. It prompts the player for a command, parses the
command, sets the built-in parser varaibles and exits. Then
the BEFORE handler is called. It is called before the
action associated with the command to allow the adventure
author to inspect the parser variables before proceeding to
the action itself. After the BEFORE handler completes, the
action itself is called (or whatever action is stored in the
built-in variable $ACTION when the BEFORE handler
completes). When the action completes, the AFTER handler is
called to give the author a chance to handle events that
happen only at the end of a successful turn. The ERROR
handler is called when the parser detects an error.
ADVSYS An Adventure Writing System Page 8
The handler loop:
INIT
|
v
UPDATE<----------+
| |
v |
parse--->ERROR---+
| |
v |
BEFORE |
| |
v |
action |
| |
v |
AFTER------------+
ADVSYS An Adventure Writing System Page 9
The parser:
The parser handles all commands from the player. It prompts
the player when it is ready for a new command. The prompt
is the colon character. When the player has typed a
command, the parser breaks the command into phrases. The
parser recognizes the following command forms:
[actor,] verb
[actor,] verb dobjects
[actor,] verb dobjects preposition iobject
[actor,] verb iobject dobjects
Where:
actor ==> a noun phrase
verb ==> the verb phrase (1 or 2 words)
dobjects ==> dobject [conjunction dobject]*
dobject ==> a noun phrase
preposition ==> a preposition
iobject ==> a noun phrase
noun phrase ==> [article] [adjective]* noun
Examples:
Look
Take the red magic sword
Take the red sword and the blue bottle
Give the troll the red sword
Give the red sword to the troll
Troll, give me the sword
Notes:
Square brackets enclose optional phrases. An asterisk
indicates zero or more of the preceeding element.
The fourth form above is treated as if the player had
typed:
[actor,] verb dobject "to" iobject
Once the parser has broken the command into phrases, it
assigns each noun phrase a number. It stores the number of
the actor noun phrase in the built-in variable $ACTOR. It
stores the first direct object noun phrase number in the
variable $DOBJECT. It stores the number of direct objects
in the variable $NDOBJECTS. It stores the indirect object
noun phrase number in the variable $IOBJECT. If any of the
noun phrases is missing from the command, the corresponding
variable is set to NIL. The parser saves the verb phrase
and preposition to use when determining which action to use
to handle the command.
ADVSYS An Adventure Writing System Page 10
Action definition statement:
Actions are used to handle player commands. Each time the
parser finishes parsing a new command, it uses the verb
phrase and the preposition to locate an action to handle the
command. Each action specifies a kind of template that must
match the command in order for the action to be called. The
template consists of the words used in the verb phrase and
preposition and the existance of the actor, direct object
and indirect object noun phrases. Once the parser finds an
action that matches the command, it stores the action in the
built-in variable $ACTION and exits.
(ACTION action-name astat*)
astat:
(ACTOR [flag])
(VERB verb*)
(DIRECT-OBJECT [flag])
(PREPOSITION word*)
(INDIRECT-OBJECT [flag])
flag:
REQUIRED must have the corresponding np
OPTIONAL may have the corresponding np
FORBIDDEN must not have the corresponding np
verb:
word
(word word)
Example:
(ACTION take
(VERB take (pick up))
(DIRECT-OBJECT)
(CODE
(print "You can't take the ")
(print-noun $dobject)
(print "!\n")))
If the ACTOR, DIRECT-OBJECT or INDIRECT-OBJECT statements
are left out entirely, the settings of the corresponding
flags are taken from the action default definitions. If
there is no action default definition, the value FORBIDDEN
is assumed. If any of these statements is present, but no
flag is specified, it is treated as if the flag REQUIRED was
specified.
ADVSYS An Adventure Writing System Page 11
Action default definition statement:
This statement defines default values for the ACTOR, DIRECT-
OBJECT and INDIRECT-OBJECT flags.
(DEFAULT dstat*)
dstat:
(ACTOR [flag])
(DIRECT-OBJECT [flag])
(INDIRECT-OBJECT [flag])
flag:
REQUIRED
OPTIONAL
FORBIDDEN
Example:
(DEFAULT
(ACTOR OPTIONAL))
ADVSYS An Adventure Writing System Page 12
Object definition statements:
The object definition statements are used to define
individual objects and classes of objects. The most basic
way of defining an object is using the (OBJECT ...)
statement. This defines an object which has no parent
class.
It is also possible to create a class of objects that share
information. A class is defined just like a normal object.
It is given nouns, adjectives and properties. In addition,
a class may have class properties. These are properties
that are shared amongst all instances of the class. In
order to create an instance of a class, the (class-name ...)
form is used. This creates an instance of the named class.
An instance will inherit all nouns and adjectives from its
parent class. It will also inherit all class properties
defined in the parent (and its parents). Any normal
properties defined in the parent class will be copied to the
new object. The copies will have the same values that the
parent has, but it is possible for the instance to have
property definitions that override these values. Instances
may also have additional nouns, adjectives and properties.
(OBJECT object-name ostat*)
(class-name object-name ostat*)
ostat:
(NOUN word*)
(ADJECTIVE word*)
(PROPERTY [property-name value]*)
(CLASS-PROPERTY [property-name value]*)
class-name:
the name of a previously defined object
Examples:
(OBJECT sword
(NOUN sword weapon)
(CLASS-PROPERTY
is-weapon T)
(PROPERTY
weight 10
value 5
damage 20))
(sword red-sword
(ADJECTIVE red)
(PROPERTY
damage 25))
ADVSYS An Adventure Writing System Page 13
Expressions:
(+ expr expr) add
(- expr expr) subtract
(* expr expr) multiply
(/ expr expr) divide
(% expr expr) remainder
(& expr expr) bit-wise and
(| expr expr) bit-wise or
(~ expr) bit-wise complement
These arithmetic functions operate on integers. As it turns
out, every data type in the system is represented by an
integer, so these functions will work with any type of
arguments. They are probably only useful with integers,
however.
(AND expr*) logical and (short circuits)
(OR expr*) logical or (short circuits)
(NOT expr) logical not
These functions operate on logical values. In this system,
any value that is not equal to NIL (or zero) is considered
true. NIL and zero are considered false. AND and OR
evaluate their arguments from left to right and stop as soon
as the value of the entire expression can be determined. In
other words, AND stops when it encounters a false value, OR
stops when it encounters a true value.
(< expr expr) less than
(= expr expr) equal to
(> expr expr) greater than
These functions compare integers. They cannot be used to
compare strings.
(GETP obj property-name) get the value of a property
(SETP obj property-name value) set the value of a property
These functions manipulate object properties. They are used
to find the value of a property or to set the value of a
property. They will also find and set the values of
inherited properties. If GETP is used to find the value of
a property that doesn't exist for the specified object, NIL
is returned. If SETP is used to set the value of a property
that doesn't exist, the operation is ignored.
ADVSYS An Adventure Writing System Page 14
(CLASS obj)
This function returns the class of an object. If the object
was defined with an (OBJECT ...) statement, NIL will be
returned. If the object was defined with the (class-name
...) statement, the class object will be returned.
(MATCH obj noun-phrase-number)
This function matches an object with a noun phrase. An
object matches a noun phrase if it includes all of the
adjectives specified in the noun phrase and also includes
the noun mentioned. Both nouns and adjectives can be
inherited.
(YES-OR-NO) get a yes or no answer from the player
This function waits for the player to type a line. If the
line begins with a 'Y' or a 'y', the function returns T. If
the line begins with anything else, the function returns
NIL.
(PRINT expr) print a string
(PRINT-NUMBER expr) print a number
(PRINT-NOUN noun-phrase-number) print a noun phrase
(TERPRI) terminate the print line
These functions perform various sorts of output. PRINT
prints strings, PRINT-NUMBER prints numbers and PRINT-NOUN
prints a noun phrase.
(FINISH) exit and continue with the AFTER handler
(CHAIN) exit and continue with the next handler
(ABORT) exit and continue with the UPDATE handler
(RESTART) exit and restart the current game
(EXIT) exit to the operating system
These functions cause the immediate termination of the
current handler. FINISH causes execution to proceed with
the AFTER handler, CHAIN causes execution to proceed with
the next handler in the normal sequence, ABORT causes
execution to proceed with the UPDATE handler (effectively
aborting the current turn), RESTART restores the game to its
original state and starts over with the INIT handler and
EXIT causes an immediate exit back to the operating system.
(SAVE) save the current game position
(RESTORE) restore a saved game position
These functions allow the player to save and restore
positions in the game. They prompt the player for a file
name and either read a saved game position from the file or
write the current game position to the file.
ADVSYS An Adventure Writing System Page 15
(function-name expr*)
This expression invokes a user defined function. There
should be one expression for each of the formal arguments of
the user function. The value of the expression is the value
of the last expression in the body of the user function or
the value passed to a RETURN statement within the function.
(SETQ variable value)
This expression sets the value of a user variable.
(COND [(test expr*)]*) execute conditionally
(IF test then-expr [else-expr]) traditional if-then-else
(WHILE test expr*) conditional iteration
(PROGN expr*) block construct
(RETURN [expr]) return from a function
These statements are control constructs.
ADVSYS An Adventure Writing System Page 16
Primary expressions:
integer (digits preceeded by an optional sign)
string (characters enclosed in double quotes)
action-name (an action name)
object-name (an object or class name)
property-name (a property name)
constant-name (a defined constant or function)
variable-name (a variable name)
Since an adventure description contains a large quantity of
running text, the format for specifying string constants is
somewhat extended from normal programming languages. In
this system, a string is enclosed in double quotes. If the
end of line occurs before the closing quote within a string,
it is treated as if it were a space. Any number of
consecutive spaces is collapsed into a single space. Also,
the character pair "\n" is used to represent the "end of
line" character, the pair "\t" is used to represent the tab
character and the pair "\\" is used to represent the
backslash character.
Examples:
"This is a string.\n"
"This
is
a
string.\n"
Both of the examples above represent the same string.
ADVSYS An Adventure Writing System Page 17
Definitions of symbols used above:
expr an expression
value an expression
test an expression (NIL means false, anything else is true)
then-expr an expression
else-expr an expression
obj an expression that evaluates to an object
property-name an expression that evaluates to a property name
noun-phrase-number an expression that evaluates to a noun phrase number
variable a variable name
T true
NIL false
Built-in variables set by the parser:
$ACTOR (actor noun phrase number)
$ACTION (action)
$DOBJECT (first direct object noun phrase number)
$NDOBJECTS (number of direct object noun phrases)
$IOBJECT (indirect object noun phrase number)
Other built-in variables:
$OCOUNT (total number of objects in the system)
CURRENT COMPILER LIMITS
500 words
500 objects
20 properties per object
200 actions or functions
16384 bytes of code
16384 bytes of data
262144 bytes of text
