1. Purpose of GDL

2. Specification
     Syntax    Keywords    Rules    Players    Initial state    Legal moves    Game state update    Termination    Goal states  

3. Extension

4. See also

5. References

6. External links

Game Description Language, or GDL, is a logic programming language^[1] designed by Michael Genesereth as part of the General Game Playing Project at Stanford University, California. GDL describes the state of a game as a series of facts, and the game mechanics as logical rules.

Purpose of GDL

Quoted in an article in New Scientist, Genesereth pointed out that although Deep Blue is able to play chess at a grandmaster level, it is incapable of playing checkers at all because it is a specialized game player.^[2] Both chess and checkers can be described in GDL. This enables general game players to be built that can play both of these games and any other game that can be described using GDL.

Specification

Syntax

GDL is a variant of Datalog, and the syntax is largely the same. It is usually given in prefix notation. Variables begin with "?".

Keywords

The following is the list of keywords in GDL, along with brief descriptions of their functions:

distinct

This predicate is used to require that two terms be syntactically different.

does

The predicate does(?r,?m) means that player (or role) ?r makes move ?m in the current game state.

goal

The predicate goal(?r,?n) is used to define goal value ?n (usually a natural number between 0 and 100) for role ?r in the current state.

init

This predicate refers to a true fact about the initial game state.

legal

The predicate legal(?r,?m) means that ?m is a legal move for role ?r in the current state.

next

This predicate refers to a true fact about the next game state.

role

This predicate is used to add the name of a player.

terminal

This predicate means that the current state is terminal.

true

This predicate refers to a true fact about the current game state.

Rules

A game description in GDL provides complete rules for each of the following elements of a game.

Players

Facts that define the roles in a game. The following example is from a GDL description of the two-player game Tic-tac-toe:

(role xplayer)(role oplayer)

Initial state

Rules that entail all facts about the initial game state. An example is:

(init (cell 1 1 blank))...(init (cell 3 3 blank))(init (control xplayer))

Legal moves

Rules that describe each move by the conditions on the current position under which it can be taken by a player. An example is:

(<= (legal ?player (mark ?m ?n))

    (true (cell ?m ?n blank))    (true (control ?player)))

Game state update

Rules that describe all facts about the next state relative to the current state and the moves taken by the players. An example is:

(<= (next (cell ?m ?n x))

(<= (next (cell ?m ?n o))

Termination

Rules that describe the conditions under which the current state is a terminal one. An example is:

(<= terminal    (line x))(<= terminal    (line o))(<= terminal    not boardopen)

Goal states

The goal values for each player in a terminal state. An example is:

(<= (goal xplayer 100)

(<= (goal oplayer 0)

Extension

With GDL one can describe finite games with an arbitrary numbers of players. However, GDL can not describe games which contain an element of chance (for example, rolling of dice) or games where players have incomplete information about the current state of the game (for example, in many card games the opponents' cards are not visible). GDL-II, the Game Description Language for Incomplete Information games, extends GDL by two keywords that allow for the description of elements of chance and incomplete information:

sees

The predicate sees(?r,?p) means that role ?r perceives ?p in the next game state.

random

This constant refers to a pre-defined player who chooses moves randomly.

The following is an example from a GDL-II description of the card game Texas hold 'em:

(<= (sees ?player ?card)

(<= (sees ?r ?card)

    (role ?r)    (does random (deal_river ?card)))

See also

• General Game Playing
• Artificial Intelligence

References

External links

• Game Description Language Specification
• Refereed paper introducing GDL-II

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• Chain-O-Lakes, MO
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