GEL (Goal Expression Language)

In this repo are the parser and compiler implementations:

• src/ GEL implemented entirely in core golang, with no external dependencies. Other repositories within this organization depend on the parsing and compiling into a more fluent intermediate representation suitable for traversing and value unifying on.

• cmd/ contains tools for code generation and an interactive REPL. These use the golang library unless prefixed by ts-. Most commands for typescript build/test/eval are done using npm build or npm install from within ts/.

GEL and its relation to other General Game Playing environments

This project builds on work done in academia in the field of General Game Playing, especially the foundational work by Stanford and GGP.org in their design of the GDL language and holding the International General Game Playing Competition. It builds on that work by extending the language to simplify some relations (such as the integers and common board/equipment templates), and by extending GDL in ways similar to CGDL, RBG, TGDL, EGDL and the GDL-II and GDL-III extensions.

There are other systems that have tried to include as wide a range of game types (most notably OpenSpiel and Ludii) but they are only partially implemented in a logical representation and have a large component in a procedural language such as C++ or Java, so an AI reading the rules has an incomplete picture of what makes up the game definition.

This project, and the related projects for hosting a game-playing service, aim to build fully logical definitions. There is part of the GEL language which depends on an externality, and that is the user interface for gathering user inputs and distributing move and state updates to the players. These borrow from concepts expressed in GDL games and in the ontology of Ludii's ludemes, but the level of abstraction is chosen to enable the code-generation of a web service endpoint able to (efficiently) evaluate the player moves for legality and update the game state consistently.

Being able to code-generate the web service endpoint opens the door to also code-generating a user guide, an AI player, even a user interface (with the help of some utility components provided in a platform-specific client library).

Being able to code-generate the UI means we can code-generate an accessible UI for vision impaired or mobility impaired players, if not fully automatically then at least fully integrated with the game playing service. Having the API and the game logic generated from the same logical description eliminates an entire class of bugs frequently introduced when making manual changes to a service API and game playing protocol and its data schema alignment. It also provides a sound basis for separating concerns -- GM logic and client logic and game data schema and player data schema are clearly defined responsibilities of individual components of the system.

GEL and related repositories

• gel: (this repo) A parser and compiler for .ggd source files (GEL's file type), a superset of GDL and compatible with several of the language's variants. It can compile GEL down to GDL-II (or, if necessary, rtGDL) and will compile to GDL-I if able. Contains a parser/compiler for both golang and TypeScript, developed in tandem. This repo has github actions for compiling the parser and benchmarking its parse times for priority operations (e.g. ground relations used in player moves).

• gel-api: the common protocols and data types that all players (human and AI), as well as game master and lobby interfaces, use to interact. These are defined as a separate repo because some environments do not need an entire GEL compiler to play (e.g., the game clients that have been code generated from rules). This is not to say that clients should not depend on gel, but that there are benefits to building clients that only need to depend on the API. This repo has no github actions, but has hooks into build and test that check for internal consistency.

• ggp-gm: a golang service adhering to the Game Manager for hosting games. It coordinates the simultaneous play of moves, shares game updates with players and spectators, and validates legal play adhering to the game's rules. Though it depends on gel so that it can arbitrate any bespoke games, some of the game rules may be a compiled into go plugins for native compilation into Go, compiled into an intermediate notation that the service can snapshot and read back in when play resumes. This repo has github actions for staging the GM to be released.

• ggp-db: maintains the list of games, game metadata and game history for recent matches. Is defined in a game-agnostic way and compatible with OpenAuth ID servers to provide ACLs across any arbitrary game definition or game state representation. Clients do not need to know how the metadata and replay data are stored, only how to retrieve and parse it as described in gel-api. This is the reference implementation for database-related operations in the API.

• ggp-games: a private repository containing the game rulesets and backup of the wiki content residing at GGP Dojo. This is the backing store for games while they are in progress and is periodically synced with the servers running the ggpdojo site. Game metadata will include a 'status' property indicating whether it should be shown in the index or only available to beta testers, etc. This has no github actions and is mainly a staging area for games which are not released yet.

• ggp-site: web site frontend for ggpdojo.com and OAuth callback receiver. It exposes the interface for viewing current games, creating new ones or joining a lobby invitation. Thus it only needs to depend on gel-api and ggp-db, it is the GM's responsibility to arbitrate game logic decisions. There is also a dependency on vue-client because ggpdojo.com is a convenient location to deploy it from, but that dependency is at deployment time, not during build. It could be interesting to implement an SPA from within a game description...
  This repo also has the FrontEnd server's github actions for staging a release.

Dependency structure

graph LR;
    gel-->ggp-db;
    gel-->ggp-gm;
    ggp-db--->ggp-site;
    gel-api-->ggp-gm;
    ggp-games--->ggp-site

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