Writing .lud Descriptions – Basics

The .lud Format

Game descriptions for Ludii are written in text files with a .lud extension. The language used to describe games for Ludii is defined by a class grammar approach; it is automatically derived from the ludeme classes available in Ludii’s .jar file.


A full, detailed Ludii Language Reference may be downloaded from Ludii’s downloads page.

The basic premise of the language is that ludemes are described as their name, followed by a whitespace-separated list of arguments, all wrapped up in a pair of parentheses:

(ludemeName arg1 arg2 arg3 ...)

Generally, the “outer” ludeme (the first one that is visible in a game description file) will be of the type (game ...). Arguments may be of any of the following types:

  1. Ludemes: many ludemes can be used as arguments of other ludemes, which ultimately results in games being described as trees of ludemes.
  2. Strings: typically used to provide meaningful names to games, pieces, regions, etc. Strings are always written in a pair of double quotes, for example: "Pawn". By convention, names usually start with an uppercase symbol.
  3. Booleans: the boolean constants true and false may be used for any boolean (function) parameters.
  4. Integers: integer constants can simply be written directly in any .lud descriptions, without requiring any special syntax: 1, -1, 100, etc.
  5. Floats: any number containing a dot will be interpreted as a float constant. For example: 0.5, -1.2, 5.5, etc. In ludemes that expect floats as argument, numbers without dots (such as just 1) cannot be used, and the same number should be written to include a decimal component instead (e.g., 1.0).

As a first example, the following code shows the full game description for Tic-Tac-Toe:

(game "Tic-Tac-Toe"
   (players 2)
         (board (square 3))
         (piece "Disc" P1)
         (piece "Cross" P2)
      (play (move Add (to (sites Empty))))
      (end (if (is Line 3) (result Mover Win)))

Viewing Ludii’s Built-in Game Files

The Ludii.jar file is not only a runnable program, but also an archive containing files. It can be extracted or opened like any regular .zip archive, which allows for the individual files inside it to be inspected. One of the top-level directories inside it is the /lud/ directory. Under this directory, all of the .lud files for all the built-in Ludii games can be found. They may all serve as examples for game designers.