Context-free grammar

From The Battle for Wesnoth Wiki

Context-free grammar can be used to generate random strings, from short words to entire paragraphs. If you are interested in its theory, read more on [1].


In WML, it uses its own simple syntax. It is usually a translatable string and should be in <<French quotation marks>>. The code is composed from nonterminals. They are written on separate lines. Each nonterminal is substituted by one of its possibilities separated by the | symbol, the name is separated from possibilities with =. The nonterminal that makes the result is named main. The possibilities can contain other nonterminals, marked by {curly brackets}.


name_generator= _ <<
    main=Hello world|Hi world|Ahoi world

The result is either Hello world or Hi world or Ahoi world.

name_generator= _ <<
    main=Hello {noun}|Hi {noun}

The result is either Hello or Hi, followed by one of the four possible nouns, world, planet, Universe or Earth.

Other rules

A nonterminal can reference itself, which can, under the right circumstances, lead to a loop that expands indefinitely, causing a stack overflow. Avoid this unless you really know what you are doing.

Each nonterminal has a reduced probability for possibilities that were chosen the last time.

A production may include vertical bars or curly braces by using the following predefined nonterminals:

  • {!} - expands to a |
  • {(} - expands to a {
  • {)} - expands to a }
This page was last edited on 29 July 2016, at 15:25.