Modular, parameterized grammars

[pczarn]

Let's allow composition of grammars. That means you will be able to invoke symbols of one grammar from another grammar. Rules can be reused across many grammars.

There is a problem with the alphabet of terminals. Grammars must agree on their terminal symbol sets in order to be composed. I believe terminal sets should be declared. For example, having common sets such as ascii and unicode could enable simpler composition. Right now, lexers have implicit terminal sets.

Composition of languages is becoming popular*. Nevertheless, parser generators have limited support of modularity.

Related: Allow grammar extension at runtime [#17]

* for example, React's JSX in Javascript.