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In v2, we used an inheritance mechanism that was really a glorified dumb include. After discussing with the number of people including Ari Steinberg (who has a lot of experience with large SQL grammars), I have formulated the following mechanism with Kay. The mechanism is based on the idea of delegation rather than inheritance, however, rephrase it as an import that pulls in all rules making them available to the grammar that imports them.
Parsers
Imagine a simple grammar with three rules:
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Notice that you cannot use combined grammars for this; lexers have to be handled with a separate delegation.
Lexers
Lexer composition works exactly the same way. The import in the abstract copies all of the rules into the new lexer:
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lexer grammar L;
import S,T;
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Precedence of rules for the same name is given to the grammar listed first in the import statement; e.g., ID, INT, WS, etc... All rules would be copied in because there's no way to optimize out which ones you want.
We will add the ability to specify the implicitly-defined Tokens rule so that you can specify only the set of tokens you want from the merged lexers.
Propogating grammar changes
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