Course of Raku / Regexes and grammars / Grammars / Grammars, classes, and inheritance / Exercises / Inherit a grammar

Solution: Inherit a grammar

Here is a possible solution to the task.

Code

grammar Animal {
    token TOP   { <sound> }
    token sound { \w+ }
}

grammar Dog is Animal {
    token sound { 'woof' }
}

grammar Cat is Animal {
    token sound { 'meow' }
}

say Dog.parse('woof').defined;
say Cat.parse('meow').defined;
say Dog.parse('meow').defined;

🦋 You can find the source code in the file inherit-a-grammar.raku.

Output

True
True
False

Comments

1. Both Dog is Animal and Cat is Animal inherit the TOP token from the base unchanged; each supplies only its own sound.

2. So one base grammar is extended two different ways. Dog matches only woof and Cat only meow — which is why Dog.parse('meow') fails: a dog keeps its own overridden sound, exactly as overridden methods behave on objects.

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