Course of Raku / Regexes and grammars / Grammars / Grammars, classes, and inheritance / Exercises / A boolean with proto

Solution: A boolean with proto

Here is a possible solution to the task.

Code

grammar Bool {
    token TOP { <bool> }

    proto token bool {*}
    token bool:sym<true>  { 'true' }
    token bool:sym<false> { 'false' }
}

say Bool.parse('true').defined;
say Bool.parse('false').defined;
say Bool.parse('unknown').defined;

🦋 You can find the source code in the file proto-bool.raku.

Output

True
True
False

Comments

1. The proto token bool has two named variants, true and false.

2. Each parse selects the matching variant, so both 'true' and 'false' succeed. A proto token is the grammar’s tidy way of saying “one of these named alternatives”.

3. 'unknown' matches neither variant, so there is nothing for the proto token to dispatch to and the parse fails — False. The proto accepts exactly the alternatives you list, and nothing else.

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