Course of Raku / Addendum 🆕 / Types and text machines / Regexes and grammars / Exercises / A grammar that adds

Solution: A grammar that adds

Here is a possible solution to the task.

Code

grammar Sum {
    token TOP    { <number>+ % '+' }
    token number { \d+ }
}

class SumActions {
    method TOP($/) { make [+] $<number>.map(*.Int) }
}

say Sum.parse('3+4+5', actions => SumActions).made;

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

Output

12

Comments

  1. <number>+ % '+' matches one or more numbers separated by + signs — the % modifier describes the separator between repetitions.

  2. The action method runs when TOP matches. make attaches a computed value — the sum of the numbers — which .made reads back after parsing.

  3. A token never skips spaces, so this grammar is strict about its input: '3+4+5' parses, but '3 + 4 + 5' does not (.parse returns Nil). To accept spaces around the plus signs, make TOP a rule — and detach the quantifier from its atom:

    grammar Sum {
    rule TOP     { <number> + % '+' }
    token number { \d+ }
}

    In a rule, whitespace in the pattern stands for an implicit <.ws> call. Written as <number> + % '+', with a space before the + quantifier, that implied whitespace covers the whole repetition — separators included — so both '3+4+5' and '3 + 4 + 5' are parsed, and the sum is 12 either way. (If you prefer to stay with a token, spell the spaces out in the separator: <number>+ % [ \s* '+' \s* ].)

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