Course of Raku / Regexes and grammars / Grammars / Action classes

Writing an action class

An action class is an ordinary class with one method per grammar token. Each method receives the match for that token as $/, and calls make to attach a value — exactly what the inline block did, but kept apart from the pattern:

grammar Sum {
    token TOP { <a> '+' <b> }
    token a   { \d+ }
    token b   { \d+ }
}

class SumActions {
    method TOP($/) { make $<a>.made + $<b>.made }
    method a($/)   { make $/.Int }
    method b($/)   { make $/.Int }
}

The grammar now contains only the pattern; all the meaning lives in SumActions. Notice how the methods cooperate: a and b each make an integer, and TOP reads those with $<a>.made and $<b>.made and makes their sum.

To use the action class, pass an instance to .parse with the actions named argument, then read the top result with .made:

say Sum.parse('2+3', actions => SumActions.new).made; # 5

As the grammar parses, each token’s method runs automatically and builds the value from the bottom up. The result on the top match is the value you want — here, the number 5.

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