Course of Raku / Regexes and grammars / Grammars / The parse tree, make and made

make and made

Instead of digging values out of the tree afterwards, you can attach a value to a match while parsing. Within a token’s body you can embed a code block — an inner { … } written among the pattern — which runs as soon as matching reaches it. Inside that block, the function make stores a value on the current match:

grammar OneNum {
    token TOP    { <number> { make $<number>.Int } }
    token number { \d+ }
}

When TOP matches, the inner { make $<number>.Int } block runs — the outer braces are just the token’s body — and stores the integer on the match. You read it back afterwards with made (or its alias .ast):

say OneNum.parse('42').made; # 42

Because we stored $<number>.Int, the value handed back by made is a real Int — the number itself, not the match object:

say OneNum.parse('42').made + 1; # 43

Note that this particular + 1 would give 43 even without the .Int, since a match numifies to its digits in arithmetic. The point of calling .Int is that the stored value genuinely is a number — which is what you want once it is kept in a data structure or passed on, rather than the raw match.

The stored value can be anything — a number, a string, an array, an object. A token can also combine the values its sub-tokens made. For example, summing two captured numbers:

grammar Sum {
    token TOP { <a> '+' <b> { make $<a>.Int + $<b>.Int } }
    token a   { \d+ }
    token b   { \d+ }
}

say Sum.parse('2+3').made; # 5

make and made are the bridge from “it matched” to “here is the meaning”. Putting the blocks inline works, but it mixes the pattern with the logic; the next section moves that logic into a separate action class.

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