Course of Raku / Regexes and grammars / Grammars / Action classes / Exercises / One grammar, two actions

Solution: One grammar, two actions

Here is a possible solution to the task.

Code

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

class Sum  { method TOP($/) { make $<a>.Int + $<b>.Int } }
class Diff { method TOP($/) { make $<a>.Int - $<b>.Int } }

say Pair.parse('10,20', actions => Sum.new).made;
say Pair.parse('10,20', actions => Diff.new).made;

🦋 You can find the source code in the file adder-action.raku.

Output

30
-10

Comments

The grammar only describes the shape number,number; it knows nothing about what to compute.
The two action classes attach different meanings to the same parse — one sums the numbers, the other subtracts them. Passing a different actions object to .parse is all it takes to get a different result, without changing the grammar at all.

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