changing_actions_for_grammar_rules.md

Changing Actions For Grammar Rules

Recall that a grammar rule may have a few productions. To change an action when a substring is parsed as a production, we change the corresponding fn of the production. For example:

S: S Plus S {left}
  | A
  ;

terminals

Plus: '+';
A: /\d+/;

S has a production A and the corresponding action fn of the production is

pub fn s_a(_ctx: &Ctx, a: A) -> S {
    S::A(a)
}

We can add a println! to the fn such as

pub fn s_a(_ctx: &Ctx, a: A) -> S {
    println!("{a}");
    S::A(a)
}

By running the parser on input 1 + 2 + 3, we can see 1, 2 and 3 accordingly in the output.

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