disambiguation_by_priority.md

Disambiguation By Priority

Another type of ambiguous grammar looks like this:

S: A S
  | A S B
  | EMPTY
  ;

terminals

A: /a/;
B: /b/;

By running rcomp with this grammar, we also get the error: Grammar is not deterministic. This is because the two productions A S and A S B are so similar that rcomp cannot distinguish them and thus cannot decide which one should be used.

To solve this problem, we can give every production a priority. In Rustemo, the default priority for a production is 10. So, any priorities larger than 10 tell rcomp to consider using that production first.

The priority number should be put in a pair of braces at the end of a production. In the example, we can change the priority of production A S B to 11 as presented below:

S: A S
  | A S B {11}
  | EMPTY
  ;

terminals

A: /a/;
B: /b/;

This makes rcomp to give preference to A S B (instead of A S). The string a a b is parsed as a (a b), which can be seen from the following output:

Accept
Ok(
  Some(
    C1(
      SC1 {
        a: "a",
        s: Some(
          C2(
            SC2 {
              a: "a",
              s: None,
              b: "b",
            },
          ),
        ),
      },
    ),
  ),
)

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