Collecting elements between branches

[oovm]

I have a requirement like this in my grammar: if it matches branch a, collect the variables of branch a, if it matches branch b, collect the variables of branch b, and finally merge them together

Roughly similar to the following hypothetical grammar:

struct Expr {
    a: Option<char>,
    b: Option<char>,
}
peg::parser!( grammar example() for str {
    pub rule select() -> Expr
        = a:a() / b:b() {
        Expr {
            a: Some(a),
            b: Some(b),
        }
    }
    rule a() -> char = "a" { 'a' }
    rule b() -> char = "b" { 'b' }
});

I would like to know how to desugar my requirement in rust-peg.

In actual use, there are much more complex nested grammar, such as (a / b) c

[kevinmehall]

I'd probably write that as

struct Expr {
    a: Option<char>,
    b: Option<char>,
}
peg::parser!( grammar example() for str {
    pub rule select() -> Expr
        = a:a() { Expr { a: Some(a), b: None } }
        / b:b() { Expr { a: None, b: Some(b) } }
   
    rule a() -> char = "a" { 'a' }
    rule b() -> char = "b" { 'b' }
});

[oovm]

Sorry, I didn't notice that each branch can have a handler.

I'm trying to see if each of my rules can be expanded and rewritten using this pattern.

[oovm]

OK, I found that all cases can be solved by two means

• Use the distributive law to expand / patterns
• Extract a part as a partial match
  1. If it appears in all branches, it must be T
  2. Otherwise it must be Option, initialized to None in the missing branch
  3. Use the .. destructuring syntax to merge to the upper layer