Matching an enum that contains a String

[bigyihsuan]

I want a rule that should match specifically the value Token::Element("H".to_string()), and a rule that matches every other string. The Token enum has a variant Token::Element(String). What I have currently is the following:

pub rule element<S: Symbol, B: Backend<S>>(interner: &mut StringInterner<B>) -> Element<S>
    = [Token::Element(e)]
    { let e = interner.get_or_intern(e); Element(e) } // ok

pub rule hydrogen<S: Symbol, B: Backend<S>>(interner: &mut StringInterner<B>) -> Element<S>
    = [Token::Element("H".to_string())] // error here
    { let h = interner.get_or_intern("H"); Element(h) }

However, I get the following errors from the compiler:

cannot find tuple struct or tuple variant `to_string` in this scope
not found in this scope rustc[E0531]

this pattern has 2 fields, but the corresponding tuple variant has 1 field
expected 1 field, found 2 rustc[E0023]

tok.rs(52, 13): tuple variant has 1 field // this is correct, Token::Element only has 1 field

How can I write a rule that specifically matches Token::Element("H".to_string()) and nothing else?

[kevinmehall]

The [] syntax expands into a the pattern of a match statement like

match token {
    Token::Element("H".to_string()) => { let h = interner.get_or_intern("H"); Element(h) }
     ...
}

which produces the same error because the .to_string() is an expression, but not valid as a pattern on the left hand side of a match.

There's a Rust proposal to make match patterns dereference a String to into a str that you can match with a string literal pattern like "H". Looks like it may even be implemented for String already if you turn on #![feature(string_deref_patterns)] on Nightly.

What you can do on stable Rust right now though is use an if guard, which rust-peg passes through onto the match arm. The part after the if is an expression returning a boolean that can access variables bound in the pattern.

[Token::Element(e) if e == "H"]

[bigyihsuan]

It didn't occur to me that [] is just a match pattern, this is extremely helpful info! Thanks!