Example of higher order parser generating functions.

[nejucomo]

I'm having trouble implementing higher order functions that create parsers. My latest attempt looks like this:

pub fn keyword(kw: &'static str) -> Box<Fn(&str) -> ParseResult<(), &str>>
{
    use combine::{ParserExt, string, value};

    assert!(KEYWORDS.contains(&kw));

    Box::new(|input| {
        string(kw)
            .with(value(()))
            .parse_state(input)
    })
}

Later I want to compose keyword in different ways. Here's an attempted use in a unit test:

parser(keyword("let")).skip(eof()).parse(test_case)

Variations on the types I've tried include:

• dropping Box<…> from the return value of keyword - Sized is not satisfied.
• dereferencing the result of keyword("let") - Sized is not satisfied.

How can I make this work?

Note: if there's a better way to implement keyword itself, that's of secondary interest. My primary interest is to learn how to create higher order parser generators.

[nejucomo]

Ah, I used to be generic over the stream type, but elsewhere I followed an example for the then docs that had this snippet:

// Force input to be a &str
let _: &str = input;

This is with 2.0.0-beta.

[Marwes]

It appears that Parser is only implemented for FnMut not Fn which I believe is why that does not just work. That being said it is not really needed (at least in this case), an easier way of writing that would be.

// Fn -> Parser
pub fn keyword(kw: &'static str) -> Box<Parser<Input = &str, Output = ()>
{
    use combine::{ParserExt, string, value};

    assert!(KEYWORDS.contains(&kw));

// Remove the closure
    Box::new(string(kw)
            .with(value(())))
}

Just be aware that boxing incurs a runtime cost (allocation + indirection) so if you want the best performance you should avoid it as much as possible (once rust-lang/rust#35091 becomes stable creating higher order parsers should be much more convenient as well!).

[nejucomo]

Thanks, this indeed fixes all type errors. This is my first use of a trait object.

[Marwes]

Happy to be of help!