How do I collect an iterator of parsers to a single parser?

[mtvjr]

I have a large list of keywords that I was hoping to represent as an enum called Keyword. To reduce error-prone string duplication, each value can generate its own parser and I can iterate over the values to generate all the parsers I need. How would I collect the parser iterator into a single parser?

My current attempt:

// The following code is released under MIT license
impl Keyword {
    fn make_parser(&self) -> impl Parser<char, Keyword, Error = Simple<char>> {
        text::keyword(self.to_string()).to(self.clone())
    }

    pub fn keyword_parser() -> impl Parser<char, Keyword, Error = Simple<char>> {
        let iter = Keyword::iter().map(|k| k.make_parser());
        let first = iter.next();
        iter.fold(first.unwrap(), |l, r| l.or(r)) // This appears to fail since the type changes during the combination
    }
}

[zesterer]

Given that Keyword::iter() has a length known only at runtime, you'll probably need to box the parsers for them to have a consistent type, i.e: l.or(r).boxed().

[mtvjr]

When I tried the boxed, it was giving me lifetime errors.

fn combine_keyword_parser(l: impl Parser<char, Keyword, Error = Simple<char>>, r: impl Parser<char, Keyword, Error = Simple<char>> ) -> impl Parser<char, Keyword, Error = Simple<char>> {
    l.or(r).boxed()
}

error[E0310]: the parameter type `impl Parser<char, Keyword, Error = Simple<char>>` may not live long enough
   --> src/keyword.rs:118:5
    |
118 |     l.or(r).boxed()
    |     ^^^^^^^^^^^^^^^ ...so that the type `impl Parser<char, Keyword, Error = Simple<char>>` will meet its required lifetime bounds
    |
help: consider adding an explicit lifetime bound...
    |
117 | fn combine_keyword_parser(l: impl Parser<char, Keyword, Error = Simple<char>>, r: impl Parser<char, Keyword, Error = Simple<char>> + 'static ) -> impl Parser<char, Keyword, Error = Simple<char>> {
    |                                                                                                                                    +++++++++

[mtvjr]

Additionally, there may just be an issue with reducing that it can't handle combining different types that implement the same trait.

    pub fn parser() -> impl Parser<char, Keyword, Error = Simple<char>> {
        Keyword::iter().map(|s| s.make_parser())
            .reduce(combine_keyword_parser).unwrap()
    }

error[E0271]: type mismatch resolving `<fn(impl chumsky::Parser<char, Keyword, Error = chumsky::error::Simple<char>>, impl chumsky::Parser<char, Keyword, Error = chumsky::error::Simple<char>>) -> impl chumsky::Parser<char, Keyword, Error = chumsky::error::Simple<char>> {combine_keyword_parser::<impl chumsky::Parser<char, Keyword, Error = chumsky::error::Simple<char>>, impl chumsky::Parser<char, Keyword, Error = chumsky::error::Simple<char>>>} as FnOnce<(impl chumsky::Parser<char, Keyword, Error = chumsky::error::Simple<char>>, impl chumsky::Parser<char, Keyword, Error = chumsky::error::Simple<char>>)>>::Output == impl chumsky::Parser<char, Keyword, Error = chumsky::error::Simple<char>>`
    --> src/keyword.rs:142:14
     |
117  | ..._keyword_parser(l: impl Parser<char, Keyword, Error = Simple<char>>, r: impl Parser<char, Keyword, Error = Simple<char>> ) -> impl Parser<char, Keyword, Error = Simple<char>...
     |                                                                                                                                  ------------------------------------------------ one of the found opaque types
...
136  | ... make_parser(&self) -> impl Parser<char, Keyword, Error = Simple<char>> {
     |                           ------------------------------------------------
     |                           |
     |                           the expected opaque type
     |                           one of the found opaque types
...
142  | ...  .reduce(combine_keyword_parser).unwrap()
     |       ^^^^^^ expected opaque type, found a different opaque type
     |
     = note: expected opaque type `impl chumsky::Parser<char, Keyword, Error = chumsky::error::Simple<char>>` (opaque type at <src/keyword.rs:136:34>)
                found opaque type `impl chumsky::Parser<char, Keyword, Error = chumsky::error::Simple<char>>` (opaque type at <src/keyword.rs:117:137>)
     = note: distinct uses of `impl Trait` result in different opaque types
note: required by a bound in `reduce`
    --> /home/mtvjr/snap/rustup/common/rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/iter/traits/iterator.rs:2408:45
     |
2408 |         F: FnMut(Self::Item, Self::Item) -> Self::Item,
     |                                             ^^^^^^^^^^ required by this bound in `reduce`

[zesterer]

You'll need more boxing (possibly after the call to make_parser too), and probably a + 'static bound on the returned impl Parser, I think.