FlatMap does not properly implement Parser

[hsfzxjy]

• Rust version : 1.76.0
• nom version : 7.1.3
• nom compilation features used: std

Example test case:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=38ea81309c5b6c70c5ded853cbb30a34

use nom::character::complete::anychar;
use nom::Parser;

fn main() {
    anychar.flat_map(|_| anychar).parse("ab").unwrap();
}

nom/src/internal.rs

Line 262 in 869f897

G: FnMut(O) -> H,

nom/src/internal.rs

Line 364 in 869f897

impl<'a, I, O1, O2, E, F: Parser<I, O1, E>, G: Fn(O1) -> H, H: Parser<I, O2, E>> Parser<I, O2, E>

Seems like a mismatch between above lines (notice Fn vs FnMut), making the flat_map unusable.

However I saw it fixed in current main branch

nom/src/internal.rs

Line 673 in e87c7da

G: FnMut(<F as Parser<I>>::Output) -> H,

Will this fix also be ported to 7.x?

[gl-yziquel]

I'm not sure, but it seems to me that backporting such fixes would break semantic versioning.

I myself just upgraded from 7.1.3 to 8.0.0-alpha2 because I really needed the more flexible typing of 8 for everything flatmap related. (Related to on-the-fly parsing of function declarations w/r arguments for a lambda-calculus dialect.)

7.1.3 IMO has a broken flat_map implementation: the return type should be a Parser, and not an FnMut, especially in a context where it is so hard to give explicit names for anonymous closure types in rust.

I'd rather push for a 8.0.0 release or pre-release available on crates.io rather than a backport. Because of semver.

[gl-yziquel]

Oh ! 8.0.0-alpha2 is usable !! Great !