Documentation example giving error

[alper]

I'm doing the example from the docs here: https://docs.rs/nom/latest/nom/combinator/fn.map_res.html

let mut parse = map_res(digit1, |s: &str| s.parse::<u8>());

That gives:

error[E0283]: type annotations needed
   --> src/main.rs:31:24
    |
31  |     let base = map_res(digit1, |s: &str| s.parse::<u32>());
    |                ------- ^^^^^^ cannot infer type of the type parameter `E` declared on the function `digit1`
    |                |
    |                type must be known at this point
    |
    = note: cannot satisfy `_: FromExternalError<&str, ParseIntError>`
note: required by a bound in `map_res`

No idea why the example from the documentation would be failing here.

[alper]

This does work:

let (base, e): (&str, u32) = map_res(digit1, str::parse)(input)?;

So there's a big gap between calling the parser and assigning it to a local variable.