Is there a way to easily require and parse a specific length of input with nom?

Question

Given a sequence of numbers, b"12345678...", is there an easy way to have nom parse just the first n digits of the input into a type, such as u16, then the subsequent m digits into another type, and so on. Presently, nom consumes the entire contiguous sequence of digits into a single value.

The current application is for a sequence of digits representing datetime, as in, YYYYMMDDHHmmSS, from the input, (D:20230416140523Z00'00)

My attempts combining take(n) have not been successful. For instance...

use nom::bytes::complete::take;
use nom::character::complete::u16;
use nom::combinator::{flat_map};
type IResult<'a, T> = nom::IResult<&'a [u8], T, ()>;

fn parse_fixed_length_prefix<'a>(input: &[u8], n: usize) -> IResult<u16> {
    flat_map(take(n), u16::<_, ()>)(input)
}

#[test]
fn test() {
    let output = parse_fixed_length_prefix(b"123456789", 4);
    assert_eq!(output, Ok((b"56789".as_ref(), 1234u16)));
    // expected: Ok(([56789], 1234))
}

The above approach results in the following compile error, however I suspect there is a better approach that would obviate this...

   Compiling playground v0.0.1 (/playground)
error[E0277]: expected a `FnMut<(_,)>` closure, found `Result<(_, u16), nom::Err<()>>`
   --> src/main.rs:7:5
    |
7   |     flat_map(take(n), u16::<_, ()>)(input)
    |     ^^^^^^^^ expected an `FnMut<(_,)>` closure, found `Result<(_, u16), nom::Err<()>>`
    |
    = help: the trait `FnMut<(_,)>` is not implemented for `Result<(_, u16), nom::Err<()>>`
    = help: the following other types implement trait `Parser<I, O, E>`:
              <And<F, G> as Parser<I, (O1, O2), E>>
              <AndThen<F, G, O1> as Parser<I, O2, E>>
              <Box<(dyn Parser<I, O, E> + 'a)> as Parser<I, O, E>>
              <Or<F, G> as Parser<I, O, E>>
              <nom::FlatMap<F, G, O1> as Parser<I, O2, E>>
              <nom::Into<F, O1, O2, E1, E2> as Parser<I, O2, E2>>
              <nom::Map<F, G, O1> as Parser<I, O2, E>>
    = note: required for `Result<(_, u16), nom::Err<()>>` to implement `Parser<_, _, _>`
note: required by a bound in `nom::combinator::flat_map`
   --> /playground/.cargo/registry/src/github.com-1ecc6299db9ec823/nom-7.1.3/src/combinator/mod.rs:213:6
    |
213 |   H: Parser<I, O2, E>,
    |      ^^^^^^^^^^^^^^^^ required by this bound in `flat_map`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` due to previous error

Answer 1

Don't use nom:combination::flat_map. Use nom:combination::map_res to transform the output of take(n) to the desired type, u16, then use map of the Result type to transform the result to the value, Ok<u16>, that map_res can integrate into its output, in this case IResult<&[u8], u16, ()>.

fn parse_fixed_length_prefix<'a>(input: &[u8], n: usize) -> IResult<u16> {
    map_res(take(n), |v| u16::<_,()>(v).map(|(_,a)|a))(input)
}

One of the things I like about Rust is that if it compiles it works, even more so than Haskell.

Update solution per the recommendations of @ChayimFriedman

fn parse_fixed_length_prefix<'a>(input: &[u8], n: usize) -> IResult<u16> {
    map_parser(take(n), u16)(input)
}