Function nom::combinator::map_res

pub fn map_res<I: Clone, O1, O2, E: ParseError<I>, E2, F, G>(
    first: F, 
    second: G
) -> impl Fn(I) -> IResult<I, O2, E> where
    F: Fn(I) -> IResult<I, O1, E>,
    G: Fn(O1) -> Result<O2, E2>, 

applies a function returning a Result over the result of a parser

use nom::character::complete::digit1;
use nom::combinator::map_res;

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

// the parser will convert the result of digit1 to a number
assert_eq!(parse("123"), Ok(("", 123)));

// this will fail if digit1 fails
assert_eq!(parse("abc"), Err(Err::Error(("abc", ErrorKind::Digit))));

// this will fail if the mapped function fails (a `u8` is too small to hold `123456`)
assert_eq!(parse("123456"), Err(Err::Error(("123456", ErrorKind::MapRes))));