Macro nom::map_res

macro_rules! map_res {
    (__impl $i:expr, $submac:ident!( $($args:tt)* ), $submac2:ident!( $($args2:tt)* )) => { ... };
    ($i:expr, $submac:ident!( $($args:tt)* ), $g:expr) => { ... };
    ($i:expr, $submac:ident!( $($args:tt)* ), $submac2:ident!( $($args2:tt)* )) => { ... };
    ($i:expr, $f:expr, $g:expr) => { ... };
    ($i:expr, $f:expr, $submac:ident!( $($args:tt)* )) => { ... };
}

map_res!(I -> IResult<I, O>, O -> Result<P>) => I -> IResult<I, P> maps a function returning a Result on the output of a parser

use nom::character::complete::digit1;

named!(parse<&str, u8>, 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(error_position!("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(error_position!("123456", ErrorKind::MapRes))));