Macro nom::map_opt

macro_rules! map_opt {
    (__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_opt!(I -> IResult<I, O>, O -> Option<P>) => I -> IResult<I, P> maps a function returning an Option on the output of a parser

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

named!(parser<&str, u8>, map_opt!(digit1, |s: &str| s.parse::<u8>().ok()));

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

// this will fail if digit1 fails
assert_eq!(parser("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!(parser("123456"), Err(Err::Error(("123456", ErrorKind::MapOpt))));