nom::tap!

macro_rules! tap (
  ($i:expr, $name:ident : $submac:ident!( $($args:tt)* ) => $e:expr) => (
    {
      match $submac!($i, $($args)*) {
        $crate::IResult::Done(i,o)     => {
          let $name = o;
          $e;
          $crate::IResult::Done(i, $name)
        },
        $crate::IResult::Error(a)      => $crate::IResult::Error(a),
        $crate::IResult::Incomplete(i) => $crate::IResult::Incomplete(i)
      }
    }
  );
  ($i:expr, $name: ident: $f:expr => $e:expr) => (
    tap!($i, $name: call!(f) => $e);
  );
);

tap!(name: I -> IResult<I,O> => { block }) => I -> IResult<I, O> allows access to the parser's result without affecting it

 named!(ptag, tap!(res: tag!( "abcd" ) => { println!("recognized {}", str::from_utf8(res).unwrap()) } ) );

 let r = ptag(&b"abcdefgh"[..]);
 assert_eq!(r, Done(&b"efgh"[..], &b"abcd"[..]));