Macro peel_ip::prelude::peek

macro_rules! peek {
    ( $ i : expr , $ submac : ident ! ( $ ( $ args : tt ) * ) ) => { ... };
    ( $ i : expr , $ f : expr ) => { ... };
}

peek!(I -> IResult<I,O>) => I -> IResult<I, O> returns a result without consuming the input

the embedded parser may return Incomplete

 named!(ptag, peek!( tag!( "abcd" ) ) );

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