nom::opt [−] [src]

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

opt!(I -> IResult<I,O>) => I -> IResult<I, Option<O>> make the underlying parser optional

returns an Option of the returned type. This parser returns Some(result) if the child parser succeeds,None if it fails, and Incomplete if it did not have enough data to decide

 named!( o<&[u8], Option<&[u8]> >, opt!( tag!( "abcd" ) ) );

 let a = b"abcdef";
 let b = b"bcdefg";
 assert_eq!(o(&a[..]), Done(&b"ef"[..], Some(&b"abcd"[..])));
 assert_eq!(o(&b[..]), Done(&b"bcdefg"[..], None));