Macro nom::do_parse

macro_rules! do_parse {
    (__impl $i:expr, ( $($rest:expr),* )) => { ... };
    (__impl $i:expr, $field:ident : $submac:ident!( $($args:tt)* ) ) => { ... };
    (__impl $i:expr, $submac:ident!( $($args:tt)* ) ) => { ... };
    (__impl $i:expr, $field:ident : $submac:ident!( $($args:tt)* ) ~ $($rest:tt)* ) => { ... };
    (__impl $i:expr, $submac:ident!( $($args:tt)* ) ~ $($rest:tt)* ) => { ... };
    (__impl $i:expr, $field:ident : $e:ident ~ $($rest:tt)*) => { ... };
    (__impl $i:expr, $e:ident ~ $($rest:tt)*) => { ... };
    (__impl $i:expr, $e:ident >> $($rest:tt)*) => { ... };
    (__impl $i:expr, $submac:ident!( $($args:tt)* ) >> $($rest:tt)*) => { ... };
    (__impl $i:expr, $field:ident : $e:ident >> $($rest:tt)*) => { ... };
    (__impl $i:expr, $field:ident : $submac:ident!( $($args:tt)* ) >> $($rest:tt)*) => { ... };
    (__impl $i:expr, $e:ident >> ( $($rest:tt)* )) => { ... };
    (__impl $i:expr, $submac:ident!( $($args:tt)* ) >> ( $($rest:tt)* )) => { ... };
    (__impl $i:expr, $field:ident : $e:ident >> ( $($rest:tt)* )) => { ... };
    (__impl $i:expr, $field:ident : $submac:ident!( $($args:tt)* ) >> ( $($rest:tt)* )) => { ... };
    ($i:expr, $($rest:tt)*) => { ... };
    ($submac:ident!( $($args:tt)* ) >> $($rest:tt)* ) => { ... };
    ($e:ident! >> $($rest:tt)* ) => { ... };
}

do_parse!(I->IResult<I,A> >> I->IResult<I,B> >> ... I->IResult<I,X> , ( O ) ) => I -> IResult<I, O> do_parse applies sub parsers in a sequence. it can store intermediary results and make them available for later parsers

The input type I must implement nom::InputLength.

This combinator will count how much data is consumed by every child parser and take it into account if there is not enough data

use nom::be_u8;

// this parser implements a common pattern in binary formats,
// the TAG-LENGTH-VALUE, where you first recognize a specific
// byte slice, then the next bytes indicate the length of
// the data, then you take that slice and return it
//
// here, we match the tag 42, take the length in the next byte
// and store it in `length`, then use `take!` with `length`
// to obtain the subslice that we store in `bytes`, then return
// `bytes`
// you can use other macro combinators inside do_parse (like the `tag`
// one here), or a function (like `be_u8` here), but you cannot use a
// module path (like `nom::be_u8`) there, because of limitations in macros
named!(tag_length_value,
  do_parse!(
    tag!( &[ 42u8 ][..] ) >>
    length: be_u8         >>
    bytes:  take!(length) >>
    (bytes)
  )
);

let a: Vec<u8>        = vec!(42, 2, 3, 4, 5);
let result_a: Vec<u8> = vec!(3, 4);
let rest_a: Vec<u8>   = vec!(5);
assert_eq!(tag_length_value(&a[..]), Ok((&rest_a[..], &result_a[..])));

// here, the length is 5, but there are only 3 bytes afterwards (3, 4 and 5),
// so the parser will tell you that you need 7 bytes: one for the tag,
// one for the length, then 5 bytes
let b: Vec<u8>     = vec!(42, 5, 3, 4, 5);
assert_eq!(tag_length_value(&b[..]), Err(Err::Incomplete(Needed::Size(5))));

the result is a tuple, so you can return multiple sub results, like this: do_parse!(I->IResult<I,A> >> I->IResult<I,B> >> ... I->IResult<I,X> , ( O, P ) ) => I -> IResult<I, (O,P)>

use nom::be_u8;
named!(tag_length_value<(u8, &[u8])>,
  do_parse!(
    tag!( &[ 42u8 ][..] ) >>
    length: be_u8         >>
    bytes:  take!(length) >>
    (length, bytes)
  )
);