Crate nom_derive[][src]

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License: MIT Apache License 2.0 Build Status Version


nom-derive is a custom derive attribute, to derive nom parsers automatically from the structure definition.

It is not meant to replace nom, but to provide a quick and easy way to generate parsers for structures, especially for simple structures. This crate aims at simplifying common cases. In some cases, writing the parser manually will remain more efficient.

  • API documentation
  • The docs::Nom pseudo-module. This is the main documentation for the Nom attribute, with all possible options and many examples.

Feedback welcome !


This crate exposes a single custom-derive macro Nom which implements parse for the struct it is applied to.

The goal of this project is that:

  • derive(Nom) should be enough for you to derive nom parsers for simple structures easily, without having to write it manually
  • it allows overriding any parsing method by your own
  • it allows using generated parsing functions along with handwritten parsers and combining them without efforts
  • it remains as fast as nom

nom-derive adds declarative parsing to nom. It also allows mixing with procedural parsing easily, making writing parsers for byte-encoded formats very easy.

For example:

use nom_derive::*;

struct S {
  a: u32,
  b: u16,
  c: u16

This generates an implementation of the Parse trait to S. The generated code looks like (code simplified):

impl<'a> Parse<&'a> for S {
    pub fn parse_be(i: &'a [u8]) -> nom::IResult(&'a [u8], S) {
        let (i, a) = be_u32(i)?;
        let (i, b) = be_u16(i)?;
        let (i, c) = be_u16(i)?;
        Ok((i, S{ a, b, c }))
    pub fn parse_le(i: &'a [u8]) -> nom::IResult(&'a [u8], S) {
        let (i, a) = le_u32(i)?;
        let (i, b) = le_u16(i)?;
        let (i, c) = le_u16(i)?;
        Ok((i, S{ a, b, c }))
    pub fn parse(i: &'a [u8]) -> nom::IResult(&'a [u8], S) {

To parse input, just call let res = S::parse_be(input);.

If the endianness of the struct is fixed (for ex. using the top-level BigEndian or LittleEndian attributes, or the NomBE and NomLE custom derive), then the implementation always uses this endianness, and all 3 functions are equivalent.

For extensive documentation of all attributes and examples, see the documentation of docs::Nom custom derive attribute.

Many examples are provided, and more can be found in the project tests.

Combinators visibility

All inferred parsers will generate code with absolute type path, so there is no need to add use statements for them. However, if you use any combinator directly (or in a Parse statement, for ex.), it has to be imported as usual.

That is probably not going to change, since

  • a proc_macro cannot export items other than functions tagged with #[proc_macro_derive]
  • there are variants of combinators with the same names (complete/streaming, bits/bytes), so re-exporting them would create side-effects.

Debug tips

  • If the generated parser does not compile, add #[nom(DebugDerive)] to the structure. It will dump the generated parser to stderr.
  • If the generated parser fails at runtime, try adding #[nom(Debug)] to the structure or to fields. It wraps subparsers in dbg_dmp and will print the field name and input to stderr if the parser fails.


pub use nom;


The docs pseudo-module contains nom-derive documentation. Objects from this module are only used to add documentation, and are not used in the crate.



Abstract method to calculate the input length

Common trait for all parsers in nom-derive

Slicing operations using ranges.

Type Definitions

Derive Macros

The Nom derive automatically generates a parse function for the structure using nom parsers. It will try to infer parsers for primitive of known types, but also allows you to specify parsers using custom attributes.

The NomBE acts like the Nom attribute, but sets the endianness to big-endian for the current object. This can be overriden locally at the field-level.

The NomLE acts like the Nom attribute, but sets the endianness to little-endian for the current object. This can be overriden locally at the field-level.