Crate nom8

Expand description

nom, eating data byte by byte

NOTE: This is an unofficial, short-lived fork. I’m nominating nom8 to be nom v8.

nom is a parser combinator library, supporting:

String (&str), byte (&[u8]), and custom input types
Streaming parsing
Zero copy parsing

Example

use nom8::prelude::*;
use nom8::bytes::{tag, take_while_m_n};

#[derive(Debug,PartialEq)]
pub struct Color {
  pub red:     u8,
  pub green:   u8,
  pub blue:    u8,
}

fn from_hex(input: &str) -> Result<u8, std::num::ParseIntError> {
  u8::from_str_radix(input, 16)
}

fn is_hex_digit(c: char) -> bool {
  c.is_digit(16)
}

fn hex_primary(input: &str) -> IResult<&str, u8> {
  take_while_m_n(2, 2, is_hex_digit).map_res(from_hex).parse(input)
}

fn hex_color(input: &str) -> IResult<&str, Color> {
  let (input, _) = tag("#")(input)?;
  let (input, (red, green, blue)) = (hex_primary, hex_primary, hex_primary).parse(input)?;

  Ok((input, Color { red, green, blue }))
}

fn main() {
  let result = hex_color("#2F14DF").finish();
  assert_eq!(result, Ok(Color {
    red: 47,
    green: 20,
    blue: 223,
  }));
}

The code is available on Github

There are a few guides with more details about how to write parsers, or the error management system. You can also check out the _cookbook module that contains examples of common patterns.

Looking for a specific combinator? Read the “choose a combinator” guide

If you are upgrading to nom 5.0, please read the migration document.

Parser combinators are an approach to parsers that is very different from software like lex and yacc. Instead of writing the grammar in a separate syntax and generating the corresponding code, you use very small functions with very specific purposes, like “take 5 bytes”, or “recognize the word ‘HTTP’”, and assemble them in meaningful patterns like “recognize ‘HTTP’, then a space, then a version”. The resulting code is small, and looks like the grammar you would have written with other parser approaches.

This gives us a few advantages:

The parsers are small and easy to write
The parsers components are easy to reuse (if they’re general enough, please add them to nom!)
The parsers components are easy to test separately (unit tests and property-based tests)
The parser combination code looks close to the grammar you would have written
You can build partial parsers, specific to the data you need at the moment, and ignore the rest

Here is an example of one such parser, to recognize text between parentheses:

use nom8::{
  IResult,
  sequence::delimited,
  bytes::take_till1
};

fn parens(input: &str) -> IResult<&str, &str> {
  delimited('(', take_till1(")"), ')')(input)
}

It defines a function named parens which will recognize a sequence of the character (, the longest byte array not containing ), then the character ), and will return the byte array in the middle.

Here is another parser, written without using nom’s combinators this time:

use nom8::{IResult, Err, Needed};

fn take4(i: &[u8]) -> IResult<&[u8], &[u8]>{
  if i.len() < 4 {
    Err(Err::Incomplete(Needed::new(4)))
  } else {
    Ok((&i[4..], &i[0..4]))
  }
}

This function takes a byte array as input, and tries to consume 4 bytes. Writing all the parsers manually, like this, is dangerous, despite Rust’s safety features. There are still a lot of mistakes one can make. That’s why nom provides a list of functions to help in developing parsers.

With functions, you would write it like this:

use nom8::{IResult, bytes::take, input::Streaming};
fn take4(input: Streaming<&str>) -> IResult<Streaming<&str>, &str> {
  take(4u8)(input)
}

A parser in nom is a function which, for an input type I, an output type O and an optional error type E, will have the following signature:

fn parser(input: I) -> IResult<I, O, E>;

Or like this, if you don’t want to specify a custom error type (it will be (I, ErrorKind) by default):

fn parser(input: I) -> IResult<I, O>;

IResult is an alias for the Result type:

use nom8::{Needed, error::Error};

type IResult<I, O, E = Error<I>> = Result<(I, O), Err<E>>;

enum Err<E> {
  Incomplete(Needed),
  Error(E),
  Failure(E),
}

It can have the following values:

A correct result Ok((I,O)) with the first element being the remaining of the input (not parsed yet), and the second the output value;
An error Err(Err::Error(c)) with c an error that can be built from the input position and a parser specific error
An error Err(Err::Incomplete(Needed)) indicating that more input is necessary. Needed can indicate how much data is needed
An error Err(Err::Failure(c)). It works like the Error case, except it indicates an unrecoverable error: We cannot backtrack and test another parser

Please refer to the “choose a combinator” guide for an exhaustive list of parsers. See also the rest of the documentation here.

Making new parsers with function combinators

nom is based on functions that generate parsers, with a signature like this: (arguments) -> impl Fn(Input) -> IResult<Input, Output, Error>. The arguments of a combinator can be direct values (like take which uses a number of bytes or character as argument) or even other parsers (like delimited which takes as argument 3 parsers, and returns the result of the second one if all are successful).

Here are some examples:

use nom8::IResult;
use nom8::bytes::{tag, take};
fn abcd_parser(i: &str) -> IResult<&str, &str> {
  tag("abcd")(i) // will consume bytes if the input begins with "abcd"
}

fn take_10(i: &[u8]) -> IResult<&[u8], &[u8]> {
  take(10u8)(i) // will consume and return 10 bytes of input
}

Combining parsers

There are higher level patterns, like the alt combinator, which provides a choice between multiple parsers. If one branch fails, it tries the next, and returns the result of the first parser that succeeds:

use nom8::IResult;
use nom8::branch::alt;
use nom8::bytes::tag;

let mut alt_tags = alt((tag("abcd"), tag("efgh")));

assert_eq!(alt_tags(&b"abcdxxx"[..]), Ok((&b"xxx"[..], &b"abcd"[..])));
assert_eq!(alt_tags(&b"efghxxx"[..]), Ok((&b"xxx"[..], &b"efgh"[..])));
assert_eq!(alt_tags(&b"ijklxxx"[..]), Err(nom8::Err::Error((&b"ijklxxx"[..], nom8::error::ErrorKind::Tag))));

The opt combinator makes a parser optional. If the child parser returns an error, opt will still succeed and return None:

use nom8::{IResult, combinator::opt, bytes::tag};
fn abcd_opt(i: &[u8]) -> IResult<&[u8], Option<&[u8]>> {
  opt(tag("abcd"))(i)
}

assert_eq!(abcd_opt(&b"abcdxxx"[..]), Ok((&b"xxx"[..], Some(&b"abcd"[..]))));
assert_eq!(abcd_opt(&b"efghxxx"[..]), Ok((&b"efghxxx"[..], None)));

many0 applies a parser 0 or more times, and returns a vector of the aggregated results:

use nom8::{IResult, multi::many0, bytes::tag};
use std::str;

fn multi(i: &str) -> IResult<&str, Vec<&str>> {
  many0(tag("abcd"))(i)
}

let a = "abcdef";
let b = "abcdabcdef";
let c = "azerty";
assert_eq!(multi(a), Ok(("ef",     vec!["abcd"])));
assert_eq!(multi(b), Ok(("ef",     vec!["abcd", "abcd"])));
assert_eq!(multi(c), Ok(("azerty", Vec::new())));

Here are some basic combinators available:

opt: Will make the parser optional (if it returns the O type, the new parser returns Option<O>)
many0: Will apply the parser 0 or more times (if it returns the O type, the new parser returns Vec<O>)
many1: Will apply the parser 1 or more times

There are more complex (and more useful) parsers like tuples, which is used to apply a series of parsers then assemble their results.

Example with tuples:

use nom8::prelude::*;
use nom8::{
    error::ErrorKind, Needed,
    number::be_u16,
    bytes::{tag, take},
    input::Streaming,
};

let mut tpl = (be_u16, take(3u8), tag("fg"));

assert_eq!(
  tpl.parse(Streaming(&b"abcdefgh"[..])),
  Ok((
    Streaming(&b"h"[..]),
    (0x6162u16, &b"cde"[..], &b"fg"[..])
  ))
);
assert_eq!(tpl.parse(Streaming(&b"abcde"[..])), Err(nom8::Err::Incomplete(Needed::new(2))));
let input = &b"abcdejk"[..];
assert_eq!(tpl.parse(Streaming(input)), Err(nom8::Err::Error((Streaming(&input[5..]), ErrorKind::Tag))));

But you can also use a sequence of combinators written in imperative style, thanks to the ? operator:

use nom8::{IResult, bytes::tag};

#[derive(Debug, PartialEq)]
struct A {
  a: u8,
  b: u8
}

fn ret_int1(i:&[u8]) -> IResult<&[u8], u8> { Ok((i,1)) }
fn ret_int2(i:&[u8]) -> IResult<&[u8], u8> { Ok((i,2)) }

fn f(i: &[u8]) -> IResult<&[u8], A> {
  // if successful, the parser returns `Ok((remaining_input, output_value))` that we can destructure
  let (i, _) = tag("abcd")(i)?;
  let (i, a) = ret_int1(i)?;
  let (i, _) = tag("efgh")(i)?;
  let (i, b) = ret_int2(i)?;

  Ok((i, A { a, b }))
}

let r = f(b"abcdefghX");
assert_eq!(r, Ok((&b"X"[..], A{a: 1, b: 2})));