Expand description
§nom, eating data byte by byte
nom is a parser combinator library with a focus on safe parsing, streaming patterns, and as much as possible zero copy.
§Example
use nom::{
IResult,
Parser,
bytes::complete::{tag, take_while_m_n},
combinator::map_res
};
#[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> {
map_res(
take_while_m_n(2, 2, is_hex_digit),
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() {
assert_eq!(hex_color("#2F14DF"), 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 recipes 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
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 nom::{
IResult,
Parser,
sequence::delimited,
// see the "streaming/complete" paragraph lower for an explanation of these submodules
character::complete::char,
bytes::complete::is_not
};
fn parens(input: &str) -> IResult<&str, &str> {
delimited(char('('), is_not(")"), char(')')).parse(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 nom::{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 nom::{IResult, bytes::streaming::take};
fn take4(input: &str) -> IResult<&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 nom::{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))
withc
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 theError
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 nom::IResult;
use nom::bytes::complete::{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 nom::{IResult, Parser};
use nom::branch::alt;
use nom::bytes::complete::tag;
let mut alt_tags = alt((tag("abcd"), tag("efgh")));
assert_eq!(alt_tags.parse(&b"abcdxxx"[..]), Ok((&b"xxx"[..], &b"abcd"[..])));
assert_eq!(alt_tags.parse(&b"efghxxx"[..]), Ok((&b"xxx"[..], &b"efgh"[..])));
assert_eq!(alt_tags.parse(&b"ijklxxx"[..]), Err(nom::Err::Error((&b"ijklxxx"[..], nom::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 nom::{IResult, Parser, combinator::opt, bytes::complete::tag};
fn abcd_opt(i: &[u8]) -> IResult<&[u8], Option<&[u8]>> {
opt(tag("abcd")).parse(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 nom::{IResult, Parser, multi::many0, bytes::complete::tag};
use std::str;
fn multi(i: &str) -> IResult<&str, Vec<&str>> {
many0(tag("abcd")).parse(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 theO
type, the new parser returnsOption<O>
)many0
: Will apply the parser 0 or more times (if it returns theO
type, the new parser returnsVec<O>
)many1
: Will apply the parser 1 or more times
There are more complex (and more useful) parsers like tuples, which are used to apply a series of parsers then assemble their results.
Example with a tuple of parsers:
use nom::{
error::ErrorKind,
Needed,
Parser,
number::streaming::be_u16,
bytes::streaming::{tag, take}};
let mut tpl = (be_u16, take(3u8), tag("fg"));
assert_eq!(
tpl.parse(&b"abcdefgh"[..]),
Ok((
&b"h"[..],
(0x6162u16, &b"cde"[..], &b"fg"[..])
))
);
assert_eq!(tpl.parse(&b"abcde"[..]), Err(nom::Err::Incomplete(Needed::new(2))));
let input = &b"abcdejk"[..];
assert_eq!(tpl.parse(input), Err(nom::Err::Error((&input[5..], ErrorKind::Tag))));
But you can also use a sequence of combinators written in imperative style,
thanks to the ?
operator:
use nom::{IResult, bytes::complete::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})));
§Streaming / Complete
Some of nom’s modules have streaming
or complete
submodules. They hold
different variants of the same combinators.
A streaming parser assumes that we might not have all of the input data. This can happen with some network protocol or large file parsers, where the input buffer can be full and need to be resized or refilled.
A complete parser assumes that we already have all of the input data. This will be the common case with small files that can be read entirely to memory.
Here is how it works in practice:
use nom::{IResult, Err, Needed, error::{Error, ErrorKind}, bytes, character};
fn take_streaming(i: &[u8]) -> IResult<&[u8], &[u8]> {
bytes::streaming::take(4u8)(i)
}
fn take_complete(i: &[u8]) -> IResult<&[u8], &[u8]> {
bytes::complete::take(4u8)(i)
}
// both parsers will take 4 bytes as expected
assert_eq!(take_streaming(&b"abcde"[..]), Ok((&b"e"[..], &b"abcd"[..])));
assert_eq!(take_complete(&b"abcde"[..]), Ok((&b"e"[..], &b"abcd"[..])));
// if the input is smaller than 4 bytes, the streaming parser
// will return `Incomplete` to indicate that we need more data
assert_eq!(take_streaming(&b"abc"[..]), Err(Err::Incomplete(Needed::new(1))));
// but the complete parser will return an error
assert_eq!(take_complete(&b"abc"[..]), Err(Err::Error(Error::new(&b"abc"[..], ErrorKind::Eof))));
// the alpha0 function recognizes 0 or more alphabetic characters
fn alpha0_streaming(i: &str) -> IResult<&str, &str> {
character::streaming::alpha0(i)
}
fn alpha0_complete(i: &str) -> IResult<&str, &str> {
character::complete::alpha0(i)
}
// if there's a clear limit to the recognized characters, both parsers work the same way
assert_eq!(alpha0_streaming("abcd;"), Ok((";", "abcd")));
assert_eq!(alpha0_complete("abcd;"), Ok((";", "abcd")));
// but when there's no limit, the streaming version returns `Incomplete`, because it cannot
// know if more input data should be recognized. The whole input could be "abcd;", or
// "abcde;"
assert_eq!(alpha0_streaming("abcd"), Err(Err::Incomplete(Needed::new(1))));
// while the complete version knows that all of the data is there
assert_eq!(alpha0_complete("abcd"), Ok(("", "abcd")));
Modules§
- Bit level parsers
- Choice combinators
- Parsers recognizing bytes streams
- Character specific parsers and combinators
- General purpose combinators
- Error management
- Lib module to re-export everything needed from
std
orcore
/alloc
. This is howserde
does it, albeit there it is not public. - Combinators applying their child parser multiple times
- Parsers recognizing numbers
- Common recipes to build nom parsers
- Combinators applying parsers in sequence
Macros§
- Creates a parse error from a
nom::ErrorKind
, the position in the input and the next error in the parsing tree - Creates a parse error from a
nom::ErrorKind
and the position in the input
Structs§
- Implementation of
Parser::and
- Implementation of
Parser::and_then
- Applies the parser, but do not a produce a value
- Indicates that the input data is complete: no more data may be added later
- Produces a value. This is the default behaviour for parsers
- Implementation of
Parser::flat_map
- Implementation of
Parser::into
- Implementation of
Parser::map
- Implementation of
Parser::map_opt
- Implementation of
Parser::map_res
- Implementation of
Parser::or
- A saturating iterator for usize.
- Indicates that the input data is streaming: more data may be available later
Enums§
- Indicates whether a comparison was successful, an error, or if more data was needed
- The
Err
enum indicates the parser was not successful - Contains information on needed data if a parser returned
Incomplete
Traits§
- Helper trait for types that can be viewed as a byte slice
- Transforms common types to a char for basic token parsing
- Abstracts comparison operations
- Equivalent From implementation to avoid orphan rules in bits parsers
- Abstracts something which can extend an
Extend
. Used to build modified input slices inescaped_transform
- Look for a substring in self
- Look for a token in self
- Helper trait to convert a parser’s result to a more manageable type
- HexDisplay
std
Helper trait to show a byte slice as a hex dump - Parser input types must implement this trait
- Specifies the behaviour when a parser encounters an error that could be due to partial ata
- Parser mode: influences how combinators build values
- Abstractions for range-like types.
- Useful functions to calculate the offset between slices and show a hexdump of a slice
- Trait Defining the parser’s execution
- Used to integrate
str
’sparse()
method - All nom parsers implement this trait
- Helper trait to convert numbers to usize.
Type Aliases§
- Holds the result of parsing functions
- Parser result type