Expand description
This crate contains parser combinators, roughly based on the Haskell libraries parsec and attoparsec.
A parser in this library can be described as a function which takes some input and if it
is successful, returns a value together with the remaining input.
A parser combinator is a function which takes one or more parsers and returns a new parser.
For instance the many
parser can be used to convert a parser for single digits into one that
parses multiple digits. By modeling parsers in this way it becomes easy to compose complex
parsers in an almost declarative way.
Overview
combine
limits itself to creating LL(1) parsers
(it is possible to opt-in to LL(k) parsing using the attempt
combinator) which makes the
parsers easy to reason about in both function and performance while sacrificing
some generality. In addition to you being able to reason better about the parsers you
construct combine
the library also takes the knowledge of being an LL parser and uses it to
automatically construct good error messages.
extern crate combine;
use combine::{Parser, EasyParser};
use combine::stream::position;
use combine::parser::char::{digit, letter};
const MSG: &'static str = r#"Parse error at line: 1, column: 1
Unexpected `|`
Expected `digit` or `letter`
"#;
fn main() {
// Wrapping a `&str` with `State` provides automatic line and column tracking. If `State`
// was not used the positions would instead only be pointers into the `&str`
if let Err(err) = digit().or(letter()).easy_parse(position::Stream::new("|")) {
assert_eq!(MSG, format!("{}", err));
}
}
This library is currently split into a few core modules:
-
parser
is where you will find all the parsers that combine provides. It contains the coreParser
trait as well as several submodules such assequence
orchoice
which each contain several parsers aimed at a specific niche. -
stream
contains the second most important trait next toParser
. Streams represent the data source which is being parsed such as&[u8]
,&str
or iterators. -
easy
contains combine’s default “easy” error and stream handling. If you use theeasy_parse
method to start your parsing these are the types that are used. -
error
contains the types and traits that make up combine’s error handling. Unless you need to customize the errors your parsers return you should not need to use this module much.
Examples
extern crate combine;
use combine::parser::char::{spaces, digit, char};
use combine::{many1, sep_by, Parser, EasyParser};
use combine::stream::easy;
fn main() {
//Parse spaces first and use the with method to only keep the result of the next parser
let integer = spaces()
//parse a string of digits into an i32
.with(many1(digit()).map(|string: String| string.parse::<i32>().unwrap()));
//Parse integers separated by commas, skipping whitespace
let mut integer_list = sep_by(integer, spaces().skip(char(',')));
//Call parse with the input to execute the parser
let input = "1234, 45,78";
let result: Result<(Vec<i32>, &str), easy::ParseError<&str>> =
integer_list.easy_parse(input);
match result {
Ok((value, _remaining_input)) => println!("{:?}", value),
Err(err) => println!("{}", err)
}
}
If we need a parser that is mutually recursive or if we want to export a reusable parser the
parser!
macro can be used. In effect it makes it possible to return a parser without naming
the type of the parser (which can be very large due to combine’s trait based approach). While
it is possible to do avoid naming the type without the macro those solutions require either allocation
(Box<dyn Parser< Input, Output = O, PartialState = P>>
) or nightly rust via impl Trait
. The
macro thus threads the needle and makes it possible to have non-allocating, anonymous parsers
on stable rust.
#[macro_use]
extern crate combine;
use combine::parser::char::{char, letter, spaces};
use combine::{between, choice, many1, parser, sep_by, Parser, EasyParser};
use combine::error::{ParseError, StdParseResult};
use combine::stream::{Stream, Positioned};
use combine::stream::position;
#[derive(Debug, PartialEq)]
pub enum Expr {
Id(String),
Array(Vec<Expr>),
Pair(Box<Expr>, Box<Expr>)
}
// `impl Parser` can be used to create reusable parsers with zero overhead
fn expr_<Input>() -> impl Parser< Input, Output = Expr>
where Input: Stream<Token = char>,
// Necessary due to rust-lang/rust#24159
Input::Error: ParseError<Input::Token, Input::Range, Input::Position>,
{
let word = many1(letter());
// A parser which skips past whitespace.
// Since we aren't interested in knowing that our expression parser
// could have accepted additional whitespace between the tokens we also silence the error.
let skip_spaces = || spaces().silent();
//Creates a parser which parses a char and skips any trailing whitespace
let lex_char = |c| char(c).skip(skip_spaces());
let comma_list = sep_by(expr(), lex_char(','));
let array = between(lex_char('['), lex_char(']'), comma_list);
//We can use tuples to run several parsers in sequence
//The resulting type is a tuple containing each parsers output
let pair = (lex_char('('),
expr(),
lex_char(','),
expr(),
lex_char(')'))
.map(|t| Expr::Pair(Box::new(t.1), Box::new(t.3)));
choice((
word.map(Expr::Id),
array.map(Expr::Array),
pair,
))
.skip(skip_spaces())
}
// As this expression parser needs to be able to call itself recursively `impl Parser` can't
// be used on its own as that would cause an infinitely large type. We can avoid this by using
// the `parser!` macro which erases the inner type and the size of that type entirely which
// lets it be used recursively.
//
// (This macro does not use `impl Trait` which means it can be used in rust < 1.26 as well to
// emulate `impl Parser`)
parser!{
fn expr[Input]()(Input) -> Expr
where [Input: Stream<Token = char>]
{
expr_()
}
}
fn main() {
let result = expr()
.parse("[[], (hello, world), [rust]]");
let expr = Expr::Array(vec![
Expr::Array(Vec::new())
, Expr::Pair(Box::new(Expr::Id("hello".to_string())),
Box::new(Expr::Id("world".to_string())))
, Expr::Array(vec![Expr::Id("rust".to_string())])
]);
assert_eq!(result, Ok((expr, "")));
}
Modules
Stream wrapper which provides an informative and easy to use error type.
Error types and traits which define what kind of errors combine parsers may emit
A collection of both concrete parsers as well as parser combinators.
Streams are similar to the Iterator
trait in that they represent some sequential set of items
which can be retrieved one by one. Where Stream
s differ is that they are allowed to return
errors instead of just None
and if they implement the RangeStreamOnce
trait they are also
capable of returning multiple items at the same time, usually in the form of a slice.
Macros
Takes a number of parsers and tries to apply them each in order. Fails if all the parsers fails or if an applied parser fails after it has committed to its parse.
Parses an instance of std::io::Read
as a &[u8]
without reading the entire file into
memory.
Parses an instance of tokio::io::AsyncRead
as a &[u8]
without reading the entire file into
memory.
dispatch!
allows a parser to be constructed depending on earlier input, without forcing each
branch to have the same type of parser
Declares a named parser which can easily be reused.
Sequences multiple parsers and builds a struct out of them.
Enums
A Result
type which has the committed status flattened into the result.
Conversions to and from std::result::Result
can be done using result.into()
or
From::from(result)
Traits
Provides the easy_parse
method which provides good error messages by default
Trait which defines a combine parse error.
By implementing the Parser
trait a type says that it can be used to parse an input stream
into the type Output
.
A type which has a position.
A RangeStream
is an extension of Stream
which allows for zero copy parsing.
A RangeStream
is an extension of StreamOnce
which allows for zero copy parsing.
A stream of tokens which can be duplicated
StreamOnce
represents a sequence of items that can be extracted one by one.
Functions
Parses any token.
attempt(p)
behaves as p
except it always acts as p
peeked instead of committed on its
parse.
Parses open
followed by parser
followed by close
.
Returns the value of parser
.
Parses p
1 or more times separated by op
. The value returned is the one produced by the
left associative application of the function returned by the parser op
.
Parses p
one or more times separated by op
. The value returned is the one produced by the
right associative application of the function returned by op
.
Takes a tuple, a slice or an array of parsers and tries to apply them each in order. Fails if all the parsers fails or if an applied parser consumes input before failing.
Parses parser
from zero up to count
times.
Parses parser
from min
to max
times (including min
and max
).
Succeeds only if the stream is at end of input, fails otherwise.
Takes a parser that outputs a string like value (&str
, String
, &[u8]
or Vec<u8>
) and parses it
using std::str::FromStr
. Errors if the output of parser
is not UTF-8 or if
FromStr::from_str
returns an error.
look_ahead(p)
acts as p
but doesn’t consume input on success.
Parses p
zero or more times returning a collection with the values from p
.
Parses p
one or more times returning a collection with the values from p
.
Extract one token and succeeds if it is not part of tokens
.
Succeeds only if parser
fails.
Never consumes any input.
Extract one token and succeeds if it is part of tokens
.
Parses parser
and outputs Some(value)
if it succeeds, None
if it fails without
consuming any input. Fails if parser
fails after having committed some input.
Wraps a function, turning it into a parser.
Parser which just returns the current position in the stream.
Always returns the value produced by calling f
.
Parses a token and succeeds depending on the result of predicate
.
Parses a token and passes it to predicate
. If predicate
returns Some
the parser succeeds
and returns the value inside the Option
. If predicate
returns None
the parser fails
without consuming any input.
Parses parser
zero or more time separated by separator
, returning a collection with the
values from p
.
Parses parser
one or more time separated by separator
, returning a collection with the
values from p
.
Parses parser
zero or more times separated and ended by separator
, returning a collection
with the values from p
.
Parses parser
one or more times separated and ended by separator
, returning a collection
with the values from p
.
Parses parser
from zero up to count
times skipping the output of parser
.
Parses parser
from min
to max
times (including min
and max
)
skipping the output of parser
.
Parses p
zero or more times ignoring the result.
Parses p
one or more times ignoring the result.
Parses a character and succeeds if the character is equal to c
.
Parses multiple tokens.
Parses multiple tokens.
Always fails with message
as an unexpected error.
Never consumes any input.
Always fails with message
as an unexpected error.
Never consumes any input.
Always returns the value v
without consuming any input.
Type Definitions
A type alias over the specific Result
type used by parsers to indicate whether they were
successful or not.
O
is the type that is output on success.
Input
is the specific stream type used in the parser.