pom

PEG parser combinators created using operator overloading without macros.

Document

• Tutorial
• API Reference
• Learning Parser Combinators With Rust - By Bodil Stokke

What is PEG?

PEG stands for parsing expression grammar, is a type of analytic formal grammar, i.e. it describes a formal language in terms of a set of rules for recognizing strings in the language. Unlike CFGs, PEGs cannot be ambiguous; if a string parses, it has exactly one valid parse tree. Each parsing function conceptually takes an input string as its argument, and yields one of the following results:

• success, in which the function may optionally move forward or consume one or more characters of the input string supplied to it, or
• failure, in which case no input is consumed.

Read more on Wikipedia.

What is parser combinator?

A parser combinator is a higher-order function that accepts several parsers as input and returns a new parser as its output. Parser combinators enable a recursive descent parsing strategy that facilitates modular piecewise construction and testing.

Parsers built using combinators are straightforward to construct, readable, modular, well-structured and easily maintainable. With operator overloading, a parser combinator can take the form of an infix operator, used to glue different parsers to form a complete rule. Parser combinators thereby enable parsers to be defined in an embedded style, in code which is similar in structure to the rules of the formal grammar. And the code is easier to debug than macros.

The main advantage is that you don't need to go through any kind of code generation step, you're always using the vanilla language underneath. Aside from build issues (and the usual issues around error messages and debuggability, which in fairness are about as bad with macros as with code generation), it's usually easier to freely intermix grammar expressions and plain code.

List of predefined parsers and combinators

The choice of operators is established by their operator precedence, arity and "meaning". Use * to ignore the result of first operand on the start of an expression, + and - can fulfill the need on the rest of the expression.

For example, A * B * C - D + E - F will return the results of C and E as a pair.

Example code

extern crate pom;
use pom::parser::*;

let input = b"abcde";
let parser = sym(b'a') * none_of(b"AB") - sym(b'c') + seq(b"de");
let output = parser.parse(input);
assert_eq!(output, Ok( (b'b', vec![b'd', b'e']) ) );

Example JSON parser

extern crate pom;
use pom::parser::*;
use pom::Parser;

use std::collections::HashMap;
use std::str::{self, FromStr};

#[derive(Debug, PartialEq)]
pub enum JsonValue {
	Null,
	Bool(bool),
	Str(String),
	Num(f64),
	Array(Vec<JsonValue>),
	Object(HashMap<String,JsonValue>)
}

fn space() -> Parser<u8, ()> {
	one_of(b" \t\r\n").repeat(0..).discard()
}

fn number() -> Parser<u8, f64> {
	let integer = one_of(b"123456789") - one_of(b"0123456789").repeat(0..) | sym(b'0');
	let frac = sym(b'.') + one_of(b"0123456789").repeat(1..);
	let exp = one_of(b"eE") + one_of(b"+-").opt() + one_of(b"0123456789").repeat(1..);
	let number = sym(b'-').opt() + integer + frac.opt() + exp.opt();
	number.collect().convert(str::from_utf8).convert(|s|f64::from_str(&s))
}

fn string() -> Parser<u8, String> {
	let special_char = sym(b'\\') | sym(b'/') | sym(b'"')
		| sym(b'b').map(|_|b'\x08') | sym(b'f').map(|_|b'\x0C')
		| sym(b'n').map(|_|b'\n') | sym(b'r').map(|_|b'\r') | sym(b't').map(|_|b'\t');
	let escape_sequence = sym(b'\\') * special_char;
	let string = sym(b'"') * (none_of(b"\\\"") | escape_sequence).repeat(0..) - sym(b'"');
	string.convert(String::from_utf8)
}

fn array() -> Parser<u8, Vec<JsonValue>> {
	let elems = list(call(value), sym(b',') * space());
	sym(b'[') * space() * elems - sym(b']')
}

fn object() -> Parser<u8, HashMap<String, JsonValue>> {
	let member = string() - space() - sym(b':') - space() + call(value);
	let members = list(member, sym(b',') * space());
	let obj = sym(b'{') * space() * members - sym(b'}');
	obj.map(|members|members.into_iter().collect::<HashMap<_,_>>())
}

fn value() -> Parser<u8, JsonValue> {
	( seq(b"null").map(|_|JsonValue::Null)
	| seq(b"true").map(|_|JsonValue::Bool(true))
	| seq(b"false").map(|_|JsonValue::Bool(false))
	| number().map(|num|JsonValue::Num(num))
	| string().map(|text|JsonValue::Str(text))
	| array().map(|arr|JsonValue::Array(arr))
	| object().map(|obj|JsonValue::Object(obj))
	) - space()
}

pub fn json() -> Parser<u8, JsonValue> {
	space() * value() - end()
}

fn main() {
	let input = br#"
	{
        "Image": {
            "Width":  800,
            "Height": 600,
            "Title":  "View from 15th Floor",
            "Thumbnail": {
                "Url":    "http://www.example.com/image/481989943",
                "Height": 125,
                "Width":  100
            },
            "Animated" : false,
            "IDs": [116, 943, 234, 38793]
        }
    }"#;

	println!("{:?}", json().parse(input));
}

You can run this example with the following command:

cargo run --example json

Benchmark

Lifetimes and files

String literals have a static lifetime so they can work with the static version of Parser imported from pom::Parser. Input read from a file has a shorter lifetime. In this case you should import pom::parser::Parser and declare lifetimes on your parser functions. So

fn space() -> Parser<u8, ()> {
    one_of(b" \t\r\n").repeat(0..).discard()
}

would become

fn space<'a>() -> Parser<'a, u8, ()> {
    one_of(b" \t\r\n").repeat(0..).discard()
}