prsc

Tiny parser combinators library for JavaScript and TypeScript. Heavily inspired by nom.

Installation

The prsc library can be installed using npm or yarn:

npm install --save prsc

or

yarn add prsc

The package includes both a UMD bundle (dist/prsc.js), compatible with Node.js, and an ES6 module (dist/prsc.mjs). TypeScript typings are included (dist/prsc.d.ts) and should work automatically in most cases.

Usage

This library exports a number of functions that make it easy to build parsers for input represented as a string. Start from primitive parsers such as token or your own functions matching the Parser type, transform results using map and filter and combine them using the other functions such as then and star.

Example

The following parser accepts a simple arithmetic language consisting of the + and * operators:

// Create a primitive parser that accepts a single digit
const digit = (input, offset) => {
	if (/^[0-9]$/.test(input[offset])) {
		return ok(offset + 1);
	}
	return error(offset, ['digit']);
};

// Use that to accept a string of one or more digits
const digits = plus(digit);

// Then use recognize to get the matching string and use map to parse that into a number
const number = map(recognize(digits), (str) => parseInt(str, 10));

// Multiplication
const term = then(
	number,
	star(preceded(token('*'), number)),
	// Multiply everything together
	(num, factors) => factors.reduce((product, factor) => product * factor, num)
);

// Addition
const expression = then(term, star(preceded(token('+'), term)), (num, terms) =>
	terms.reduce((sum, term) => sum + term, num)
);

// Parsing some input
console.log(expression('2*3+4*5', 0));
// > { success: true, offset: 7, value: 26 }

Tips

Use functions to provide a layer of indirection for recursive definitions:

const term = then(number, optional(preceded(token('*'), termIndirect)), ...);
function termIndirect(input, offset) {
	return term(input, offset);
}

Use the typings when working in TypeScript for strongly-typed parsers:

const digit: Parser<string> = ...;
const digits = plus(digit); // Parser<string[]>
const number = map(
	digits,
	strs => parseInt(strs.join(''), 10)
); // Parser<number>