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ES6 flavoured parser combinators.


Read the API documentation.


Eulalie works on the principle of constructing parsers from smaller parsers using various combinator functions.

A parser is a function which takes an input Stream, and returns a ParseResult object if it matched the provided input, or a ParseError object if it didn't.

Data Types

The Stream object just contains a string, and an index into this string. We use this structure instead of passing strings around as input because string operations are expensive, while any operation on the Stream object can be performed in linear time, and while many Stream objects will be created during a parse operation, we only ever keep a single copy of the string they wrap.

A ParseResult contains four properties: the value we parsed (an arbitrary value), the next input to be parsed (a Stream), the point in the stream where we started parsing (also a Stream), and the substring that was matched by this parser (a string).

Finally, a ParseError simply contains an input property (a Stream) which points to the exact position where the parsing failed, and an optional message (a string).

Parser Combinators

The most basic parsers form the building blocks from which you can assemble more complex parsers:

  • unit(value) makes a parser which doesn't consume input, just returns the provided value wrapped in a ParseResult.
  • fail is a parser which consumes no input and returns a ParseError.
  • item is a parser which consumes one arbitrary character and returns it as a ParseResult.

The two fundamental parser combinators are:

  • seq(parser, next) is used to combine multiple parsers in a sequence. It takes a parser, and a function next(value) which will be called with the result of the parser if it succeeded, and must return another parser, which will be run on the remaining input. The result of the combined parser will be the result of this last parser, or the first error encountered.

  • either(parser1, parser2) makes a parser which will first try the first provided parser, and returns its result if it succeeds. If it fails, it will run the second parser on the same input, and return its result directly, whether or not it succeeded.

Using these, you can construct more advanced parser combinators. Some particularly useful combinators are predefined:

  • sat(predicate) makes a parser which will match one character only if the provided predicate function returns true for it.
  • char(c) makes a parser which matches the single character c.
  • many(parser) makes a parser which will match the provided parser zero or more times.
  • many1(parser) works just like many, but requires at minimum one match.
  • string(s) makes a parser which matches the literal string s.

Other predefined parsers are digit, space, alphanum, letter, upper and lower, which match one character of their respective types, and their inverse counterparts, notDigit, notSpace, notAlphanum, notLetter, notUpper and notLower. There are also whitespace matchers spaces and spaces1, and their opposites, notSpaces and notSpaces1.

Generator Functions

The basic combinators seq and either can also take a single generator function, which helps keep callback chains to a minimum. The job of this function is to yield parsers, which will pass their ParseResults back to the generator. The final result value of the parser will be what you return from the generator function. This becomes very useful for the seq combinator:

// without generators:
p.seq(p.notSpaces1, (left) =>
  p.seq(p.spaces1, () =>
    p.seq(p.notSpaces1, (right) =>
      p.unit({left, right}))));

// with generators:
p.seq(function*() {
  const {value: left} = yield p.notSpaces1;
  yield p.spaces1;
  const {value: right} = yield p.notSpaces1;
  return {left, right};

While you could also pass a generator to either, it will, unlike seq, accept any iterable, and just passing an array of parsers to try in sequence is generally better:


The result will be the result of the first parser to succeed.

If you do provide a generator function to either, yield will not receive any values, and what you return will be ignored in favour of the result of the first parser you yield to succeed. But, seriously, just use arrays instead.

A Working Example

This is how you might write a parser for the first line of an HTTP request:

import * as p from "eulalie";

// Combine a sequence of parsers into a bigger parser using a generator
const parser = p.seq(function*() {
  // Parse a sequence of 1 or more upper case letters
  const {value: method} = yield p.many1(p.upper);
  // Consume 1 or more spaces
  yield p.spaces1;
  // Parse a sequence of 1 or more non-whitespace characters
  const {value: path} = yield p.notSpaces1;
  // Consume 1 or more spaces
  yield p.spaces1;
  // Match the string "HTTP/"
  yield p.string("HTTP/");
  // Parse a new sequence
  const {value: version} = yield p.seq(function*() {
    // Parse 1 or more digits
    const {value: left} = yield p.many1(p.digit);
    // Match a period
    yield p.char(".");
    // Parse 1 or more digits
    const {value: right} = yield p.many1(p.digit);
    // Assemble the parsed values
    return `${left}.${right}`;
  // Return the final parsed value
  return {method, path, version};

const result = p.parse(parser,"GET /lol.gif HTTP/1.0"));

assert.deepEqual(result.value, {
  method: "GET",
  path: "/lol.gif",
  version: "1.0"


Copyright 2015 Bodil Stokke

This program is free software: you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License along with this program. If not, see


ES6 flavoured parser combinators




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