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Overview

Kessel is a parser combinator library inspired heavily by the Parsec library written in Haskell and its F# counterpart FParsec. It shares many of the features of these libraries, brought to JavaScript, and includes some others.

  • Full UTF-8 support, including 3- and 4-byte characters that JavaScript doesn't regularly support
  • Detailed error messages and facilities to customize error messages
  • Optimized implementations of combinators to help make up for the relative slowness of parser combinators in general
  • Written in a consistent functional style, which makes it natural to create new parsers that look and work just like the packaged ones
  • Selectable backtracking
  • Ability to write arbitrary functions that contain other parsers and can be used as parsers themselves

To illustrate, here's a short parser definition for a CSV parser. This is a fully featured parser which can handle quoted commas, escaped quotes, and the like. This example is taken from a similar example in the online book Real World Haskell. It's shorter than the Haskell/Parsec version, but JavaScript's lack of custom operators means it's perhaps a bit more dense.

import { 
  alt, char, end, join, many, newline, noneOf, run, second, sep,
  seqB, string, value,
} from '@barandis/kessel'

const quotedChar = alt(noneOf('"'), value(string('""'), '"'))

const quotedCell = second(seqB(
  char('"'),
  join(many(quotedChar)),
  char('"', 'quote at end of cell'),
))

const cell = alt(quotedCell, join(many(noneOf(',\n\r'))))
const line = sep(cell, char(','))
const csv = end(line, newline)

const parseCsv = input => run(csv, input)

// CSV input is from Wikipedia's article on comma-separated values
// https://en.wikipedia.org/wiki/Comma-separated_values#Example

const text = `Year,Make,Model,Description,Price
1997,Ford,E350,"ac, abs, moon",3000.00
1999,Chevy,"Venture ""Extended Edition""","",4900.00
1999,Chevy,"Venture ""Extended Edition, Very Large""",,5000.00
1996,Jeep,Grand Cherokee,"MUST SELL! air, moon roof, loaded",4799.00`

const result = parseCsv(text)

// `result` is equal to the following:
// [
//   ['Year', 'Make', 'Model', 'Description', 'Price'],
//   ['1997', 'Ford', 'E350', 'ac, abs, moon', '3000.00'],
//   ['1999', 'Chevy', 'Venture "Extended Edition"', '', '4900.00'],
//   ['1999', 'Chevy', 'Venture "Extended Edition, Very Large"', '', '5000.00'], 
//   ['1996', 'Jeep', 'Grand Cherokee', 'MUST SELL! air, moon roof, loaded', '4799.00']
// ]