A monad for disjunctions. (e.g.: computations that may fail)


The Either(a, b) structure represents the logical disjunction between a and b. In other words, Either may contain either a value of type a or a value of type b, at any given time. This particular implementation is biased on the right value (b), thus projections will take the right value over the left one.

A common use of this structure is to represent computations that may fail, when you want to provide additional information on the failure. This can force failures and their handling to be explicit, and avoid the problems associated with throwing exceptions — non locality, abnormal exits, etc.

Furthermore, being a monad, Either(a, b) can be composed in manners similar to other monads, by using the generic sequencing and composition operations provided for the common interface in [Fantasy Land][].

// Returns the contents of the file at `path`, if it exists. 
//  read : String -> Either(Error, String) 
function read(path) {
  return exists(path)?     Either.Right(fread(path))
  :      /* otherwise */   Either.Left("Non-existing file: " + path)
var intro = read('intro.txt')  // => Right(...) 
var outro = read('outro.txt')  // => Right(...) 
var nope  = read('nope.txt')   // => Left("Non-existing file: nope.txt") 
// We can concatenate things without knowing if they exist at all, and 
// failures are handled for us 
intro.chain(function(a) {
  return {
    return a + b
// => Right(...) 
intro.chain(function(a) {
  return {
    return a + b
// => Left("Non-existing file: nope.txt") 

The easiest way is to grab it from NPM. If you're running in a Browser environment, you can use [Browserify][]

$ npm install data.either

If you're not using NPM, [Download the latest release][release], and require the data.either.umd.js file:

var Either = require('data.either')

[Download the latest release][release], and require the data.either.umd.js file:

require(['data.either'], function(Either) {
  ( ... )

[Download the latest release][release], and load the data.either.umd.js file. The properties are exposed in the global object:

<script src="/path/to/data.either.umd.js"></script>

If you want to compile this library from the source, you'll need [Git][], [Make][], [Node.js][], and run the following commands:

$ git clone git://
$ cd data.either
$ npm install
$ make bundle

This will generate the dist/data.either.umd.js file, which you can load in any JavaScript environment.

You can [read the documentation online][docs] or build it yourself:

$ git clone git://
$ cd data.either
$ npm install
$ make documentation

Then open the file docs/index.html in your browser.

This library assumes an ES5 environment, but can be easily supported in ES3 platforms by the use of shims. Just include [es5-shim][] :)

Copyright (c) 2013 Quildreen Motta.

Released under the MIT licence.

[Fantasy Land]: [Browserify]: [Git]: [Make]: [Node.js]: [es5-shim]: [docs]: [release]: