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

npm install monads.either
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Monads: Either

Build Status NPM version Dependencies Status experimental

The Either(a, b) monad 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 monad 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.


var Either = require('monads.either')

// + type: String -> Either(Error, String)
function read(path) {
  return exists(path)?    Either.Right(readFile(path))
  :      /* otherwise */  Either.Left(new Error(path + ' does not exist.'))

var intro = read('intro.json')
var outro = read('outro.json') { {
    console.log(intro + outro)


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

$ npm install monads.either

Using with CommonJS

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

var Either = require('monads.either')

Using with AMD

Download the latest release, and require the monads.either.umd.js file:

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

Using without modules

Download the latest release, and load the monads.either.umd.js file. The properties are exposed in the global folktale.monads.Either object:

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

Compiling from source

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 monads.either
$ npm install
$ make bundle

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


You can read the documentation online or build it yourself:

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

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

Platform support

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.

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