A monad for disjunctions. (e.g.: computations that may fail)
Last updated 2 years ago by robotlolita .
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$ cnpm install data.either 
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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

Using with CommonJS

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

var Either = require('data.either')

Using with AMD

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

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

Using without modules

Download the latest 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>

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 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 or build it yourself:

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

Then open the file docs/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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