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Monads: Validation

Build Status NPM version Dependencies Status experimental

The Validation(a, b) is a disjunction that's more appropriate for validating inputs, or any use case where you want to aggregate failures. Not only the Validation monad provides a better terminology for working with such cases (Failure and Success versus Left and Right), it also allows one to easily aggregate failures and successes as an Applicative Functor.


var Validation = require('monads.validation')
var Success = Validation.Success
var Failure = Validation.Failure
function isPasswordLongEnough(a) {
  return a.length > 6?    Success(a)
  :      /* otherwise */  Failure("Password must have more than 6 characters")
function isPasswordStrongEnough(a) {
  return /[\W]/.test(a)?  Success(a)
  :      /* otherwise */  Failure("Password must contain special characters")
function isPasswordValid(a) {
  return [isPasswordLongEnough(a), isPasswordStrongEnough(a)]
           .map(function(x){ return x.bimap(liftNel, k) })
           .reduce(function(a, b) { return a.ap(b) })
function liftNel(a) {
  return [a]
function k(a){ return function(b) {
  return a
// => Validation.Failure([ 
//      "Password must have more than 6 characters.", 
//      "Password must contain special characters." 
//    ]) 
// => Validation.Failure([ 
//      "Password must contain special characters." 
//    ]) 
// => Validation.Success("rosesarered$andstuff") 


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

$ npm install monads.validation

Using with CommonJS

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

var Validation = require('monads.validation')

Using with AMD

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

require(['monads.validation'], function(Validation) {
  ( ... )

Using without modules

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

<script src="/path/to/monads.validation.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.validation
$ npm install
$ make bundle

This will generate the dist/monads.validation.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.validation
$ 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.