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Monads: Maybe

Build Status NPM version Dependencies Status experimental

A Monad for values that may not be present, or computations that may fail. Maybe(a) explicitly models the effects that implicit in Nullable types, thus has none of the problems associated with using null or undefined — like NullPointerException or TypeError.

Furthermore, being a Monad, Maybe(a) 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 Maybe = require('monads.maybe')
// :: [a], (a -> Bool) -> Maybe(a) 
function find(collection, predicate) {
  for (var i = 0; i < collection.length; ++i) {
    var item = collection[i]
    if (predicate(item))  return Maybe.Just(item)
  return Maybe.Nothing()
var numbers = [-2, -1, 0, 1, 2]
var a = find(numbers, function(a){ return a > 5 })
var b = find(numbers, function(a){ return a === 0 })
// Call a function only if both a and b 
// have values (sequencing) 
a.chain(function(x) {
  return b.chain(function(y) {
    doSomething(x, y)
// Transform values only if they're available:{ return x + 1 })
// => Maybe.Nothing{ return x + 1 })
// => Maybe.Just(1) 
// Use a default value if no value is present 
a.orElse(function(){ return Maybe.Just(-1) })
// => Maybe.Just(-1) 
b.orElse(function(){ return Maybe.Just(-1) })
// => Maybe.Just(0) 


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

$ npm install monads.maybe

Using with CommonJS

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

var Maybe = require('monads.maybe')

Using with AMD

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

require(['monads.maybe'], function(Maybe) {
  ( ... )

Using without modules

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

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

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