Monads: Maybe
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.
Example
var Maybe = require('monads.maybe')
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 })
a.chain(function(x) {
return b.chain(function(y) {
doSomething(x, y)
})
})
a.map(function(x){ return x + 1 })
b.map(function(x){ return x + 1 })
a.orElse(function(){ return Maybe.Just(-1) })
b.orElse(function(){ return Maybe.Just(-1) })
Installing
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://github.com/folktale/monads.maybe.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.
Documentation
You can read the documentation online or build it yourself:
$ git clone git://github.com/folktale/monads.maybe.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 :)
Licence
Copyright (c) 2013 Quildreen Motta.
Released under the MIT licence.