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3.1.2 • Public • Published


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The Task(a, b) structure represents values that depend on time. This allows one to model time-based effects explicitly, such that one can have full knowledge of when they're dealing with delayed computations, latency, or anything that can not be computed immediately.

A common use for this monad is to replace the usual Continuation-Passing Style form of programming, in order to be able to compose and sequence time-dependent effects using the generic and powerful monadic operations.


var Task = require('data.task')
var fs = require('fs')
// read : String -> Task(Error, Buffer)
function read(path) {
  return new Task(function(reject, resolve) {
    fs.readFile(path, function(error, data) {
      if (error)  reject(error)
      else        resolve(data)
// decode : Task(Error, Buffer) -> Task(Error, String)
function decode(task) {
  return {
    return buffer.toString('utf-8')
var intro = decode(read('intro.txt'))
var outro = decode(read('outro.txt'))
// You can use `.chain` to sequence two asynchronous actions, and
// `.map` to perform a synchronous computation with the eventual
// value of the Task.
var concatenated = intro.chain(function(a) {
                     return {
                       return a + b
// But the implementation of Task is pure, which means that you'll
// never observe the effects by using `chain` or `map` or any other
// method. The Task just records the sequence of actions that you
// wish to observe, and defers the playing of that sequence of actions
// for your application's entry-point to call.
// To observe the effects, you have to call the `fork` method, which
// takes a callback for the rejection, and a callback for the success.
  function(error) { throw error }
, function(data)  { console.log(data) }


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

$ npm install data.task

Using with CommonJS

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

var Task = require('data.task')

Using with AMD

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

require(['data.task'], function(Task) {
  ( ... )

Using without modules

Download the latest release, and load the data.task.umd.js file. The properties are exposed in the global Task object:

<script src="/path/to/data.task.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.task
$ npm install
$ make bundle

This will generate the dist/data.task.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.task
$ npm install
$ make documentation

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) 2013-2015 Quildreen Motta.

Released under the MIT licence.

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