Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Type Name Latest commit message Commit time
Failed to load latest commit information.
lib handle case when Task is rejected in `ap` Apr 8, 2016
tools Whitespace cleanup Oct 27, 2014
.gitmodules Remove benchmarks for new branch Dec 13, 2014
.hgtags Added tag v2.2.0 for changeset 8143f7409e54 Jun 21, 2014
.travis.yml Rewrite in JS to use JSDoc Apr 12, 2014 ... Jan 12, 2015
LICENCE Rename to task Mar 22, 2015
Makefile test: Adds tests for new Task#ap Apr 8, 2016 Update Dec 6, 2017
package.json Bump to 3.1.2 Sep 4, 2018


Build statusNPM versionDependencies statusLicenceStable API

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.