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Fluture offers a control structure similar to Promises, Tasks, Deferreds, and what-have-you. Let's call them Futures.

Much like Promises, Futures represent the value arising from the success or failure of an asynchronous operation (I/O). Though unlike Promises, Futures are lazy and adhere to the monadic interface.

Some of the features provided by Fluture include:

For more information:


With NPM

$ npm install --save fluture

Bundled from a CDN

To load Fluture directly into a browser, a code pen, or Deno, use one of the following downloads from the JSDelivr content delivery network. These are single files that come with all of Fluture's dependencies pre-bundled.

  • Fluture Script: A JavaScript file that adds Fluture to the global scope. Ideal for older browsers and code pens.
  • Fluture Script Minified: The same as above, but minified.
  • Fluture Module: An EcmaScript module with named exports. Ideal for Deno or modern browsers.
  • Fluture Module Minified: A minified EcmaScript module without TypeScript typings. Not recommended for Deno.


EcmaScript Module

Fluture is written as modular JavaScript.

  • On Node 12 and up, Fluture can be loaded directly with import 'fluture'.
  • On some older (minor) Node versions, you may need to import from 'fluture/index.js' instead, and/or pass --experimental-modules to node.
  • On Node versions below 12, the esm loader can be used. Alternatively, there is a CommonJS Module available.
  • Modern browsers can run Fluture directly. If you'd like to try this out, I recommend installing Fluture with Pika or Snowpack. You can also try the bundled module to avoid a package manager.
  • For older browsers, use a bundler such as Rollup or WebPack. Besides the module system, Fluture uses purely ES5-compatible syntax, so the source does not have to be transpiled after bundling. Alternatively, there is a CommonJS Module available.
import {readFile} from 'fs'
import {node, encase, chain, map, fork} from 'fluture'

const getPackageName = file => (
  node (done => { readFile (file, 'utf8', done) })
  .pipe (chain (encase (JSON.parse)))
  .pipe (map (x =>

getPackageName ('package.json')
.pipe (fork (console.error) (console.log))

CommonJS Module

Although the Fluture source uses the EcmaScript module system, the main file points to a CommonJS version of Fluture.

On older environments one or more of the following functions may need to be polyfilled: Object.create, Object.assign and Array.isArray.

const fs = require ('fs')
const Future = require ('fluture')

const getPackageName = function (file) {
  return Future.node (function (done) { fs.readFile (file, 'utf8', done) })
  .pipe (Future.chain (Future.encase (JSON.parse)))
  .pipe ( (function (x) { return }))

getPackageName ('package.json')
.pipe (Future.fork (console.error) (console.log))


Table of contents

Creating new Futures
Converting between Nodeback APIs and Futures
Converting between Promises and Futures
Transforming and combining Futures
Consuming/forking Futures
Concurrency related utilities and data structures
Resource management
Other utilities


The name "Fluture" is a conjunction of "FL" (the acronym to Fantasy Land) and "future". Fluture means butterfly in Romanian: A creature one might expect to see in Fantasy Land.

Credit goes to Erik Fuente for styling the logo, and WEAREREASONABLEPEOPLE for sponsoring the project.


Fantasy Land

  • Future implements Fantasy Land 1.0+ -compatible Alt, Bifunctor, Monad, and ChainRec (of, ap, alt, map, bimap, chain, chainRec).
  • Future.Par implements Fantasy Land 3 -compatible Alternative (of, zero, map, ap, alt).
  • The Future and ConcurrentFuture representatives contain @@type properties for Sanctuary Type Identifiers.
  • The Future and ConcurrentFuture instances contain @@show properties for Sanctuary Show.

Type signatures

The various function signatures are provided in a small language referred to as Hindley-Milner notation.

In summary, the syntax is as follows: InputType -> OutputType. Now, because functions in Fluture are curried, the "output" of a function is often another function. In Hindley-Milner that's simply written as InputType -> InputToSecondFunction -> OutputType and so forth.

By convention, types starting with an upper-case letter are concrete types. When they start with a lower-case letter they're type variables. You can think of these type variables as generic types. So a -> b denotes a function from generic type a to generic type b.

Finally, through so-called constraints, type variables can be forced to conform to an "interface" (or Type Class in functional jargon). For example, MyInterface a => a -> b, denotes a function from generic type a to generic type b, where a must implement MyInterface.

You can read in depth about Hindley-Milner in JavaScript here.


The concrete types you will encounter throughout this documentation:

  • Future - Instances of Future provided by compatible versions of Fluture.
  • ConcurrentFuture - Futures wrapped with (Future.Par).
  • Promise a b - Values which conform to the Promises/A+ specification and have a rejection reason of type a and a resolution value of type b.
  • Nodeback a b - A Node-style callback; A function of signature (a | Nil, b) -> x.
  • Pair a b - An array with exactly two elements: [a, b].
  • Iterator - Objects with next-methods which conform to the Iterator protocol.
  • Cancel - The nullary cancellation functions returned from computations.
  • Throwing e a b - A function from a to b that may throw an exception e.
  • List - Fluture's internal linked-list structure: { head :: Any, tail :: List }.
  • Context - Fluture's internal debugging context object: { tag :: String, name :: String, stack :: String }.

Type classes

Some signatures contain constrained type variables. Generally, these constraints express that some value must conform to a Fantasy Land-specified interface.


Cancellation is a system whereby running Futures get an opportunity to stop what they're doing and release resources that they were holding, when the consumer indicates it is no longer interested in the result.

To cancel a Future, it must be unsubscribed from. Most of the consumption functions return an unsubscribe function. Calling it signals that we are no longer interested in the result. After calling unsubscribe, Fluture guarantees that our callbacks will not be called; but more importantly: a cancellation signal is sent upstream.

The cancellation signal travels all the way back to the source (with the exception of cached Futures - see cache), allowing all parties along the way to clean up.

With the Future constructor, we can provide a custom cancellation handler by returning it from the computation. Let's see what this looks like:

// We use the Future constructor to create a Future instance.
const eventualAnswer = Future (function computeTheAnswer (rej, res) {

  // We give the computer time to think about the answer, which is 42.
  const timeoutId = setTimeout (res, 60000, 42)

  // Here is how we handle cancellation. This signal is received when nobody
  // is interested in the answer any more.
  return function onCancel () {
    // Clearing the timeout releases the resources we were holding.
    clearTimeout (timeoutId)


// Now, let's fork our computation and wait for an answer. Forking gives us
// the unsubscribe function.
const unsubscribe = fork (log ('rejection')) (log ('resolution')) (eventualAnswer)

// After some time passes, we might not care about the answer any more.
// Calling unsubscribe will send a cancellation signal back to the source,
// and trigger the onCancel function.
unsubscribe ()

Many natural sources in Fluture have cancellation handlers of their own. after, for example, does exactly what we've done just now: calling clearTimeout.

Finally, Fluture unsubscribes from Futures that it forks for us, when it no longer needs the result. For example, both Futures passed into race are forked, but once one of them produces a result, the other is unsubscribed from, triggering cancellation. This means that generally, unsubscription and cancellation is fully managed for us behind the scenes.

Stack safety

Fluture interprets our transformations in a stack safe way. This means that none of the following operations result in a RangeError: Maximum call stack size exceeded:

> const add1 = x => x + 1

> let m = resolve (1)

> for (let i = 0; i < 100000; i++) {
.   m = map (add1) (m)
. }

> fork (log ('rejection')) (log ('resolution')) (m)
[resolution]: 100001
> const m = (function recur (x) {
.   const mx = resolve (x + 1)
.   return x < 100000 ? chain (recur) (mx) : mx
. }(1))

> fork (log ('rejection')) (log ('resolution')) (m)
[resolution]: 100001

To learn more about memory and stack usage under different types of recursion, see (or execute) scripts/test-mem.


First and foremost, Fluture type-checks all of its input and throws TypeErrors when incorrect input is provided. The messages they carry are designed to provide enough insight to figure out what went wrong.

Secondly, Fluture catches exceptions that are thrown asynchronously, and exposes them to you in one of two ways:

  1. By throwing an Error when it happens.
  2. By calling your exception handler with an Error.

The original exception isn't used because it might have been any value. Instead, a regular JavaScript Error instance whose properties are based on the original exception is created. Its properties are as follows:

  • name: Always just "Error".
  • message: The original error message, or a message describing the value.
  • reason: The original value that was caught by Fluture.
  • context: A linked list of "context" objects. This is used to create the stack property, and you generally don't need to look at it. If debug mode is not enabled, the list is always empty.
  • stack: The stack trace of the original exception if it had one, or the Error's own stack trace otherwise. If debug mode (see below) is enabled, additional stack traces from the steps leading up to the crash are included.
  • future: The instance of Future that was being consumed when the exception happened. Often printing it as a String can yield useful information. You can also try to consume it in isolation to better identify what's going wrong.

Finally, as mentioned, Fluture has a debug mode wherein additional contextual information across multiple JavaScript ticks is collected, included as an extended "async stack trace" on Errors, and exposed on Future instances.

Debug mode can have a significant impact on performance, and uses up memory, so I would advise against using it in production.

Casting Futures to String

There are multiple ways to print a Future to String. Let's take a simple computation as an example:

const add = a => b => a + b;
const eventualAnswer = ap (resolve (22)) (map (add) (resolve (20)));
  1. Casting it to String directly by calling String(eventualAnswer) or eventualAnswer.toString() will yield an approximation of the code that was used to create the Future. In this case:

    "ap (resolve (22)) (map (a => b => a + b) (resolve (20)))"
  2. Casting it to String using JSON.stringify(eventualAnswer, null, 2) will yield a kind of abstract syntax tree.

      "$": "fluture/Future@5",
      "kind": "interpreter",
      "type": "transform",
      "args": [
          "$": "fluture/Future@5",
          "kind": "interpreter",
          "type": "resolve",
          "args": [
            "$": "fluture/Future@5",
            "kind": "transformation",
            "type": "ap",
            "args": [
                "$": "fluture/Future@5",
                "kind": "interpreter",
                "type": "resolve",
                "args": [
            "$": "fluture/Future@5",
            "kind": "transformation",
            "type": "map",
            "args": [


When using this module with Sanctuary Def (and Sanctuary by extension) one might run into the following issue:

> import S from 'sanctuary'

> import {resolve} from 'fluture'

> S.I (resolve (1))
! TypeError: Since there is no type of which all the above values are members,
. the type-variable constraint has been violated.

This happens because Sanctuary Def needs to know about the types created by Fluture to determine whether the type-variables are consistent.

To let Sanctuary know about these types, we can obtain the type definitions from fluture-sanctuary-types and pass them to S.create:

> import sanctuary from 'sanctuary'

> import {env as flutureEnv} from 'fluture-sanctuary-types'

> import {resolve} from 'fluture'

> const S = sanctuary.create ({checkTypes: true, env: sanctuary.env.concat (flutureEnv)})

> fork (log ('rejection'))
.      (log ('resolution'))
.      (S.I (resolve (42)))
[resolution]: 42

Incompatible Fluture Versions

Most versions of Fluture understand how to consume instances from most other versions, even across Fluture's major releases. This allows for different packages that depend on Fluture to interact.

However, sometimes it's unavoidable that a newer version of Fluture is released that can no longer understand older versions, and vice-versa. This only ever happens on a major release, and will be mentioned in the breaking change log. When two incompatible versions of Fluture meet instances, they do their best to issue a clear error message about it.

When this happens, you need to manually convert the older instance to a newer instance of Future. When isFuture returns false, a conversion is necessary. You can also apply this trick if the Future comes from another library similar to Fluture.

const NoFuture = require ('incompatible-future')

const incompatible = NoFuture.of ('Hello')

const compatible = Future ((rej, res) => {
  return NoFuture.fork (rej) (res) (incompatible)

both (compatible) (resolve ('world'))

Creating Futures


Future :: ((a -> Undefined, b -> Undefined) -> Cancel) -> Future a b

Creates a Future with the given computation. A computation is a function which takes two callbacks. Both are continuations for the computation. The first is reject, commonly abbreviated to rej; The second is resolve, or res. When the computation is finished (possibly asynchronously) it may call the appropriate continuation with a failure or success value.

Additionally, the computation must return a nullary function containing cancellation logic. See Cancellation.

If you find that there is no way to cancel your computation, you can return a noop function as a cancellation function. However, at this point there is usually a more fitting way to create that Future (like for example via node).

> fork (log ('rejection'))
.      (log ('resolution'))
.      (Future (function computation (reject, resolve) {
.        const t = setTimeout (resolve, 20, 42)
.        return () => clearTimeout (t)
.      }))
[resolution]: 42


resolve :: b -> Future a b

Creates a Future which immediately resolves with the given value.

> fork (log ('rejection'))
.      (log ('resolution'))
.      (resolve (42))
[answer]: 42


reject :: a -> Future a b

Creates a Future which immediately rejects with the given value.

> fork (log ('rejection'))
.      (log ('resolution'))
.      (reject ('It broke!'))
[rejection]: "It broke!"


after :: Number -> b -> Future a b

Creates a Future which resolves with the given value after the given number of milliseconds.

> fork (log ('rejection'))
.      (log ('resolution'))
.      (after (20) (42))
[resolution]: 42


rejectAfter :: Number -> a -> Future a b

Creates a Future which rejects with the given reason after the given number of milliseconds.

> fork (log ('rejection'))
.      (log ('resolution'))
.      (rejectAfter (20) ('It broke!'))
[rejection]: "It broke!"


go :: (() -> Iterator) -> Future a b

A way to do async/await with Futures, similar to Promise Coroutines or Haskell Do-notation.

Takes a function which returns an Iterator, commonly a generator-function, and chains every produced Future over the previous.

> fork (log ('rejection')) (log ('resolution')) (go (function*() {
.   const thing = yield after (20) ('world')
.   const message = yield after (20) ('Hello ' + thing)
.   return message + '!'
. }))
[resolution]: "Hello world!"

A rejected Future short-circuits the whole coroutine.

> fork (log ('rejection')) (log ('resolution')) (go (function*() {
.   const thing = yield reject ('It broke!')
.   const message = yield after (20) ('Hello ' + thing)
.   return message + '!'
. }))
[rejection]: "It broke!"

To handle rejections inside the coroutine, we need to coalesce the error into our control domain.

I recommend using coalesce with an Either.

> const control = coalesce (S.Left) (S.Right)

> fork (log ('rejection')) (log ('resolution')) (go (function*() {
.   const thing = yield control (reject ('It broke!'))
.   return S.either (x => `Oh no! ${x}`)
.                   (x => `Yippee! ${x}`)
.                   (thing)
. }))
[resolution]: "Oh no! It broke!"


attempt :: Throwing e Undefined r -> Future e r

Creates a Future which resolves with the result of calling the given function, or rejects with the error thrown by the given function.

Short for encase (f) (undefined).

> const data = {foo: 'bar'}

> fork (log ('rejection'))
.      (log ('resolution'))
.      (attempt (() =>
[rejection]: new TypeError ("Cannot read property 'baz' of undefined")


attemptP :: (Undefined -> Promise a b) -> Future a b

Create a Future which when forked spawns a Promise using the given function and resolves with its resolution value, or rejects with its rejection reason.

Short for encaseP (f) (undefined).

> fork (log ('rejection'))
.      (log ('resolution'))
.      (attemptP (() => Promise.resolve (42)))
[resolution]: 42


node :: (Nodeback e r -> x) -> Future e r

Creates a Future which rejects with the first argument given to the function, or resolves with the second if the first is not present.

Note that this function does not support cancellation.

> fork (log ('rejection'))
.      (log ('resolution'))
.      (node (done => done (null, 42)))
[resolution]: 42


encase :: Throwing e a r -> a -> Future e r

Takes a function and a value, and returns a Future which when forked calls the function with the value and resolves with the result. If the function throws an exception, it is caught and the Future will reject with the exception.

Applying encase with a function f creates a "safe" version of f. Instead of throwing exceptions, the encased version always returns a Future.

> fork (log ('rejection'))
.      (log ('resolution'))
.      (encase (JSON.parse) ('{"foo" = "bar"}'))
[rejection]: new SyntaxError ('Unexpected token =')


encaseP :: (a -> Promise e r) -> a -> Future e r

Turns Promise-returning functions into Future-returning functions.

Takes a function which returns a Promise, and a value, and returns a Future. When forked, the Future calls the function with the value to produce the Promise, and resolves with its resolution value, or rejects with its rejection reason.

> encaseP (fetch) ('')
. .pipe (chain (encaseP (res => res.json ())))
. .pipe (map (user =>
. .pipe (fork (log ('rejection')) (log ('resolution')))
[resolution]: "Aldwin Vlasblom"

Transforming Futures


map :: Functor m => (a -> b) -> m a -> m b

Transforms the resolution value inside the Future or Functor, and returns a Future or Functor with the new value. The transformation is only applied to the resolution branch: if the Future is rejected, the transformation is ignored.

See also chain and mapRej.

> fork (log ('rejection'))
.      (log ('resolution'))
.      (map (x => x + 1) (resolve (41)))
[resolution]: 42

For comparison, an approximation with Promises is:

> Promise.resolve (41)
. .then (x => x + 1)
. .then (log ('resolution'), log ('rejection'))
[resolution]: 42


bimap :: Bifunctor m => (a -> c) -> (b -> d) -> m a b -> m c d

Maps the left function over the rejection reason, or the right function over the resolution value, depending on which is present. Can be used on any Bifunctor.

> fork (log ('rejection'))
.      (log ('resolution'))
.      (bimap (x => x + '!') (x => x + 1) (resolve (41)))
[resolution]: 42

> fork (log ('rejection'))
.      (log ('resolution'))
.      (bimap (x => x + '!') (x => x + 1) (reject ('It broke!')))
[rejection]: "It broke!!"

For comparison, an approximation with Promises is:

> Promise.resolve (41)
. .then (x => x + 1, x => Promise.reject (x + '!'))
. .then (log ('resolution'), log ('rejection'))
[resolution]: 42

> Promise.reject ('It broke!')
. .then (x => x + 1, x => Promise.reject (x + '!'))
. .then (log ('resolution'), log ('rejection'))
[rejection]: "It broke!!"


chain :: Chain m => (a -> m b) -> m a -> m b

Sequence a new Future or Chain using the resolution value from another. Similarly to map, chain expects a function. But instead of returning the new value, chain expects a Future (or instance of the same Chain) to be returned.

The transformation is only applied to the resolution branch: if the Future is rejected, the transformation is ignored.

See also chainRej.

> fork (log ('rejection'))
.      (log ('resolution'))
.      (chain (x => resolve (x + 1)) (resolve (41)))
[resolution]: 42

For comparison, an approximation with Promises is:

> Promise.resolve (41)
. .then (x => Promise.resolve (x + 1))
. .then (log ('resolution'), log ('rejection'))
[resolution]: 42


bichain :: (a -> Future c d) -> (b -> Future c d) -> Future a b -> Future c d

Sequence a new Future using either the resolution or the rejection value from another. Similarly to bimap, bichain expects two functions. But instead of returning the new value, bichain expects Futures to be returned.

> fork (log ('rejection'))
.      (log ('resolution'))
.      (bichain (resolve) (x => resolve (x + 1)) (resolve (41)))
[resolution]: 42

> fork (log ('rejection'))
.      (log ('resolution'))
.      (bichain (x => resolve (x + 1)) (resolve) (reject (41)))
[resolution]: 42

For comparison, an approximation with Promises is:

> Promise.resolve (41)
. .then (x => Promise.resolve (x + 1), Promise.resolve)
. .then (log ('resolution'), log ('rejection'))
[resolution]: 42

> Promise.reject (41)
. .then (Promise.resolve, x => Promise.resolve (x + 1))
. .then (log ('resolution'), log ('rejection'))
[resolution]: 42


swap :: Future a b -> Future b a

Swap the rejection and resolution branches.

> fork (log ('rejection'))
.      (log ('resolution'))
.      (swap (resolve (42)))
[rejection]: 42

> fork (log ('rejection'))
.      (log ('resolution'))
.      (swap (reject (42)))
[resolution]: 42


mapRej :: (a -> c) -> Future a b -> Future c b

Map over the rejection reason of the Future. This is like map, but for the rejection branch.

> fork (log ('rejection'))
.      (log ('resolution'))
.      (mapRej (s => `Oh no! ${s}`) (reject ('It broke!')))
[rejection]: "Oh no! It broke!"

For comparison, an approximation with Promises is:

> Promise.reject ('It broke!')
. .then (null, s => Promise.reject (`Oh no! ${s}`))
. .then (log ('resolution'), log ('rejection'))
[rejection]: "Oh no! It broke!"


chainRej :: (a -> Future c b) -> Future a b -> Future c b

Chain over the rejection reason of the Future. This is like chain, but for the rejection branch.

> fork (log ('rejection'))
.      (log ('resolution'))
.      (chainRej (s => resolve (`${s} But it's all good.`)) (reject ('It broke!')))
[resolution]: "It broke! But it's all good."

For comparison, an approximation with Promises is:

> Promise.reject ('It broke!')
. .then (null, s => `${s} But it's all good.`)
. .then (log ('resolution'), log ('rejection'))
[resolution]: "It broke! But it's all good."


coalesce :: (a -> c) -> (b -> c) -> Future a b -> Future d c

Applies the left function to the rejection value, or the right function to the resolution value, depending on which is present, and resolves with the result.

This provides a convenient means to ensure a Future is always resolved. It can be used with other type constructors, like S.Either, to maintain a representation of failure.

> fork (log ('rejection'))
.      (log ('resolution'))
.      (coalesce (S.Left) (S.Right) (resolve ('hello'))
[resolution]: Right ("hello")

> fork (log ('rejection'))
.      (log ('resolution'))
.      (coalesce (S.Left) (S.Right) (reject ('It broke!'))
[resolution]: Left ("It broke!")

For comparison, an approximation with Promises is:

> Promise.resolve ('hello')
. .then (S.Right, S.Left)
. .then (log ('resolution'), log ('rejection'))
[resolution]: Right ("hello")

> Promise.reject ('It broke!')
. .then (S.Right, S.Left)
. .then (log ('resolution'), log ('rejection'))
[resolution]: Left ("It broke!")

Combining Futures


ap :: Apply m => m a -> m (a -> b) -> m b

Applies the function contained in the right-hand Future or Apply to the value contained in the left-hand Future or Apply. This process can be repeated to gradually fill out multiple function arguments of a curried function, as shown below.

Note that the Futures will be executed in sequence - not in parallel* - because of the Monadic nature of Futures. The execution order is, as specified by Fantasy Land, m (a -> b) first followed by m a. So that's right before left.

* Have a look at pap for an ap function that runs its arguments in parallel. If you must use ap (because you're creating a generalized function), but still want Futures passed into it to run in parallel, then you could use ConcurrentFuture instead.

> fork (log ('rejection'))
.      (log ('resolution'))
.      (ap (resolve (7)) (ap (resolve (49)) (resolve (x => y => x - y))))
[resolution]: 42


pap :: Future a b -> Future a (b -> c) -> Future a c

Has the same signature and function as ap, but runs the two Futures given to it in parallel. See also ConcurrentFuture for a more general way to achieve this.

> fork (log ('rejection'))
.      (log ('resolution'))
.      (pap (resolve (7)) (pap (resolve (49)) (resolve (x => y => x - y))))
[resolution]: 42


alt :: Alt f => f a -> f a -> f a

Select one of two Alts.

Behaves like logical or on Future instances, returning a new Future which either resolves with the first resolution value, or rejects with the last rejection reason. We can use it if we want a computation to run only if another has failed.

Note that the Futures will be executed in sequence - not in parallel* - because of the Monadic nature of Futures. The right Future is evaluated before the left Future.

See also and and lastly.

* If you'd like to use a parallel implementation of alt, you could simply use race. Alternatively you could wrap your Future instances with Par before passing them to alt.

> fork (log ('rejection'))
.      (log ('resolution'))
.      (alt (resolve ('left')) (resolve ('right')))
[resolution]: "right"

> fork (log ('rejection'))
.      (log ('resolution'))
.      (alt (resolve ('left')) (reject ('It broke!')))
[resolution]: "left"


and :: Future a c -> Future a b -> Future a c

Logical and for Futures.

Returns a new Future which either rejects with the first rejection reason, or resolves with the last resolution value once and if both Futures resolve. We can use it if we want a computation to run only after another has succeeded. The right Future is evaluated before the left Future.

See also alt and lastly.

> fork (log ('rejection'))
.      (log ('resolution'))
.      (and (resolve ('left')) (resolve ('right')))
[resolution]: "left"

> fork (log ('rejection'))
.      (log ('resolution'))
.      (and (resolve ('left')) (reject ('It broke!')))
[rejection]: "It broke!"


lastly :: Future a c -> Future a b -> Future a b

Run a second Future after the first settles (successfully or unsuccessfully). Rejects with the rejection reason from the first or second Future, or resolves with the resolution value from the first Future. This can be used to run a computation after another settles, successfully or unsuccessfully.

If you're looking to clean up resources after running a computation which acquires them, you should use hook, which has many more fail-safes in place.

See also and and alt.

> fork (log ('rejection'))
.      (log ('resolution'))
.      (lastly (encase (log ('lastly')) ('All done!')) (resolve (42)))
[lastly]: "All done!"
[resolution]: 42

Consuming Futures


fork :: (a -> Any) -> (b -> Any) -> Future a b -> Cancel

Execute the computation represented by a Future, passing reject and resolve callbacks to continue once there is a result.

This function is called fork because it literally represents a fork in our program: a point where a single code-path splits in two. It is recommended to keep the number of calls to fork at a minimum for this reason. The more forks, the higher the code complexity.

Generally, one only needs to call fork in a single place in the entire program.

After we fork a Future, the computation will start running. If the program decides halfway through that it's no longer interested in the result of the computation, it can call the unsubscribe function returned by fork. See Cancellation.

If an exception was encountered during the computation, it will be re-thrown by fork and likely not be catchable. You can handle it using process.on('uncaughtException') in Node, or use forkCatch.

Almost all code examples in Fluture use fork to run the computation. There are some variations on fork that serve different purposes below.


forkCatch :: (Error -> Any) -> (a -> Any) -> (b -> Any) -> Future a b -> Cancel

An advanced version of fork that allows us to react to a fatal error in a custom way. Fatal errors occur when unexpected exceptions are thrown, when the Fluture API is used incorrectly, or when resources couldn't be disposed.

The exception handler will always be called with an instance of Error, independent of what caused the crash.

Using this function is a trade-off;

Generally it's best to let a program crash and restart when an a fatal error occurs. Restarting is the surest way to restore the memory that was allocated by the program to an expected state.

By using forkCatch, we can keep our program alive after a fatal error, which can be very beneficial when the program is being used by multiple clients. However, since fatal errors might indicate that something, somewhere has entered an invalid state, it's probably still best to restart our program upon encountering one.

See Debugging for information about the Error object that is passed to your exception handler.

> forkCatch (log ('fatal error'))
.           (log ('rejection'))
.           (log ('resolution'))
.           (map (x => (resolve (null)))
[fatal error]: new Error ("Cannot read property 'foo' of null")


value :: (b -> Any) -> Future a b -> Cancel

Like fork but for the resolution branch only. Only use this function if you are sure the Future is going to be resolved, for example; after using coalesce. If the Future rejects, value will throw an Error.

As with fork, value returns an unsubscribe function. See Cancellation.

> value (log ('resolution')) (resolve (42))
[resolution]: 42


done :: Nodeback a b -> Future a b -> Cancel

Run the Future using a Nodeback as the continuation.

This is like fork, but instead of taking two unary functions, it takes a single binary function.

As with fork, done returns an unsubscribe function. See Cancellation.

> done ((err, val) => log ('resolution') (val)) (resolve (42))
[resolution]: 42


promise :: Future Error a -> Promise Error a

Run the Future and get a Promise to represent its continuation.

Returns a Promise which resolves with the resolution value, or rejects with the rejection reason of the Future.

If an exception was encountered during the computation, the promise will reject with it. I recommend using coalesce before promise to ensure that exceptions and rejections are not mixed into the Promise rejection branch.

Cancellation capabilities are lost when using promise to consume the Future.

> promise (resolve (42)) .then (log ('resolution'))
[resolution]: 42

> promise (reject ('failure')) .then (log ('resolution'), log ('rejection'))
[rejection]: "failure"



race :: Future a b -> Future a b -> Future a b

Race two Futures against each other. Creates a new Future which resolves or rejects with the resolution or rejection value of the first Future to settle.

When one Future settles, the other gets cancelled automatically.

> fork (log ('rejection'))
.      (log ('resolution'))
.      (race (after (15) ('left')) (after (30) ('right')))
[resolution]: "left"


both :: Future a b -> Future a c -> Future a (Pair b c)

Run two Futures in parallel and get a Pair of the results. When either Future rejects, the other Future will be cancelled and the resulting Future will reject.

> fork (log ('rejection'))
.      (log ('resolution'))
.      (both (after (15) ('left')) (after (30) ('right')))
[resolution]: ["left", "right"]


parallel :: PositiveInteger -> Array (Future a b) -> Future a (Array b)

Creates a Future which when forked runs all Futures in the given Array in parallel, ensuring no more than limit Futures are running at once.

In the following example, we're running up to 5 Futures in parallel. Every Future takes about 20ms to settle, which means the result should appear after about 40ms.

If we use 1 for the limit, the Futures would run in sequence, causing the result to appear only after 200ms.

We can also use Infinity as the limit. This would create a function similar to Promise.all, which always runs all Futures in parallel. This can easily cause the computation to consume too many resources, however, so I would advise using a number roughly equal to maximum size of Array you think your program should handle.

> fork (log ('rejection'))
.      (log ('resolution'))
.      (parallel (5) (Array.from (Array (10) .keys ()) .map (after (20))))
[resolution]: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

When one Future rejects, all currently running Futures will be cancelled and the resulting Future will reject. If you want to settle all Futures, even if some may fail, you can use parallel in combination with coalesce.

> fork (log ('rejection'))
.      (log ('resolution'))
.      (parallel (2) ([resolve (42), reject ('It broke!')]
.                     .map (coalesce (S.Left) (S.Right))))
[resolution]: [Right (42), Left ("It broke!")]


The ConcurrentFuture type is very similar to the Future type, except that it has parallel semantics where Future has sequential semantics.

These sematics are most notable in the implementation of Applicative for ConcurrentFuture. When using ap on two ConcurrentFutures, they run parallely, whereas regular Future instances would've run sequentially. This means that ConcurrentFuture cannot be a Monad, which is why we have it as a separate type.

The implementation of Alternative on ConcurrentFuture has parallel semantics as well. Whereas alt on regular Futures uses the failure effect to determine a winner, on ConcurrentFutures timing is used, and the winner will be whichever ConcurrentFuture settled first.

The idea is that we can switch back and forth between Future and ConcurrentFuture, using Par and seq, to get sequential or concurrent behaviour respectively. It's a useful type to pass to abstractions that don't know about Future-specific functions like parallel or race, but do know how to operate on Apply and Alternative.

//Some dummy values
const x = 41;
const f = a => a + 1;

//The following two are equal ways to construct a ConcurrentFuture
const parx = S.of (Par) (x)
const parf = Par (S.of (Future) (f))

//We can make use of parallel apply
value (log ('resolution')) (seq (ap (parx) (parf)))
[resolution]: 42

//Concurrent sequencing
value (log ('resolution')) (seq (S.sequence (Par) ([parx, parx, parx])))
[resolution]: [41, 41, 41]

//And concurrent alt
value (log ('resolution')) (alt (after (15) ('left')) (after (30) ('right')))
[resolution]: "left"
Par :: Future a b -> ConcurrentFuture a b

Converts a Future to a ConcurrentFuture.


Converts a ConcurrentFuture to a Future.

seq :: ConcurrentFuture a b -> Future a b

Resource management

Functions listed under this category allow for more fine-grained control over the flow of acquired values.


hook :: Future a b -> (b -> Future c d) -> (b -> Future a e) -> Future a e

Combines resource acquisition, consumption, and disposal in such a way that you can be sure that a resource will always be disposed if it was acquired, even if an exception is thrown during consumption; Sometimes referred to as bracketing.

The signature is like hook (acquire, dispose, consume), where:

  • acquire is a Future which might create connections, open files, etc.
  • dispose is a function that takes the result from acquire and should be used to clean up (close connections etc). The Future it returns must resolve, and its resolution value is ignored. If it rejects, a fatal error is raised which can only be handled with forkCatch.
  • consume is another Function takes the result from acquire, and may be used to perform any arbitrary computations using the resource.

Typically, you'd want to partially apply this function with the first two arguments (acquisition and disposal), as shown in the example.

> import {open, read, close} from 'fs'

> const withFile = hook (node (done => open ('package.json', 'r', done)))
.                       (fd => node (done => close (fd, done)))

> fork (log ('rejection'))
.      (log ('resolution'))
.      (withFile (fd => node (done => (
.        read (fd, Buffer.alloc (1), 0, 1, null, (e, _, x) => done (e, x)))
.      )))
[resolution]: <Buffer 7b>

When a hooked Future is cancelled while acquiring its resource, nothing else will happen. When it's cancelled after acquistion completes, however, the disposal will still run, and if it fails, an exception will be thrown.

If you have multiple resources that you'd like to consume all at once, you can use Fluture Hooks to combine multiple hooks into one.

Utility functions


Future.prototype.pipe :: Future a b ~> (Future a b -> c) -> c

A method available on all Futures to allow arbitrary functions over Futures to be included in a fluent-style method chain.

You can think of this as a fallback for the ESNext pipe operator (|>).

> resolve (x => y => x * y)
. .pipe (ap (after (20) (Math.PI)))
. .pipe (ap (after (20) (13.37)))
. .pipe (map (Math.round))
. .pipe (fork (log ('rejection')) (log ('resolution')))
[resolution]: 42


cache :: Future a b -> Future a b

Returns a Future which caches the resolution value or rejection reason of the given Future so that whenever it's forked, it can load the value from cache rather than re-executing the underlying computation.

This essentially turns a unicast Future into a multicast Future, allowing multiple consumers to subscribe to the same result. The underlying computation is never cancelled unless all consumers unsubscribe before it completes.

There is a glaring drawback to using cache, which is that returned Futures are no longer referentially transparent, making reasoning about them more difficult and refactoring code that uses them harder.

> import {readFile} from 'fs'

> const eventualPackageName = (
.   node (done => readFile ('package.json', 'utf8', done))
.   .pipe (chain (encase (JSON.parse)))
.   .pipe (chain (encase (x =>
.   .pipe (map (data => {
.      log ('debug') ('Read, parsed, and traversed the package data')
.      return data
.    }))
. )

> fork (log ('rejection')) (log ('resolution')) (eventualPackageName)
[debug]: "Read, parsed, and traversed the package data"
[resolution]: "Fluture"

> fork (log ('rejection')) (log ('resolution')) (eventualPackageName)
[debug]: "Read, parsed, and traversed the package data"
[resolution]: "Fluture"

> const eventualCachedPackageName = cache (eventualPackageName)

> fork (log ('rejection')) (log ('resolution')) (eventualCachedPackageName)
[debug]: "Read, parsed, and traversed the package data"
[resolution]: "Fluture"

> fork (log ('rejection')) (log ('resolution')) (eventualCachedPackageName)
[resolution]: "Fluture"


isFuture :: a -> Boolean

Returns true for Futures and false for everything else. This function (and also return true for instances of Future that were created within other contexts. It is therefore recommended to use this over instanceof, unless your intent is to explicitly check for Futures created using the exact Future constructor you're testing against.

> isFuture (resolve (42))

> isFuture (42)


never :: Future a b

A Future that never settles. Can be useful as an initial value when reducing with race, for example.


isNever :: a -> Boolean

Returns true if the given input is a never.


extractLeft :: Future a b -> Array a

Returns an array whose only element is the rejection reason of the Future. In many cases it will be impossible to extract this value; In those cases, the array will be empty. This function is meant to be used for type introspection: it is not the correct way to consume a Future.


extractRight :: Future a b -> Array b

Returns an array whose only element is the resolution value of the Future. In many cases it will be impossible to extract this value; In those cases, the array will be empty. This function is meant to be used for type introspection: it is not the correct way to consume a Future.


debugMode :: Boolean -> Undefined

Enable or disable Fluture's debug mode. Debug mode is disabled by default. Pass true to enable, or false to disable.

debugMode (true)

For more information, see Debugging and Context.


Future.prototype.context :: Future a b ~> List Context

A linked list of debugging contexts made available on every instance of Future. When debug mode is disabled, the list is always empty.

The context objects have stack properties which contain snapshots of the stacktraces leading up to the creation of the Future instance. They are used by Fluture to generate contextual stack traces.


MIT licensed