creed :: async

Sophisticated and functionally-minded async with advanced features: coroutines, promises, ES2015 iterables, fantasy-land.

Creed simplifies async by letting you write coroutines using ES2015 generators and promises, and encourages functional programming via fantasy-land. It also makes uncaught errors obvious by default, and supports other ES2015 features such as iterables.

You can also use babel and the babel-creed-async plugin to write ES7 async functions backed by creed coroutines.

Example

Using creed coroutines, ES2015, and FP to solve the async-problem:

import { runNode, all, coroutine } from 'creed'
import { readFile } from 'fs'
import { join } from 'path'

// joinPath :: String -> String -> String
const joinPath = init => tail => join(init, tail)

// readFileP :: String -> String -> Promise Error Buffer
const readFileP = encoding => file => runNode(readFile, file, {encoding})

// pipe :: (a -> b) -> (b -> c) -> (a -> c)
const pipe = (f, g) => x => g(f(x))

// concatFiles :: String -> Promise Error String
const concatFiles = coroutine(function* (dir) {
    const readUtf8P = pipe(joinPath(dir), readFileP('utf8'))

    const index = yield readUtf8P('index.txt')
    const results = yield all(index.match(/^.*(?=\n)/gm).map(readUtf8P))
    return results.join('')
})

const main = process => concatFiles(process.argv[2])
    .then(s => process.stdout.write(s))

main(process)

Get it

npm install --save creed

bower install --save creed

As a module:

// ES2015
import { resolve, reject, all, ... } from 'creed';

// Node/CommonJS
var creed = require('creed')

// AMD
define(['creed'], function(creed) { ... })

As window.creed:

<!-- Browser global: window.creed -->
<script src="creed/dist/creed.js"></script>

Try it

Creed will work anywhere ES5 works. Here's how to try it.

Creed is REPL friendly, with instant and obvious feedback. Try it out in JSBin or using ES2015 with babel, or try it in a REPL:

Note that although babel supports ES2015 import statements, the babel-node REPL doesn't. Use let + require in the REPL instead.

npm install creed
npm install -g babel-node
babel-node
> let { resolve, delay, all, race } = require('creed')
'use strict'
> resolve('hello')
Promise { fulfilled: hello }
> all([1, 2, 3].map(resolve))
Promise { fulfilled: 1,2,3 }
> let p = delay(1000, 'done!'); p
Promise { pending }
... wait 1 second ...
> p
Promise { fulfilled: done! }
> race([delay(100, 'no'), 'winner'])
Promise { fulfilled: winner }

Errors & debugging

By design, uncaught creed promise errors are fatal. They will crash your program, forcing you to fix or .catch them. You can override this behavior by registering your own error event listener.

Consider this small program, which contains a ReferenceError.

import { all, runNode } from 'creed';
import { readFile } from 'fs';

const readFileP = file => runNode(readFile, file)

const readFilesP = files => all(files.map(readFileP))

const append = (head, tail) => head + fail; // Oops, typo will throw ReferenceError

// Calling append() from nested promise causes
// a ReferenceError, but it is not being caught
readFilesP(process.argv.slice(2))
    .map(contents => contents.reduce(append, ''))
    .then(s => console.log(s))

Running this program (e.g. using babel-node) causes a fatal error, exiting the process with a stack trace:

> babel-node experiments/errors.js file1 file2 ...
/Users/brian/Projects/creed/dist/creed.js:672
		throw value;
		^

ReferenceError: fail is not defined
    at append (/Users/brian/Projects/creed/experiments/errors.js:8:39)
    at Array.reduce (native)
    at readFilesP.map.contents (/Users/brian/Projects/creed/experiments/errors.js:13:31)
    at tryCall (/Users/brian/Projects/creed/dist/creed.js:344:12)
    at Map.fulfilled (/Users/brian/Projects/creed/dist/creed.js:408:3)
    at Fulfilled._runAction (/Users/brian/Projects/creed/dist/creed.js:945:10)
    at Future.run (/Users/brian/Projects/creed/dist/creed.js:871:5)
    at TaskQueue._drain (/Users/brian/Projects/creed/dist/creed.js:131:8)
    at /Users/brian/Projects/creed/dist/creed.js:117:53
    at _combinedTickCallback (internal/process/next_tick.js:67:7)

Async traces

Experimental: V8 only

Fatal stack traces are helpful, but sometimes they aren't enough. Enable async traces for stack traces for even more detail.

Note: Enabling async traces is typically an application-level concern. Libraries that use creed should not enable them in dist builds.

Running the example above with async traces enabled yields a more helpful trace. Notably:

• asynchronous stack frames are shown: both the point at which map is called and the point in the mapping function (which is called asynchronous) are shown.
• the Map operation is called out specifically
• stack frames from within creed are omitted

> CREED_DEBUG=1 babel-node experiments/errors.js file1 file2 ...
/Users/brian/Projects/creed/dist/creed.js:672
		throw value;
		^

ReferenceError: fail is not defined
    at append (/Users/brian/Projects/creed/experiments/errors.js:8:39)
    at Array.reduce (native)
    at readFilesP.map.contents (/Users/brian/Projects/creed/experiments/errors.js:13:31)
 from Map:
    at Object.<anonymous> (/Users/brian/Projects/creed/experiments/errors.js:13:6)
    at loader (/Users/brian/Projects/creed/node_modules/babel-register/lib/node.js:144:5)
    at Object.require.extensions.(anonymous function) [as .js] (/Users/brian/Projects/creed/node_modules/babel-register/lib/node.js:154:7)

Enabling async traces

Enable async traces by:

• NODE_ENV=development or NODE_ENV=test - async traces will be enabled automatically.
• CREED_DEBUG=1 (or any non-empty string) - enables async traces even if NODE_ENV=production or NODE_ENV not set.
• enableAsyncTraces() - programatically enable async traces, e.g. for browsers, etc. where env vars aren't available.
  • disableAsyncTraces() - programatically disable async traces.

Performance impact

Async traces typically have about a 3-4x impact on performance.

That may be just fine for some applications, while not for others. Be sure to assess your application performance needs and budget before running with async traces enabled in production.

Debug events

Creed supports global window events in browsers, and process events in Node, similar to Node's 'uncaughtException' event. This allows applications to register a handler to receive events from all promise implementations that support these global events.

Errors passed to unhandled rejection event handlers will have async traces if they are enabled.

The events are:

• 'unhandledRejection': fired when an unhandled rejection is detected
• 'rejectionHandled': fired when rejection previously reported via an 'unhandledRejection' event becomes handled

Node global process events

The following example shows how to use global process events in Node.js to implement simple debug output. The parameters passed to the process event handlers:

• reason - the rejection reason, typically an Error instance.
• promise - the promise that was rejected. This can be used to correlate corresponding unhandledRejection and rejectionHandled events for the same promise.

process.on('unhandledRejection', reportRejection)
process.on('rejectionHandled', reportHandled)

function reportRejection(error, promise) {
	// Implement whatever logic your application requires
	// Log or record error state, etc.
}

function reportHandled(promise) {
	// Implement whatever logic your application requires
	// Log that error has been handled, etc.
}

Browser window events

The following example shows how to use global window events in browsers to implement simple debug output. The event object has the following extra properties:

• event.detail.reason - the rejection reason (typically an Error instance)
• event.detail.promise - the promise that was rejected. This can be used to correlate corresponding unhandledRejection and rejectionHandled events for the same promise.

window.addEventListener('unhandledRejection', event => {
	// Calling preventDefault() suppresses default rejection logging
	// in favor of your own.
	event.preventDefault()
	reportRejection(event.detail.reason, event.detail.promise)
}, false)

window.addEventListener('rejectionHandled', event => {
	// Calling preventDefault() suppresses default rejection logging
	// in favor of your own.
	event.preventDefault()
	reportHandled(event.detail.promise)
}, false)

function reportRejection(error, promise) {
	// Implement whatever logic your application requires
	// Log or record error state, etc.
}

function reportHandled(promise) {
	// Implement whatever logic your application requires
	// Log that error has been handled, etc.
}

API

Run async tasks

coroutine :: Generator a → (...* → Promise e a)

Create an async coroutine from a promise-yielding generator.

import { coroutine } from 'creed';

function fetchTextFromUrl(url) {
    // Fetch the text and return a promise for it
    return promise;
}

// Make an async coroutine from a generator
let getUserProfile = coroutine(function* (userId) {
    try {
        let profileUrl = yield getUserProfileUrlFromDB(userId)
        let text = yield fetchTextFromUrl(profileUrl)
        return text;
    } catch(e) {
        return getDefaultText()
    }
})

// Call it
getUserProfile(123)
    .then(profile => console.log(profile))

Note: In current implementations of JavaScript, it's not possible to detect with reasonable certainty if the function passed to coroutine is a generator function or not. Creed can't know until the function is executed, causing creed to throw an exception synchronously at run-time.

fromNode :: NodeApi e a → (...* → Promise e a)

type NodeApi e a = ...* → Nodeback e a → () type Nodeback e a = e → a → ()

Turn a Node API into a promise API

import { fromNode } from 'creed';
import { readFile } from 'fs';

// Make a promised version of fs.readFile
let readFileP = fromNode(readFile)

readFileP('theFile.txt', 'utf8')
    .then(contents => console.log(contents))

runNode :: NodeApi e a → ...* → Promise e a

type NodeApi e a = ...* → Nodeback e a → () type Nodeback e a = e → a → ()

Run a Node API and return a promise for its result.

import { runNode } from 'creed';
import { readFile } from 'fs';

runNode(readFile, 'theFile.txt', 'utf8')
    .then(contents => console.log(contents))

runPromise :: Producer e a → ...* → Promise e a

type Producer e a = (...* → Resolve e a → Reject e → ()) type Resolve e a = a|Thenable e a → () type Reject e = e → ()

Run a function to produce a promised result.

import { runPromise } from 'creed';

/* Run a function, threading in a url parameter */
let p = runPromise((url, resolve, reject) => {
    var xhr = new XMLHttpRequest;
    xhr.addEventListener("error", reject)
    xhr.addEventListener("load", resolve)
    xhr.open("GET", url)
    xhr.send(null)
}, 'http://...') // inject url parameter

p.then(result => console.log(result))

Parameter threading also makes it easy to create reusable tasks that don't rely on closures and scope chain capturing.

import { runPromise } from 'creed';

function xhrGet(url, resolve, reject) => {
    var xhr = new XMLHttpRequest;
    xhr.addEventListener("error", reject)
    xhr.addEventListener("load", resolve)
    xhr.open("GET", url)
    xhr.send(null)
}

runPromise(xhrGet, 'http://...')
    .then(result => console.log(result))

merge :: (...* → b) → ...Promise e a → Promise e b

Merge promises by passing their fulfillment values to a merge function. Returns a promise for the result of the merge function. Effectively liftN for promises.

import { merge, resolve } from 'creed';

merge((x, y) => x + y, resolve(123), resolve(1))
    .then(z => console.log(z)) //=> 124

Make promises

future :: () → { resolve: Resolve e a, promise: Promise e a }

type Resolve e a = a|Thenable e a → ()

Create a { resolve, promise } pair, where resolve is a function that seals the fate of promise.

import { future, reject } from 'creed';

// Fulfill
let { resolve, promise } = future()
resolve(123)
promise.then(x => console.log(x)) //=> 123

// Resolve to another promise
let anotherPromise = ...;
let { resolve, promise } = future()
resolve(anotherPromise) //=> make promise's fate the same as anotherPromise's

// Reject
let { resolve, promise } = future()
resolve(reject(new Error('oops')))
promise.catch(e => console.log(e)) //=> [Error: oops]

resolve :: a|Thenable e a → Promise e a

Coerce a value or Thenable to a promise.

import { resolve } from 'creed';

resolve(123)
    .then(x => console.log(x)) //=> 123

resolve(resolve(123))
    .then(x => console.log(x)) //=> 123

resolve(jQuery.get('http://...')) // coerce any thenable
    .then(x => console.log(x)) //=> 123

fulfill :: a → Promise e a

Lift a value into a promise.

import { fulfill, resolve } from 'creed';

fulfill(123)
    .then(x => console.log(x)) //=> 123

// Note the difference from resolve
fulfill(fulfill(123))
    .then(x => console.log(x)) //=> '[object Promise { fulfilled: 123 }]'

resolve(fulfill(123))
    .then(x => console.log(x)) //=> 123

reject :: Error e => e → Promise e a

Make a rejected promise for an error.

import { reject } from 'creed';

reject(new TypeError('oops!'))
    .catch(e => console.log(e.message)) //=> oops!

never :: Promise e a

Make a promise that remains pending forever.

import { never } from 'creed';

never()
    .then(x => console.log(x)) // nothing logged, ever

Note: never consumes virtually no resources. It does not hold references to any functions passed to then, map, chain, etc.

Transform promises

.then :: Promise e a → (a → b|Promise e b) → Promise e b

Promises/A+ then. Transform a promise's value by applying a function to the promise's fulfillment value. Returns a new promise for the transformed result.

import { resolve } from 'creed';

resolve(1)
    .then(x => x + 1) // return a transformed value
    .then(y => console.log(y)) //=> 2

resolve(1)
    .then(x => resolve(x + 1)) // return transformed promise
    .then(y => console.log(y)) //=> 2

.catch :: Promise e a → (e → b|Promise e b) → Promise e b

Catch and handle a promise error.

import { reject, resolve } from 'creed';

reject(new Error('oops!'))
    .catch(e => 123) // recover by returning a new value
    .then(x => console.log(x)) //=> 123

reject(new Error('oops!'))
    .catch(e => resolve(123)) // recover by returning a promise
    .then(x => console.log(x)) //=> 123

.finally :: Promise e a → (() → b|Promise e b) → Promise e a

Perform cleanup side effects regardless of a Promise's outcome. If a finally handler:

1. returns a non-promise, it is discarded
2. returns a promise that fulfills, it's fulfillment value is discarded
3. throws, the thrown error will take precedence
4. returns a rejected promise, the rejection will take precedence

import { reject, resolve, delay } from 'creed'

resolve(123)
  .finally(() => resolve(456)) // do some cleanup
  .then(x => console.log(x)) //=> 123

reject(new Error('oops!'))
  .finally(() => resolve(456)) // do some cleanup
  .catch(e => console.log(e.message)) //=> oops!

reject(new Error('oops!'))
  .finally(() => delay(1000, 456)) // do some cleanup
  .catch(e => console.log(e.message)) //=> oops! after 1 second

As mentioned above, errors from a finally handler take precedence:

// Errors in finally handler take precedence
reject(new Error('oops!'))
  .finally(() => reject(new Error('finally error'))) // cleanup failed!
  .catch(e => console.log(e.message)) //=> finally error

reject(new Error('oops!'))
  .finally(() => {
    throw new Error('finally error') // cleanup failed!
  })
  .catch(e => console.log(e.message)) //=> finally error

.map :: Promise e a → (a → b) → Promise e b

Transform a promise's value by applying a function. The return value of the function will be used verbatim, even if it is a promise. Returns a new promise for the transformed value.

import { resolve } from 'creed';

resolve(1)
    .map(x => x + 1) // return a transformed value
    .then(y => console.log(y)) //=> 2

.bimap :: Promise e a → (e → f) → (a → b) → Promise f b

Fantasy-land Functor. Transform a promise's error or value by applying functions to either. The first function will be applied to the error of a rejected promise, and the second function will be applied to the value of a fulfilled promise. Like map, the return value of the applied function will be used verbatim, even if it is a promise. Returns a new promise for the transformed value.

import { resolve, reject } from 'creed';

resolve(1)
    .bimap(e => new Error('not called'), x => x + 1) // transform value
    .then(y => console.log(y)) //=> 2

reject(new Error('oops'))
    .bimap(e => new Error(e.message + '!!!'), x => x + 1) // transform error
    .catch(e => console.log(e)) //=> Error: oops!!!

.ap :: Promise e (a → b) → Promise e a → Promise e b

Apply a promised function to a promised value. Returns a new promise for the result.

import { resolve } from 'creed';

resolve(x => x + 1)
    .ap(resolve(123))
    .then(y => console.log(y)) //=> 124

resolve(x => y => x+y)
    .ap(resolve(1))
    .ap(resolve(123))
    .then(y => console.log(y)) //=> 124

.chain :: Promise e a → (a → Promise e b) → Promise e b

Sequence async actions. When a promise fulfills, run another async action and return a promise for its result.

let profileText = getUserProfileUrlFromDB(userId)
    .chain(fetchTextFromUrl)

profileText.then(text => console.log(text)) //=> <user profile text>

.or :: Promise e a → Promise e a → Promise e a

(deprecated) .concat :: Promise e a → Promise e a → Promise e a

Note: The name concat is deprecated, use or instead.

Returns a promise equivalent to the earlier of two promises. Preference is given to the callee promise in the case that both promises have already settled.

import { delay, fulfill } from 'creed';

delay(200, 'bar').or(delay(100, 'foo'))
    .then(x => console.log(x)); //=> 'foo'

fulfill(123).or(fulfill(456))
    .then(x => console.log(x)); //=> 123

Control time

delay :: Int → a|Promise e a → Promise e a

Create a delayed promise for a value, or further delay the fulfillment of an existing promise. Delay only delays fulfillment: it has no effect on rejected promises.

import { delay, reject } from 'creed';

delay(5000, 'hi')
    .then(x => console.log(x)) //=> 'hi' after 5 seconds

delay(5000, delay(1000, 'hi'))
    .then(x => console.log(x)) //=> 'hi' after 6 seconds

delay(5000, reject(new Error('oops')))
    .catch(e => console.log(e.message)) //=> 'oops' immediately

timeout :: Int → Promise e a → Promise e a

Create a promise that will reject after a specified time unless it settles earlier.

import { delay } from 'creed';

timeout(2000, delay(1000, 'hi'))
    .then(x => console.log(x)) //=> 'hi' after 1 second

timeout(1000, delay(2000, 'hi')) //=> TimeoutError after 1 second

Resolve Iterables

Creed's iterable functions accept any ES2015 Iterable. Most of the examples in this section show Arrays, but Sets, generators, etc. will work as well.

all :: Iterable (Promise e a) → Promise e [a]

Await all promises from an Iterable. Returns a promise that fulfills with an array containing all input promise fulfillment values, or rejects if at least one input promise rejects.

import { all, resolve } from 'creed';

all([resolve(123), resolve(456)])
    .then(x => console.log(x)) //=> [123, 456]

let promises = new Set()
promises.add(resolve(123))
promises.add(resolve(456))

all(promises)
    .then(x => console.log(x)) //=> [123, 456]

function *generator() {
    yield resolve(123)
    yield resolve(456)
}

all(generator())
    .then(x => console.log(x)) //=> [123, 456]

race :: Iterable (Promise e a) → Promise e a

Returns a promise equivalent to the input promise that settles earliest.

If there are input promises that are already settled or settle simultaneously, race prefers the one encountered first in the iteration order.

Note the differences from any().

Note: As per the ES6-spec, racing an empty iterable returns never()

import { race, resolve, reject, delay, isNever } from 'creed';

race([delay(100, 123), resolve(456)])
    .then(x => console.log(x)) //=> 456

race([resolve(123), reject(456)])
    .then(x => console.log(x)) //=> 123

race([delay(100, 123), reject(new Error('oops'))])
    .catch(e => console.log(e)) //=> [Error: oops]

isNever(race([])) //=> true

any :: Iterable (Promise e a) → Promise e a

Returns a promise equivalent to the input promise that fulfills earliest. If all input promises reject, the returned promise rejects.

If there are input promises that are already fulfilled or fulfill simultaneously, any prefers the one encountered first in the iteration order.

Note the differences from race().

import { any, resolve, reject, delay, isNever } from 'creed';

any([delay(100, 123), resolve(456)])
    .then(x => console.log(x)); //=> 456

any([resolve(123), reject(456)])
    .then(x => console.log(x)) //=> 123

any([reject(456), resolve(123)])
    .then(x => console.log(x)); //=> 123

any([delay(100, 123), reject(new Error('oops'))])
    .catch(e => console.log(e)); //=> 123

any([reject(new Error('foo')), reject(new Error('bar'))])
    .catch(e => console.log(e)) //=> [RangeError: No fulfilled promises in input]

any([])
    .catch(e => console.log(e)) //=> [RangeError: No fulfilled promises in input]

settle :: Iterable (Promise e a) → Promise e [Promise e a]

Returns a promise that fulfills with an array of settled promises.

import { settle, resolve, reject, isFulfilled, getValue } from 'creed';

// Find all the fulfilled promises in an iterable
settle([resolve(123), reject(new Error('oops')), resolve(456)])
    .map(settled => settled.filter(isFulfilled).map(getValue))
    .then(fulfilled => console.log(fulfilled)) //=> [ 123, 456 ]

Inspect

isFulfilled :: Promise e a → boolean

Returns true if the promise is fulfilled.

import { isFulfilled, resolve, reject, delay, never } from 'creed';

isFulfilled(resolve(123))        //=> true
isFulfilled(reject(new Error())) //=> false
isFulfilled(delay(0, 123))       //=> true
isFulfilled(delay(1, 123))       //=> false
isFulfilled(never())             //=> false

isRejected :: Promise e a → boolean

Returns true if the promise is rejected.

import { isRejected, resolve, reject, delay, never } from 'creed';

isRejected(resolve(123))        //=> false
isRejected(reject(new Error())) //=> true
isRejected(delay(0, 123))       //=> false
isRejected(delay(1, 123))       //=> false
isRejected(never())             //=> false

isSettled :: Promise e a → boolean

Returns true if the promise is either fulfilled or rejected.

import { isSettled, resolve, reject, delay, never } from 'creed';

isSettled(resolve(123))        //=> true
isSettled(reject(new Error())) //=> true
isSettled(delay(0, 123))       //=> true
isSettled(delay(1, 123))       //=> false
isSettled(never())             //=> false

isPending :: Promise e a → boolean

Returns true if the promise is pending (not yet fulfilled or rejected).

import { isPending, resolve, reject, delay, never } from 'creed';

isPending(resolve(123))        //=> false
isPending(reject(new Error())) //=> false
isPending(delay(0, 123))       //=> false
isPending(delay(1, 123))       //=> true
isPending(never())             //=> true

isNever :: Promise e a → boolean

Returns true if it is known that the promise will remain pending forever. In practice, this means that the promise is one that was returned by never() or a promise that has been resolved to such.

import { isNever, resolve, reject, delay, never, race } from 'creed';

isNever(resolve(123))         //=> false
isNever(reject(new Error()))  //=> false
isNever(delay(0, 123))        //=> false
isNever(delay(1, 123))        //=> false
isNever(never())              //=> true
isNever(resolve(never()))     //=> true
isNever(delay(1000, never())) //=> true
isNever(race([]))             //=> true

getValue :: Promise e a → a

Extract the value of a fulfilled promise. Throws if called on a pending or rejected promise, so check first with isFulfilled.

import { getValue, resolve, reject, never } from 'creed';

getValue(resolve(123)) //=> 123
getValue(reject())     //=> throws TypeError
getValue(never())      //=> throws TypeError

getReason :: Promise e a → e

Extract the reason of a rejected promise. Throws if called on a pending or fulfilled promise, so check first with isRejected.

import { getReason, resolve, reject, never } from 'creed';

getReason(resolve(123))      //=> throws TypeError
getReason(reject('because')) //=> 'because'
getReason(never())           //=> throws TypeError

Debugging

enableAsyncTraces :: () → ()

Enable async traces. Can be called at any time, but will only trace promises created after it's called. If called multiple times, resets the tracing state each time.

disableAsyncTraces :: () → ()

Disable async traces.

isHandled :: Promise e a → boolean

Returns true if the promise is rejected and the rejection has been "handled", that is, .catch has been called on the promise at least once with an argument that is a Function.

Note that if .catch is called with zero arguments or with an argument that isn't a Function, it does not affect the "handled" state of the promise.

Polyfill

shim :: () → PromiseConstructor|undefined

Polyfill the global Promise constructor with an ES6-compliant creed Promise. If there was a pre-existing global Promise, it is returned.

import { shim } from 'creed';

// Install creed's ES2015-compliant Promise as global
let NativePromise = shim()

// Create a creed promise
Promise.resolve(123)

Fantasy Land

Creed implements Fantasy Land 2.1:

• Functor
• Bifunctor
• Apply
• Applicative
• Alt
• Plus
• Alternative
• Chain
• Monad
• Semigroup
• Monoid