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:
// joinPath :: String -> String -> Stringconst joinPath = // readFileP :: String -> String -> Promise Error Bufferconst readFileP = // pipe :: (a -> b) -> (b -> c) -> (a -> c)const pipe = // concatFiles :: String -> Promise Error Stringconst concatFiles = const main =
Get it
npm install --save creed
bower install --save creed
As a module:
// ES2015; // Node/CommonJSvar creed = // AMD
As window.creed
:
<!-- Browser global: window.creed -->
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
.
;; const readFileP = const readFilesP = const append = head + fail; // Oops, typo will throw ReferenceError // Calling append() from nested promise causes// a ReferenceError, but it is not being caught
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
orNODE_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 anError
instance.promise
- the promise that was rejected. This can be used to correlate correspondingunhandledRejection
andrejectionHandled
events for the same promise.
processprocess { // Implement whatever logic your application requires // Log or record error state, etc.} { // 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 anError
instance)event.detail.promise
- the promise that was rejected. This can be used to correlate correspondingunhandledRejection
andrejectionHandled
events for the same promise.
window window { // Implement whatever logic your application requires // Log or record error state, etc.} { // 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.
; { // Fetch the text and return a promise for it return promise;} // Make an async coroutine from a generatorlet getUserProfile = // Call it
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
;; // Make a promised version of fs.readFilelet readFileP =
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.
;;
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.
; /* Run a function, threading in a url parameter */let p = // inject url parameter p
Parameter threading also makes it easy to create reusable tasks that don't rely on closures and scope chain capturing.
; => { var xhr = ; xhr xhr xhr xhr}
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.
; //=> 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
.
; // Fulfilllet resolve promise = promise //=> 123 // Resolve to another promiselet anotherPromise = ...;let resolve promise = //=> make promise's fate the same as anotherPromise's // Rejectlet resolve promise = promise //=> [Error: oops]
resolve :: a|Thenable e a → Promise e a
Coerce a value or Thenable to a promise.
; //=> 123 //=> 123 // coerce any thenable //=> 123
fulfill :: a → Promise e a
Lift a value into a promise.
; //=> 123 // Note the difference from resolve //=> '[object Promise { fulfilled: 123 }]' //=> 123
reject :: Error e => e → Promise e a
Make a rejected promise for an error.
; //=> oops!
never :: Promise e a
Make a promise that remains pending forever.
; // 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.
; // return a transformed value //=> 2 // return transformed promise //=> 2
.catch :: Promise e a → (e → b|Promise e b) → Promise e b
Catch and handle a promise error.
; // recover by returning a new value //=> 123 // recover by returning a promise //=> 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:
- returns a non-promise, it is discarded
- returns a promise that fulfills, it's fulfillment value is discarded
- throws, the thrown error will take precedence
- returns a rejected promise, the rejection will take precedence
// do some cleanup //=> 123 // do some cleanup //=> oops! // do some cleanup //=> oops! after 1 second
As mentioned above, errors from a finally handler take precedence:
// Errors in finally handler take precedence // cleanup failed! //=> finally error //=> 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.
; // return a transformed value //=> 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.
; // transform value //=> 2 // transform error //=> 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.
; //=> 124 //=> 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 = profileText //=> <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.
; ; //=> 'foo' ; //=> 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.
; //=> 'hi' after 5 seconds //=> 'hi' after 6 seconds //=> 'oops' immediately
timeout :: Int → Promise e a → Promise e a
Create a promise that will reject after a specified time unless it settles earlier.
; //=> 'hi' after 1 second //=> 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.
; //=> [123, 456] let promises = promisespromises //=> [123, 456] { } //=> [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()
; //=> 456 //=> 123 //=> [Error: oops] //=> 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()
.
; ; //=> 456 //=> 123 ; //=> 123 ; //=> 123 //=> [RangeError: No fulfilled promises in input] //=> [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.
; // Find all the fulfilled promises in an iterable //=> [ 123, 456 ]
Inspect
isFulfilled :: Promise e a → boolean
Returns true if the promise is fulfilled.
; //=> true //=> false //=> true //=> false //=> false
isRejected :: Promise e a → boolean
Returns true if the promise is rejected.
; //=> false //=> true //=> false //=> false //=> false
isSettled :: Promise e a → boolean
Returns true if the promise is either fulfilled or rejected.
; //=> true //=> true //=> true //=> false //=> false
isPending :: Promise e a → boolean
Returns true if the promise is pending (not yet fulfilled or rejected).
; //=> false //=> false //=> false //=> true //=> 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.
; //=> false //=> false //=> false //=> false //=> true //=> true //=> true //=> 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
.
; //=> 123 //=> throws TypeError //=> 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
.
; //=> throws TypeError //=> 'because' //=> 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.
; // Install creed's ES2015-compliant Promise as globallet NativePromise = // Create a creed promisePromise
Fantasy Land
Creed implements Fantasy Land 2.1: