fluture

Fluture

A complete Fantasy Land compatible Future library.

npm install --save fluture ^{Requires a node 5.0.0 compatible environment like modern browsers, transpilers or Node 5+}

Usage

Using the low level, high performance method API:

const Future = require('fluture');
const program = file =>
  Future.node(done => fs.readFile(file, 'utf8', done))
  .chain(Future.encase(JSON.parse))
  .map(x => x.name)
  .fork(console.error, console.log);
program('package.json');
//> "fluture"

Or use the high level, fully curried, functional dispatch API for function composition using composers like S.pipe:

const {node, chain, encase, map, fork} = require('fluture');
const program = S.pipe([
  file => node(fs.readFile.bind(fs, file, 'utf8')),
  chain(encase(JSON.parse)),
  map(x => x.name),
  fork(console.error, console.log)
]);
program('package.json')
//> "fluture"

Motivation

Existing implementations of Future are a pain to debug. This library was made in an effort to provide great error messages when something goes wrong. The library also comes bundled with many async control utilities. To prevent these features from coming at the cost of performance, Fluture was optimized to operate at high performance. For an overview of differences between Fluture and other Future implementations, look at this wiki article.

Documentation

Type signatures

Hindley-Milner type signatures are used to document functions. A list of all types used within these signatures follows:

• Forkable - Any Object with a fork method that takes at least two arguments. This includes instances of Fluture, instances of Task from data.task or instances of Future from ramda-fantasy.
• Future - Instances of Future provided by Fluture.
• Functor - Any object that has a map method which satisfies the Fantasy Land Functor specification.
• Chain - Any object that has a chain method which satisfies the Fantasy Land Chain specification.
• Apply - Any object that has an ap method which satisfies the Fantasy Land Apply specification.

Constructors

Future :: ((a -> Void), (b -> Void) -> Void) -> Future a b

A (monadic) container which represents an eventual value. A lot like Promise but more principled in that it follows the Fantasy Land algebraic JavaScript specification.

const eventualThing = Future((reject, resolve) => {
  setTimeout(resolve, 500, 'world');
});

eventualThing.fork(
  console.error,
  thing => console.log(`Hello ${thing}!`)
);
//> "Hello world!"

of :: b -> Future a b

A constructor that creates a Future containing the given value.

const eventualThing = Future.of('world');
eventualThing.fork(
  console.error,
  thing => console.log(`Hello ${thing}!`)
);
//> "Hello world!"

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

A constructor that creates a Future containing the given value after n milliseconds.

const eventualThing = Future.after(500, 'world');
eventualThing.fork(console.error, thing => console.log(`Hello ${thing}!`));
//> "Hello world!"

cast :: Forkable a b -> Future a b

Cast any Forkable to a Future.

Future.cast(require('data.task').of('hello')).value(console.log);
//> "hello"

encase :: (a -> !b | c) -> a -> Future b c

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

const data = '{"foo" = "bar"}';
const parseJson = Future.encase(JSON.parse);
parseJson('a').fork(console.error, console.log)
//> [SyntaxError: Unexpected token =]

try :: (Void -> !a | b) -> Future a b

A constructor that creates a Future which resolves with the result of calling the given function, or rejects with the error thrown by the given function.

Sugar for Future.encase(f, undefined).

const data = {foo: 'bar'}
Future.try(() => data.foo.bar.baz)
.fork(console.error, console.log)
//> [TypeError: Cannot read property 'baz' of undefined]

node :: ((a, b -> Void) -> Void) -> Future a b

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

This is a convenience for NodeJS users who wish to easily obtain a Future from a node style callback API.

Future.node(done => fs.readFile('package.json', 'utf8', done))
.fork(console.error, console.log)
//> "{...}"

parallel :: PositiveInteger -> [Future a b] -> Future a [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.

const tenFutures = Array.from(Array(10).keys()).map(Future.after(20));

//Runs all Futures in sequence:
Future.parallel(1, tenFutures).fork(console.error, console.log);
//after about 200ms:
//> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

//Runs upto five Futures in parallel:
Future.parallel(5, tenFutures).fork(console.error, console.log);
//after about 40ms:
//> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

//Runs all Futures in parallel:
Future.parallel(Infinity, tenFutures).fork(console.error, console.log);
//after about 20ms:
//> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

If you want to settle all Futures, even if some may fail, you can use this in combination with fold:

const fourInstableFutures = Array.from(Array(4).keys()).map(
  i => Future(
    (rej, res) => setTimeout(
      () => Math.random() > 0.8 ? rej('failed') : res(i),
      20
    )
  )
);

const stabalizedFutures = fourInstableFutures.map(Future.fold(S.Left, S.Right))

Future.parallel(2, stabalizedFutures).fork(console.error, console.log);
//after about 40ms:
//> [ Right(0), Left("failed"), Right(2), Right(3) ]

cache :: Future a b -> Future a b

Returns a Future which caches the resolution value of the given Future so that whenever it's forked, it can load the value from cache rather than reexecuting the chain.

const eventualPackage = Future.cache(
  Future.node(done => {
    console.log('Reading some big data');
    fs.readFile('package.json', 'utf8', done)
  })
);

eventualPackage.fork(console.error, console.log);
//> "Reading some big data"
//> "{...}"

eventualPackage.fork(console.error, console.log);
//> "{...}"

Method API

fork :: Future a b ~> (a -> Void), (b -> Void) -> Void

Execute the Future (which up until now was merely a container for its computation), and get at the result or error.

It is the return from Fantasy Land to the real world. This function best shows the fundamental difference between Promises and Futures.

Future.of('world').fork(
  err => console.log(`Oh no! ${err.message}`),
  thing => console.log(`Hello ${thing}!`)
);
//> "Hello world!"

Future.reject(new Error('It broke!')).fork(
  err => console.log(`Oh no! ${err.message}`),
  thing => console.log(`Hello ${thing}!`)
);
//> "Oh no! It broke!"

map :: Future a b ~> (b -> c) -> Future a c

Map over the value inside the Future. If the Future is rejected, mapping is not performed.

Future.of(1)
.map(x => x + 1)
.fork(console.error, console.log);
//> 2

chain :: Future a b ~> (b -> Future a c) -> Future a c

FlatMap over the value inside the Future. If the Future is rejected, chaining is not performed.

Future.of(1)
.chain(x => Future.of(x + 1))
.fork(console.error, console.log);
//> 2

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

FlatMap over the rejection value inside the Future. If the Future is resolved, chaining is not performed.

Future.reject(new Error('It broke!')).chainRej(err => {
  console.error(err);
  return Future.of('All is good')
})
.fork(console.error, console.log)
//> "All is good"

ap :: Future a (b -> c) ~> Future a b -> Future a c

Apply the value in the Future to the value in the given Future. If the Future is rejected, applying is not performed.

Future.of(x => x + 1)
.ap(Future.of(1))
.fork(console.error, console.log);
//> 2

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

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

Future.after(100, 'hello')
.race(Future.after(50, 'bye'))
.fork(console.error, console.log)
//> "bye"

or :: Future a b ~> Future a b -> Future a b

Logical or for Futures.

Returns a new Future which either resolves with the first resolutation value, or rejects with the last rejection value once and if both Futures reject.

This behaves analogues to how JavaScript's or operator does, except both Futures run simultaneously, so it is not short-circuited. That means that if the second has side-effects, they will run even if the first resolves.

//An asynchronous version of:
//const result = planA() || planB();

const result = planA().or(planB());

In the example, assume both plans return Futures. Both plans are executed in parallel. If planA resolves, the returned Future will resolve with its value. If planA fails there is always planB. If both plans fail then the returned Future will also reject using the rejection reason of planB.

fold :: Future a b ~> (a -> c), (b -> c) -> Future _ 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 representataion of failures:

Future.of('hello')
.fold(S.Left, S.Right)
.value(console.log);
//> Right('hello')

Future.reject('it broke')
.fold(S.Left, S.Right)
.value(console.log);
//> Left('it broke')

value :: Future a b ~> (b -> Void) -> Void

Extracts the value from a resolved Future by forking it. Only use this function if you are sure the Future is going to be resolved, for example; after using .fold(). If the Future rejects and value was used, an (likely uncatchable) Error will be thrown.

Future.reject(new Error('It broke'))
.fold(S.Left, S.Right)
.value(console.log)
//> Left([Error: It broke])

promise :: Future a b ~> Promise b a

Cast the Future to a Promise. This will cause the Future to be eagerly forked. This is useful if some API wants you to give it a Promise. You shouldn't need this method for anything else.

Dispatcher API

fork :: (a -> Void) -> (b -> Void) -> Future a b -> Void

Dispatches the first and second arguments to the fork method of the third argument.

const consoleFork = Future.fork(console.error, console.log);
consoleFork(Future.of('Hello'));
//> "Hello"

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

Dispatches the first argument to the map method of the second argument. Has the same effect as R.map.

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

Dispatches the first argument to the chain method of the second argument. Has the same effect as R.chain.

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

Dispatches the first argument to the chainRej method of the second argument.

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

Dispatches the second argument to the ap method of the first argument. Has the same effect as R.ap.

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

Dispatches the first argument to the race method of the second argument.

const first = futures => futures.reduce(Future.race);
first([
  Future.after(100, 'hello'),
  Future.after(50, 'bye'),
  Future(rej => setTimeout(rej, 25, 'nope'))
])
.fork(console.error, console.log)
//> [Error nope]

or :: Future a b -> Future a b -> Future a b

Dispatches the first argument to the or method of the second argument.

const any = futures => futures.reduce(Future.or, Future.reject('Empty list!'));

any([
  Future.reject('first: nope'),
  Future.of('second: yep'),
  Future.reject('third: yep')
])
.fork(console.error, console.log);
//> "second: yep"

any([
  Future.reject('first: nope'),
  Future.reject('second: nope'),
  Future.reject('third: nope')
])
.fork(console.error, console.log);
//> "third: nope"

fold :: (a -> c) -> (b -> c) -> Future a b -> Future _ c

Dispatches the first and second arguments to the fold method of the third argument.

value :: (b -> Void) -> Future a b -> Void

Dispatches the first argument to the value method of the second argument.

Futurization

To reduce the boilerplate of making Node or Promise functions return Future's instead, one might use the Futurize library:

const Future = require('fluture');
const futurize = require('futurize').futurize(Future);
const readFile = futurize(require('fs').readFile);
readFile('README.md', 'utf8')
.map(text => text.split('\n'))
.map(lines => lines[0])
.fork(console.error, console.log);
//> "# Fluture"

Road map

• Implement Future#and
• Implement Future#or
• Implement Future.predicate
• Implement Future#promise
• Implement Future.cast
• Implement Future.encase
• Implement Profunctor (and possibly rename chainRej -> lchain)
• Fail-fast curried functions
• Wiki: Comparison between Future libs
• Wiki: Comparison Future and Promise
• Add test coverage
• Add readme badges
• A transpiled ES5 version if demand arises

Benchmarks

Simply run node ./bench/<file> to see how a specific method compares to implementations in data.task, ramda-fantasy.Future and Promise*.

* Promise is not included in all benchmarks because it tends to make the process run out of memory.

The name

A conjunction of the acronym to Fantasy Land (FL) and Future. Also "fluture" means butterfly in Romanian; A creature you might expect to see in Fantasy Land.

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MIT licensed