Experimental The api can change between versions.

exalted.future

A javascript monadic library & functional fun. fantasy-land compliant, mostly.

The style of monad object is inspired by DrBoolean course on egghead.io and his book Mostly adequate guide to FP.

The choice for cata, encase, head, tail, last is inspired by rametta's take on monads in pratica.

This is, in many ways, an evolution of oncha which I wrote with other people many years ago and is no longer maintained.

Natural Transformation

One of the main goals of the exalted.future is to make it possible to rely on natural transformation when composing Monads. So that you can write in one continuous flow your program, agnostic of the Monad you are working with. That is why flatMap, mapReduce, reduce, fold, and fork use the same language, cata. You can always call cata on an object, and it will compute your results. The following example attempts to illustrate that. Regardless that the fetch succeeds or fails the outcome will be the same, indifferent to calling cata on Maybe, Either (Left|Right), or Future.

Future.promise(fetch('https://jsonplaceholder.typicode.com/todos/1'))
  .chain(response => Either.encase(() => response.json()))
  .chain(Future.promise)
  .chain(Maybe)
  .map(todo => todo.title)
  .map(t => t.toUpperCase())
  .cata({
    Left: e => console.log('oops', e),
    Right: json => console.log(json)
  })

Install

yarn add exalted.future

Types

Name	Apply	Applicative	Setoid	Foldable	Functor	Monad	Chain
Either	✔︎	✔︎	✔︎	✔︎	✔︎	✔︎	✔︎
Future	✔︎	—	✔︎	✔︎	✔︎	—	✔︎
Identity	✔︎	✔︎	✔︎	✔︎	✔︎	✔︎	✔︎
Maybe	✔︎	✔︎	✔︎	✔︎	✔︎	✔︎	✔︎

• There is a divergence from fantasy-land where reduce is named cata and loosely based on daggy's union types.
• fold always folds on identity a => a, except when it does not like with the Future.
• Maybe.map will return Nothing if the callback function returns null. In other words Just(null) is impossible, unless you call the 'static' constructor like this Just.of(null). See this pr for some explanation.
• Left is not 100% applicative.
• Not all functions are documented, so you are encouraged to read the source code, you'll find bimap, swap, fold, foldr, foldf.

Future

A Future monad for async computation. Left is reject and Right is resolve. Because Right is always right and Left is not.

// Basic usage
Future((err, ok) => ok('Yay'))
  .map(res => res.toUpperString())
  .cata({
    Left: err => log(`Err: ${err}`),
    Right: res => log(`Res: ${res}`)
  })
//=> 'YAY'

// Handle promises
Future.promise(fetch('https://api.awesome.com/catOfTheDay'))
  .cata({
    Left: err => log('There was an error fetching the cat of the day :('),
    Right: cat => log('Cat of the day: ' + cat)
  })
//=> 'Cat of the day: Garfield'

// Chain http calls
Future.promise(fetch('https://api.awesome.com/catOfTheDay'))
  .chain(cat => Future.promise(fetch(`https://api.catfacts.com/${cat}`)))
  .cata({
    Left: err => log('There was an error fetching the cat of the day :('),
    Right: facts => log('Facts for cat of the day: ' + facts)
  })
//=> 'Facts for cat of the day: Garfield is awesome.'

Future.all

Concats all the results form the list of futures.

all :: ([Futures]) -> b

Future.all(
  Future.of('apple'),
  Future((left, right) => setTimeout(() => right('orange'), 1000)),
  Future.of('lemon')
).cata({
    Left: () => (),
    Right: ([ apple, orange, lemon ]) => console.log(apple, orange, lemon)
  }) //=> apple, orange, lemon

Id

Identity monad.

Id(5)
  .map(num => num * 7)
  .map(num => num - 1)
  .cata({
    Right: a => a
  })
//=> 34

Maybe

Maybe monad.

// Maybe of a string
Maybe('Hello exalted one')
  .map(sentence => sentence.toUpperString())
  .map(sentence => `${sentence}!`)
  .cata({
    Right: console.log
  })
//=> 'HELLO EXALTED ONE!'

// Maybe of nothing
Maybe(null)
  .map(sentence => sentence.toUpperString())
  .alt(() => 'Maybe received a null')
  .cata({
    Right: console.log
  })
//=> 'Maybe received a null'

Either

An Either monad and nullable, Left, Right.

nullable('Hello') // this will return a Right('Hello')
  .cata({
    Left: () => 'Oops',
    Right: val => `${val} world!`
  })
//=> 'Hello world!'

nullable(null) // this will return a Left()
  .cata({
    Left: () => 'Oops',
    Right: val => `${val} world!`
  })
//=> 'Oops'

const extractEmail = obj => obj.email ? Right(obj.email) : Left()
extractEmail({ email: 'test@example.com' }
  .map(extractDomain)
  .cata({
    Left: () => 'No email found!',
    Right:x => x
  })
//=> 'example.com'

extractEmail({ name: 'user' }
  .map(extractDomain) // this will not get executed
  .cata({
    Left: () => 'No email found!',
    Right: x => x
  })
//=> 'No email found!'

Functions

The following functions are common to all monads types.

alt

Sets the value to cata on.

alt :: Any -> Nothing of Any

Maybe(1).alt(5).cata({
    Right: a => a
  })
//=> 1

Maybe(null).alt(5).cata({
    Right: a => a
  })
//=> 5

ap

Apply

chain :: (a -> b) -> b

Id(5).chain(a => Id(a))
//=> Id(5)

// You can use chain to join the monads.
Id(Id(5)).chain(a => a)
//=> Id(5)

cata

Foldable

cata :: ({ Left: () -> b, Right -> a -> a }) -> a | b

Id(5).cata({
    Right: a => a
  })
//=> 5

Id(5).cata({
    Right: a => a + 1
  })
//=> 6

Right(5).cata({
    Left: a => 8 // ignored 
    Right: a => a + 1
  })
//=> 6

Left(5).cata({
    Left: a => a + 1 
    Right: a => 8 // ignored
  })
//=> 6

Maybe(5).cata({
    Right: a => a
  })
//=> 5

Maybe(5).cata({
    Left: () => { } // not called
    Right: a => a + 1
  })
//=> 6

Maybe(null).cata({
    Left: () => 'there was a null'
    Right: a => a + 1 // not called
  })
//=> there was a null

Right(5).cata({
    Left: () => 1,
    Right: a => a + 2
  })
//=> 7

Left(5).cata(a => a + 1)
//=> 6

chain

Chain

chain :: (a -> b) -> b

Id(5).chain(a => Id(a))
//=> Id(5)

// You can use chain to join the monads.
Id(Id(5)).chain(a => a)
//=> Id(5)

compose

Compose takes n functions as arguments and return a function.

const transform = compose(sentence => sentence.toUpperString(), sentence => `${sentence}!`)
const logTransform = compose(log, transform)

logTransform('Hello exalted one')
//=> 'HELLO EXALTED ONE!'

// supports miltiple arguments
compose(path.normalize, path.join)('./exalted', '/one')
//=> './exalted/one'

equals

Setoid

equals :: Id -> Boolean

Id(1).equals(Id(1))
//=> true

Id(2).equals(Id(1))
//=> false

Id(2).equals(Id(1)) === Id(1).equals(Id(1))
//=> false

inspect

inspect :: () -> String

Id(5).inspect()
//=> Id(5)

map

Functor

map :: (a -> b) -> Id of b

Id(7).map(a => a * 2)
//=> Id(14)

map (first class)

Map as partial application and first class with arity support.

map(a => a + 1, a => a * 3)([1, 2, 3])
//=> [4, 7, 10]

of

Applicative

of :: a -> Id of a

Id(5).of(6)
//=> Id(6)

Id(5).of(Id(6))
//=> Id(Id(6))

head, tail, last

Returns a Maybe.

head([1,2])
//=> Just(1)

head([])
//=> Nothing()

tail([1,2,3])
//=> Just([2,3])

tail([])
//=> Nothing()

last([1,2,3])
//=> Just(3)

last([])
//=> Nothing()

Maybe.encase, Either.encase

Returns Left | Right.

Maybe.encase(() => JSON.parse('["foo","bar","baz"]'))
//=> Just(['foo','bar','baz'])

Maybe.encase(() => JSON.parse('['))
//=> Nothing()

Either.encase(() => JSON.parse('["foo","bar","baz"]'))
//=> Right(['foo','bar','baz'])

Either.encase(() => JSON.parse('['))
//=> Left(new SyntaxError ('Unexpected end of JSON input'))