fp-ts

This site uses Just the Docs, a documentation theme for Jekyll.

Search

Task overview

interface Task<A> {
  (): Promise<A>
}

Task<A> represents an asynchronous computation that yields a value of type A and never fails. If you want to represent an asynchronous computation that may fail, please see TaskEither.

Added in v2.0.0

Table of contents

Applicative

of

Wrap a value into the type constructor.

Signature

export declare const of: <A>(a: A) => Task<A>

Added in v2.0.0

Apply

ap

Apply a function to an argument under a type constructor.

Signature

export declare const ap: <A>(fa: Task<A>) => <B>(fab: Task<(a: A) => B>) => Task<B>

Added in v2.0.0

Functor

map

map can be used to turn functions (a: A) => B into functions (fa: F<A>) => F<B> whose argument and return types use the type constructor F to represent some computational context.

Signature

export declare const map: <A, B>(f: (a: A) => B) => (fa: Task<A>) => Task<B>

Added in v2.0.0

Monad

chain

Composes computations in sequence, using the return value of one computation to determine the next computation.

Signature

export declare const chain: <A, B>(f: (a: A) => Task<B>) => (ma: Task<A>) => Task<B>

Added in v2.0.0

MonadTask

fromTask

Signature

export declare const fromTask: <A>(fa: Task<A>) => Task<A>

Added in v2.7.0

combinators

apFirst

Combine two effectful actions, keeping only the result of the first.

Derivable from Apply.

Signature

export declare const apFirst: <B>(fb: Task<B>) => <A>(fa: Task<A>) => Task<A>

Added in v2.0.0

apSecond

Combine two effectful actions, keeping only the result of the second.

Derivable from Apply.

Signature

export declare const apSecond: <B>(fb: Task<B>) => <A>(fa: Task<A>) => Task<B>

Added in v2.0.0

chainFirst

Composes computations in sequence, using the return value of one computation to determine the next computation and keeping only the result of the first.

Derivable from Monad.

Signature

export declare const chainFirst: <A, B>(f: (a: A) => Task<B>) => (ma: Task<A>) => Task<A>

Added in v2.0.0

chainIOK

Signature

export declare function chainIOK<A, B>(f: (a: A) => IO<B>): (ma: Task<A>) => Task<B>

Added in v2.4.0

delay

Creates a task that will complete after a time delay

Signature

export declare function delay(millis: number): <A>(ma: Task<A>) => Task<A>

Example

import { sequenceT } from 'fp-ts/Apply'
import * as T from 'fp-ts/Task'

async function test() {
  const log: Array<string> = []
  const append = (message: string): T.Task<void> =>
    T.fromIO(() => {
      log.push(message)
    })
  const fa = append('a')
  const fb = append('b')
  const fc = T.delay(10)(append('c'))
  const fd = append('d')
  await sequenceT(T.task)(fa, fb, fc, fd)()
  assert.deepStrictEqual(log, ['a', 'b', 'd', 'c'])
}

test()

Added in v2.0.0

flatten

Derivable from Monad.

Signature

export declare const flatten: <A>(mma: Task<Task<A>>) => Task<A>

Added in v2.0.0

fromIOK

Signature

export declare function fromIOK<A extends ReadonlyArray<unknown>, B>(f: (...a: A) => IO<B>): (...a: A) => Task<B>

Added in v2.4.0

constructors

fromIO

Signature

export declare const fromIO: <A>(ma: IO<A>) => Task<A>

Added in v2.0.0

instances

ApplicativePar

Signature

export declare const ApplicativePar: Applicative1<'Task'>

Added in v2.7.0

ApplicativeSeq

Signature

export declare const ApplicativeSeq: Applicative1<'Task'>

Added in v2.7.0

Functor

Signature

export declare const Functor: Functor1<'Task'>

Added in v2.7.0

URI

Signature

export declare const URI: 'Task'

Added in v2.0.0

URI (type alias)

Signature

export type URI = typeof URI

Added in v2.0.0

getMonoid

Lift a monoid into ‘Task’, the inner values are concatenated using the provided Monoid.

Signature

export declare function getMonoid<A>(M: Monoid<A>): Monoid<Task<A>>

Added in v2.0.0

getRaceMonoid

Monoid returning the first completed task.

Note: uses Promise.race internally.

Signature

export declare function getRaceMonoid<A = never>(): Monoid<Task<A>>

Example

import * as T from 'fp-ts/Task'

async function test() {
  const S = T.getRaceMonoid<string>()
  const fa = T.delay(20)(T.of('a'))
  const fb = T.delay(10)(T.of('b'))
  assert.deepStrictEqual(await S.concat(fa, fb)(), 'b')
}

test()

Added in v2.0.0

getSemigroup

Lift a semigroup into ‘Task’, the inner values are concatenated using the provided Semigroup.

Signature

export declare function getSemigroup<A>(S: Semigroup<A>): Semigroup<Task<A>>

Example

import * as T from 'fp-ts/Task'
import { semigroupString } from 'fp-ts/Semigroup'

async function test() {
  const S = T.getSemigroup(semigroupString)
  const fa = T.of('a')
  const fb = T.of('b')
  assert.deepStrictEqual(await S.concat(fa, fb)(), 'ab')
}

test()

Added in v2.0.0

task

Signature

export declare const task: Monad1<'Task'> & MonadTask1<'Task'>

Added in v2.0.0

taskSeq

Like task but ap is sequential

Signature

export declare const taskSeq: Monad1<'Task'> & MonadTask1<'Task'>

Added in v2.0.0

model

Task (interface)

Signature

export interface Task<A> {
  (): Promise<A>
}

Added in v2.0.0

utils

Do

Signature

export declare const Do: Task<{}>

Added in v2.9.0

apS

Signature

export declare const apS: <A, N extends string, B>(
  name: Exclude<N, keyof A>,
  fb: Task<B>
) => (fa: Task<A>) => Task<{ [K in N | keyof A]: K extends keyof A ? A[K] : B }>

Added in v2.8.0

bind

Signature

export declare const bind: <N extends string, A, B>(
  name: Exclude<N, keyof A>,
  f: (a: A) => Task<B>
) => (fa: Task<A>) => Task<{ [K in N | keyof A]: K extends keyof A ? A[K] : B }>

Added in v2.8.0

bindTo

Signature

export declare const bindTo: <N extends string>(name: N) => <A>(fa: Task<A>) => Task<{ [K in N]: A }>

Added in v2.8.0

never

A Task that never completes.

Signature

export declare const never: Task<never>

Added in v2.0.0

sequenceArray

this function works like Promise.all it will get an array of tasks and return a task of array.

this function have the same behavior of A.sequence(T.task) but it’s stack safe.

**This function run all task in parallel for sequential use sequenceSeqArray **

Signature

export declare const sequenceArray: <A>(arr: readonly Task<A>[]) => Task<readonly A[]>

Example

import * as RA from 'fp-ts/ReadonlyArray'
import { pipe } from 'fp-ts/function'
import { of, sequenceArray } from 'fp-ts/Task'

async function test() {
  const arr = RA.range(1, 10)
  assert.deepStrictEqual(await pipe(arr, RA.map(of), sequenceArray)(), arr)
}

test()

Added in v2.9.0

sequenceSeqArray

run tasks in array sequential and give a task of array

this function have the same behavior of A.sequence(T.taskSeq) but it’s stack safe.

**This function run all task sequentially for parallel use sequenceArray **

Signature

export declare const sequenceSeqArray: <A>(arr: readonly Task<A>[]) => Task<readonly A[]>

Added in v2.9.0

traverseArray

this function map array to task using provided function and transform it to a task of array.

this function have the same behavior of A.traverse(T.task) but it’s stack safe.

**This function run all task in parallel for sequential use traverseSeqArray **

Signature

export declare const traverseArray: <A, B>(f: (a: A) => Task<B>) => (arr: readonly A[]) => Task<readonly B[]>

Example

import { range } from 'fp-ts/ReadonlyArray'
import { pipe } from 'fp-ts/function'
import { of, traverseArray } from 'fp-ts/Task'
async function test() {
  const arr = range(0, 10)
  assert.deepStrictEqual(await pipe(arr, traverseArray(of))(), arr)
}

test()

Added in v2.9.0

traverseArrayWithIndex

Signature

export declare const traverseArrayWithIndex: <A, B>(
  f: (index: number, a: A) => Task<B>
) => (arr: readonly A[]) => Task<readonly B[]>

Added in v2.9.0

traverseSeqArray

runs an action for every element in array then run task sequential, and accumulates the results in the array.

this function have the same behavior of A.traverse(T.taskSeq) but it’s stack safe.

**This function run all task sequentially for parallel use traverseArray **

Signature

export declare const traverseSeqArray: <A, B>(f: (a: A) => Task<B>) => (arr: readonly A[]) => Task<readonly B[]>

Added in v2.9.0

traverseSeqArrayWithIndex

Signature

export declare const traverseSeqArrayWithIndex: <A, B>(
  f: (index: number, a: A) => Task<B>
) => (arr: readonly A[]) => Task<readonly B[]>

Added in v2.9.0