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fp-ts

TaskEither overview 

interface TaskEither<E, A> extends Task<Either<E, A>> {}

TaskEither<E, A> represents an asynchronous computation that either yields a value of type A or fails yielding an error of type E. If you want to represent an asynchronous computation that never fails, please see Task.

Added in v2.0.0

Table of contents

Alt

alt

Identifies an associative operation on a type constructor. It is similar to Semigroup, except that it applies to types of kind * -> *.

In case of TaskEither returns fa if is a Right or the value returned by that otherwise.

See also orElse.

Signature

export declare const alt: <E, A>(that: Lazy<TaskEither<E, A>>) => (fa: TaskEither<E, A>) => TaskEither<E, A>

Example

import * as E from 'fp-ts/Either'
import { pipe } from 'fp-ts/function'
import * as TE from 'fp-ts/TaskEither'

async function test() {
  assert.deepStrictEqual(
    await pipe(
      TE.right(1),
      TE.alt(() => TE.right(2))
    )(),
    E.right(1)
  )
  assert.deepStrictEqual(
    await pipe(
      TE.left('a'),
      TE.alt(() => TE.right(2))
    )(),
    E.right(2)
  )
  assert.deepStrictEqual(
    await pipe(
      TE.left('a'),
      TE.alt(() => TE.left('b'))
    )(),
    E.left('b')
  )
}

test()

Added in v2.0.0

altW

Less strict version of alt.

Signature

export declare const altW: <E2, B>(
  that: Lazy<TaskEither<E2, B>>
) => <E1, A>(fa: TaskEither<E1, A>) => TaskEither<E2 | E1, B | A>

Added in v2.9.0

Applicative

of

Wrap a value into the type constructor.

Equivalent to right.

Signature

export declare const of: <E, A>(a: A) => TaskEither<E, A>

Added in v2.0.0

Apply

ap

Apply a function to an argument under a type constructor.

Signature

export declare const ap: <E, A>(fa: TaskEither<E, A>) => <B>(fab: TaskEither<E, (a: A) => B>) => TaskEither<E, B>

Added in v2.0.0

apW

Less strict version of ap.

Signature

export declare const apW: <D, A>(
  fa: TaskEither<D, A>
) => <E, B>(fab: TaskEither<E, (a: A) => B>) => TaskEither<D | E, B>

Added in v2.8.0

Bifunctor

bimap

Map a pair of functions over the two type arguments of the bifunctor.

Signature

export declare const bimap: <E, G, A, B>(f: (e: E) => G, g: (a: A) => B) => (fa: TaskEither<E, A>) => TaskEither<G, B>

Added in v2.0.0

mapLeft

Map a function over the first type argument of a bifunctor.

Signature

export declare const mapLeft: <E, G>(f: (e: E) => G) => <A>(fa: TaskEither<E, A>) => TaskEither<G, A>

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) => <E>(fa: TaskEither<E, A>) => TaskEither<E, 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: <E, A, B>(f: (a: A) => TaskEither<E, B>) => (ma: TaskEither<E, A>) => TaskEither<E, B>

Added in v2.0.0

chainW

Less strict version of chain.

Signature

export declare const chainW: <E, A, B>(
  f: (a: A) => TaskEither<E, B>
) => <D>(ma: TaskEither<D, A>) => TaskEither<E | D, B>

Added in v2.6.0

MonadIO

fromIO

Signature

export declare const fromIO: <E, A>(fa: IO<A>) => TaskEither<E, A>

Added in v2.7.0

MonadTask

fromTask

Signature

export declare const fromTask: <E, A>(fa: T.Task<A>) => TaskEither<E, A>

Added in v2.7.0

throwError

Signature

export declare const throwError: <E, A>(e: E) => TaskEither<E, 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: <E, B>(fb: TaskEither<E, B>) => <A>(fa: TaskEither<E, A>) => TaskEither<E, 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: <E, B>(fb: TaskEither<E, B>) => <A>(fa: TaskEither<E, A>) => TaskEither<E, B>

Added in v2.0.0

chainEitherK

Signature

export declare const chainEitherK: <E, A, B>(f: (a: A) => E.Either<E, B>) => (ma: TaskEither<E, A>) => TaskEither<E, B>

Added in v2.4.0

chainEitherKW

Less strict version of chainEitherK.

Signature

export declare const chainEitherKW: <E, A, B>(
  f: (a: A) => E.Either<E, B>
) => <D>(ma: TaskEither<D, A>) => TaskEither<E | D, B>

Added in v2.6.1

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: <E, A, B>(f: (a: A) => TaskEither<E, B>) => (ma: TaskEither<E, A>) => TaskEither<E, A>

Added in v2.0.0

chainFirstW

Less strict version of chainFirst.

Derivable from Monad.

Signature

export declare const chainFirstW: <E, A, B>(
  f: (a: A) => TaskEither<E, B>
) => <D>(ma: TaskEither<D, A>) => TaskEither<E | D, A>

Added in v2.8.0

chainIOEitherK

Signature

export declare const chainIOEitherK: <E, A, B>(
  f: (a: A) => IOEither<E, B>
) => (ma: TaskEither<E, A>) => TaskEither<E, B>

Added in v2.4.0

chainIOEitherKW

Less strict version of chainIOEitherK.

Signature

export declare const chainIOEitherKW: <E, A, B>(
  f: (a: A) => IOEither<E, B>
) => <D>(ma: TaskEither<D, A>) => TaskEither<E | D, B>

Added in v2.6.1

filterOrElse

Derivable from MonadThrow.

Signature

export declare const filterOrElse: {
  <E, A, B extends A>(refinement: Refinement<A, B>, onFalse: (a: A) => E): (ma: TaskEither<E, A>) => TaskEither<E, B>
  <E, A>(predicate: Predicate<A>, onFalse: (a: A) => E): (ma: TaskEither<E, A>) => TaskEither<E, A>
}

Added in v2.0.0

flatten

Derivable from Monad.

Signature

export declare const flatten: <E, A>(mma: TaskEither<E, TaskEither<E, A>>) => TaskEither<E, A>

Added in v2.0.0

fromEitherK

Signature

export declare function fromEitherK<E, A extends ReadonlyArray<unknown>, B>(
  f: (...a: A) => Either<E, B>
): (...a: A) => TaskEither<E, B>

Added in v2.4.0

fromIOEitherK

Signature

export declare function fromIOEitherK<E, A extends ReadonlyArray<unknown>, B>(
  f: (...a: A) => IOEither<E, B>
): (...a: A) => TaskEither<E, B>

Added in v2.4.0

orElse

Returns ma if is a Right or the value returned by onLeft otherwise.

See also alt.

Signature

export declare const orElse: <E, A, M>(onLeft: (e: E) => TaskEither<M, A>) => (ma: TaskEither<E, A>) => TaskEither<M, A>

Example

import * as E from 'fp-ts/Either'
import { pipe } from 'fp-ts/function'
import * as TE from 'fp-ts/TaskEither'

async function test() {
  const errorHandler = TE.orElse((error: string) => TE.right(`recovering from ${error}...`))
  assert.deepStrictEqual(await pipe(TE.right('ok'), errorHandler)(), E.right('ok'))
  assert.deepStrictEqual(await pipe(TE.left('ko'), errorHandler)(), E.right('recovering from ko...'))
}

test()

Added in v2.0.0

swap

Signature

export declare const swap: <E, A>(ma: TaskEither<E, A>) => TaskEither<A, E>

Added in v2.0.0

tryCatchK

Converts a function returning a Promise to one returning a TaskEither.

Signature

export declare function tryCatchK<E, A extends ReadonlyArray<unknown>, B>(
  f: (...a: A) => Promise<B>,
  onRejected: (reason: unknown) => E
): (...a: A) => TaskEither<E, B>

Added in v2.5.0

constructors

fromEither

Derivable from MonadThrow.

Signature

export declare const fromEither: <E, A>(ma: E.Either<E, A>) => TaskEither<E, A>

Added in v2.0.0

fromIOEither

Signature

export declare const fromIOEither: <E, A>(fa: IOEither<E, A>) => TaskEither<E, A>

Added in v2.0.0

fromOption

Derivable from MonadThrow.

Signature

export declare const fromOption: <E>(onNone: Lazy<E>) => <A>(ma: Option<A>) => TaskEither<E, A>

Added in v2.0.0

fromPredicate

Derivable from MonadThrow.

Signature

export declare const fromPredicate: {
  <E, A, B extends A>(refinement: Refinement<A, B>, onFalse: (a: A) => E): (a: A) => TaskEither<E, B>
  <E, A>(predicate: Predicate<A>, onFalse: (a: A) => E): (a: A) => TaskEither<E, A>
}

Added in v2.0.0

left

Signature

export declare const left: <E = never, A = never>(e: E) => TaskEither<E, A>

Added in v2.0.0

leftIO

Signature

export declare const leftIO: <E = never, A = never>(me: IO<E>) => TaskEither<E, A>

Added in v2.0.0

leftTask

Signature

export declare const leftTask: <E = never, A = never>(me: T.Task<E>) => TaskEither<E, A>

Added in v2.0.0

Signature

export declare const right: <E = never, A = never>(a: A) => TaskEither<E, A>

Added in v2.0.0

rightIO

Signature

export declare const rightIO: <E = never, A = never>(ma: IO<A>) => TaskEither<E, A>

Added in v2.0.0

rightTask

Signature

export declare const rightTask: <E = never, A = never>(ma: T.Task<A>) => TaskEither<E, A>

Added in v2.0.0

tryCatch

Transforms a Promise that may reject to a Promise that never rejects and returns an Either instead.

Note: f should never throw errors, they are not caught.

Signature

export declare function tryCatch<E, A>(f: Lazy<Promise<A>>, onRejected: (reason: unknown) => E): TaskEither<E, A>

Example

import { left, right } from 'fp-ts/Either'
import { tryCatch } from 'fp-ts/TaskEither'

tryCatch(() => Promise.resolve(1), String)().then((result) => {
  assert.deepStrictEqual(result, right(1))
})
tryCatch(() => Promise.reject('error'), String)().then((result) => {
  assert.deepStrictEqual(result, left('error'))
})

Added in v2.0.0

destructors

fold

Signature

export declare const fold: <E, A, B>(
  onLeft: (e: E) => T.Task<B>,
  onRight: (a: A) => T.Task<B>
) => (ma: TaskEither<E, A>) => T.Task<B>

Added in v2.0.0

getOrElse

Signature

export declare const getOrElse: <E, A>(onLeft: (e: E) => T.Task<A>) => (ma: TaskEither<E, A>) => T.Task<A>

Added in v2.0.0

getOrElseW

Less strict version of getOrElse.

Signature

export declare const getOrElseW: <E, B>(onLeft: (e: E) => T.Task<B>) => <A>(ma: TaskEither<E, A>) => T.Task<B | A>

Added in v2.6.0

instances

Alt

Signature

export declare const Alt: Alt2<'TaskEither'>

Added in v2.7.0

ApplicativePar

Signature

export declare const ApplicativePar: Applicative2<'TaskEither'>

Added in v2.7.0

ApplicativeSeq

Signature

export declare const ApplicativeSeq: Applicative2<'TaskEither'>

Added in v2.7.0

Bifunctor

Signature

export declare const Bifunctor: Bifunctor2<'TaskEither'>

Added in v2.7.0

Functor

Signature

export declare const Functor: Functor2<'TaskEither'>

Added in v2.7.0

URI

Signature

export declare const URI: 'TaskEither'

Added in v2.0.0

URI (type alias)

Signature

export type URI = typeof URI

Added in v2.0.0

getAltTaskValidation

Signature

export declare function getAltTaskValidation<E>(SE: Semigroup<E>): Alt2C<URI, E>

Added in v2.7.0

getApplicativeTaskValidation

Signature

export declare function getApplicativeTaskValidation<E>(A: Apply1<T.URI>, SE: Semigroup<E>): Applicative2C<URI, E>

Added in v2.7.0

getApplyMonoid

Signature

export declare function getApplyMonoid<E, A>(M: Monoid<A>): Monoid<TaskEither<E, A>>

Added in v2.0.0

getApplySemigroup

Semigroup returning the left-most Left value. If both operands are Rights then the inner values are concatenated using the provided Semigroup

Signature

export declare function getApplySemigroup<E, A>(S: Semigroup<A>): Semigroup<TaskEither<E, A>>

Added in v2.0.0

getFilterable

Signature

export declare function getFilterable<E>(M: Monoid<E>): Filterable2C<URI, E>

Added in v2.1.0

getSemigroup

Semigroup returning the left-most non-Left value. If both operands are Rights then the inner values are concatenated using the provided Semigroup

Signature

export declare function getSemigroup<E, A>(S: Semigroup<A>): Semigroup<TaskEither<E, A>>

Added in v2.0.0

getTaskValidation

Signature

export declare function getTaskValidation<E>(
  SE: Semigroup<E>
): Monad2C<URI, E> & Bifunctor2<URI> & Alt2C<URI, E> & MonadTask2C<URI, E> & MonadThrow2C<URI, E>

Added in v2.0.0

taskEither

Signature

export declare const taskEither: Monad2<'TaskEither'> &
  Bifunctor2<'TaskEither'> &
  Alt2<'TaskEither'> &
  MonadTask2<'TaskEither'> &
  MonadThrow2<'TaskEither'>

Added in v2.0.0

taskEitherSeq

Like TaskEither but ap is sequential

Signature

export declare const taskEitherSeq: Monad2<'TaskEither'> &
  Bifunctor2<'TaskEither'> &
  Alt2<'TaskEither'> &
  MonadTask2<'TaskEither'> &
  MonadThrow2<'TaskEither'>

Added in v2.0.0

model

TaskEither (interface)

Signature

export interface TaskEither<E, A> extends Task<Either<E, A>> {}

Added in v2.0.0

utils

Do

Signature

export declare const Do: TaskEither<never, {}>

Added in v2.9.0

apS

Signature

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

Added in v2.8.0

apSW

Signature

export declare const apSW: <A, N extends string, D, B>(
  name: Exclude<N, keyof A>,
  fb: TaskEither<D, B>
) => <E>(fa: TaskEither<E, A>) => TaskEither<D | E, { [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, E, B>(
  name: Exclude<N, keyof A>,
  f: (a: A) => TaskEither<E, B>
) => (fa: TaskEither<E, A>) => TaskEither<E, { [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
) => <E, A>(fa: TaskEither<E, A>) => TaskEither<E, { [K in N]: A }>

Added in v2.8.0

bindW

Signature

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

Added in v2.8.0

bracket

Make sure that a resource is cleaned up in the event of an exception (*). The release action is called regardless of whether the body action throws (*) or returns.

(*) i.e. returns a Left

Derivable from MonadThrow.

Signature

export declare const bracket: <E, A, B>(
  acquire: TaskEither<E, A>,
  use: (a: A) => TaskEither<E, B>,
  release: (a: A, e: E.Either<E, B>) => TaskEither<E, void>
) => TaskEither<E, B>

Added in v2.0.0

filterOrElseW

Less strict version of filterOrElse.

Signature

export declare const filterOrElseW: {
  <A, B extends A, E2>(refinement: Refinement<A, B>, onFalse: (a: A) => E2): <E1>(
    ma: TaskEither<E1, A>
  ) => TaskEither<E2 | E1, B>
  <A, E2>(predicate: Predicate<A>, onFalse: (a: A) => E2): <E1>(ma: TaskEither<E1, A>) => TaskEither<E2 | E1, A>
}

Added in v2.9.0

sequenceArray

this function has the same behavior of A.sequence(TE.taskEither) but it’s stack safe and performs better

this function run all tasks in parallel and does not bail out, for sequential version use sequenceSeqArray

Signature

export declare const sequenceArray: <A, E>(arr: readonly TaskEither<E, A>[]) => TaskEither<E, readonly A[]>

Example

import * as TE from 'fp-ts/TaskEither'
import * as A from 'fp-ts/Array'
import { right } from 'fp-ts/Either'
import { pipe } from 'fp-ts/function'

const PostRepo = {
  findById: (id: number) => TE.of({ id, title: '' }),
}

const findAllPosts = (ids: number[]) => pipe(ids, A.map(PostRepo.findById), TE.sequenceArray)

async function test() {
  const ids = A.range(0, 10)

  assert.deepStrictEqual(
    await findAllPosts(ids)(),
    right(
      pipe(
        ids,
        A.map((id) => ({ id, title: '' }))
      )
    )
  )
}

test()

Added in v2.9.0

sequenceSeqArray

this function has the same behavior of A.sequence(TE.taskEitherSeq) but it’s stack safe and performs better

this function run all tasks in sequential order and bails out on left side of either, for parallel version use sequenceArray

Signature

export declare const sequenceSeqArray: <A, E>(arr: readonly TaskEither<E, A>[]) => TaskEither<E, readonly A[]>

Added in v2.9.0

taskify

Convert a node style callback function to one returning a TaskEither

Note. If the function f admits multiple overloadings, taskify will pick last one. If you want a different behaviour, add an explicit type annotation

// readFile admits multiple overloadings

// const readFile: (a: string) => TaskEither<NodeJS.ErrnoException, Buffer>
const readFile = taskify(fs.readFile)

const readFile2: (filename: string, encoding: string) => TaskEither<NodeJS.ErrnoException, Buffer> = taskify(
  fs.readFile
)

Signature

export declare function taskify<L, R>(f: (cb: (e: L | null | undefined, r?: R) => void) => void): () => TaskEither<L, R>
export declare function taskify<A, L, R>(
  f: (a: A, cb: (e: L | null | undefined, r?: R) => void) => void
): (a: A) => TaskEither<L, R>
export declare function taskify<A, B, L, R>(
  f: (a: A, b: B, cb: (e: L | null | undefined, r?: R) => void) => void
): (a: A, b: B) => TaskEither<L, R>
export declare function taskify<A, B, C, L, R>(
  f: (a: A, b: B, c: C, cb: (e: L | null | undefined, r?: R) => void) => void
): (a: A, b: B, c: C) => TaskEither<L, R>
export declare function taskify<A, B, C, D, L, R>(
  f: (a: A, b: B, c: C, d: D, cb: (e: L | null | undefined, r?: R) => void) => void
): (a: A, b: B, c: C, d: D) => TaskEither<L, R>
export declare function taskify<A, B, C, D, E, L, R>(
  f: (a: A, b: B, c: C, d: D, e: E, cb: (e: L | null | undefined, r?: R) => void) => void
): (a: A, b: B, c: C, d: D, e: E) => TaskEither<L, R>

Example

import { taskify } from 'fp-ts/TaskEither'
import * as fs from 'fs'

// const stat: (a: string | Buffer) => TaskEither<NodeJS.ErrnoException, fs.Stats>
const stat = taskify(fs.stat)
assert.strictEqual(stat.length, 0)

Added in v2.0.0

traverseArray

this function has the same behavior of A.traverse(TE.taskEither) but it’s stack safe and performs better

this function run all tasks in parallel and does not bail out, for sequential version use traverseSeqArray

Signature

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

Example

import * as TE from 'fp-ts/TaskEither'
import * as A from 'fp-ts/Array'
import { right } from 'fp-ts/Either'
import { pipe } from 'fp-ts/function'

const PostRepo = {
  findById: (id: number) => TE.of({ id, title: '' }),
}

const findAllPosts = (ids: number[]) => pipe(ids, TE.traverseArray(PostRepo.findById))

async function test() {
  const ids = A.range(0, 10)

  assert.deepStrictEqual(
    await findAllPosts(ids)(),
    right(
      pipe(
        ids,
        A.map((id) => ({ id, title: '' }))
      )
    )
  )
}

test()

Added in v2.9.0

traverseArrayWithIndex

Signature

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

Added in v2.9.0

traverseSeqArray

this function has the same behavior of A.traverse(TE.taskEitherSeq) but it’s stack safe and performs better

this function run all tasks in sequential order and bails out on left side of either, for parallel version use traverseArray

Signature

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

Added in v2.9.0

traverseSeqArrayWithIndex

Signature

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

Added in v2.9.0