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

combinators
- tap
- tapEither
- tapIO
- tapTask
constructors
- left
- leftIO
- leftTask
- of
- right
- rightIO
- rightTask
conversions
- fromEither
- fromIO
- fromIOEither
- fromNullable
- fromOption
- fromTask
- fromTaskOption
- toUnion
do notation
- Do
- apS
- apSW
- bind
- bindTo
- bindW
- let
error handling
- alt
- altW
- getAltTaskValidation
- getApplicativeTaskValidation
- getOrElse
- getOrElseW
- mapBoth
- mapError
- orElse
- orElseFirstIOK
- orElseFirstTaskK
- orElseW
- orLeft
- tapError
filtering
instances
- Alt
- ApplicativePar
- ApplicativeSeq
- ApplyPar
- ApplySeq
- Bifunctor
- Chain
- FromEither
- FromIO
- FromTask
- Functor
- Monad
- MonadIO
- MonadTask
- MonadThrow
- Pointed
interop
legacy
lifting
mapping
- as
- asUnit
- flap
- map
model
- TaskEither (interface)
pattern matching
- fold
- foldW
- match
- matchE
- matchEW
- matchW
sequencing
traversing
type lambdas
- URI
- URI (type alias)
utils
- ApT
- ap
- apFirst
- apFirstW
- apSecond
- apSecondW
- apW
- bracket
- bracketW
- swap
- throwError
zone of death

combinators

tap

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

Signature

export declare const tap: {
  <E1, A, E2, _>(self: TaskEither<E1, A>, f: (a: A) => TaskEither<E2, _>): TaskEither<E1 | E2, A>
  <A, E2, _>(f: (a: A) => TaskEither<E2, _>): <E1>(self: TaskEither<E1, A>) => TaskEither<E2 | E1, A>
}

Added in v2.15.0

tapEither

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

Signature

export declare const tapEither: {
  <A, E2, _>(f: (a: A) => E.Either<E2, _>): <E1>(self: TaskEither<E1, A>) => TaskEither<E2 | E1, A>
  <E1, A, E2, _>(self: TaskEither<E1, A>, f: (a: A) => E.Either<E2, _>): TaskEither<E1 | E2, A>
}

Example

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

const checkString = (value: string) =>
  pipe(
    TE.of(value),
    TE.tapEither(() => (value.length > 0 ? E.right('ok') : E.left('error')))
  )

async function test() {
  assert.deepStrictEqual(await checkString('')(), E.left('error'))
  assert.deepStrictEqual(await checkString('fp-ts')(), E.right('fp-ts'))
}

test()

Added in v2.16.0

tapIO

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

Signature

export declare const tapIO: {
  <A, _>(f: (a: A) => IO<_>): <E>(self: TaskEither<E, A>) => TaskEither<E, A>
  <E, A, _>(self: TaskEither<E, A>, f: (a: A) => IO<_>): TaskEither<E, A>
}

Example

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

// Will produce `Hello, fp-ts` to the stdout
const effectA = TE.tapIO(TE.of(1), (value) => Console.log(`Hello, ${value}`))

// No output to the stdout
const effectB = pipe(
  TE.left('error'),
  TE.tapIO((value) => Console.log(`Hello, ${value}`))
)

async function test() {
  assert.deepStrictEqual(await effectA(), E.of(1))
  assert.deepStrictEqual(await effectB(), E.left('error'))
}

test()

Added in v2.16.0

tapTask

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

Signature

export declare const tapTask: {
  <A, _>(f: (a: A) => T.Task<_>): <E>(self: TaskEither<E, A>) => TaskEither<E, A>
  <E, A, _>(self: TaskEither<E, A>, f: (a: A) => T.Task<_>): TaskEither<E, A>
}

Example

import * as TE from 'fp-ts/TaskEither'
import * as T from 'fp-ts/Task'
import * as E from 'fp-ts/Either'

const effect = TE.tapIO(TE.of(1), (value) => T.of(value + 1))

async function test() {
  assert.deepStrictEqual(await effect(), E.of(1))
}

test()

Added in v2.16.0

constructors

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

of

Signature

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

Added in v2.0.0

right

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

conversions

fromEither

Signature

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

Added in v2.0.0

fromIO

Signature

export declare const fromIO: <A, E = never>(fa: IO<A>) => TaskEither<E, A>

Added in v2.7.0

fromIOEither

Signature

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

Added in v2.0.0

fromNullable

Signature

export declare const fromNullable: <E>(e: E) => <A>(a: A) => TaskEither<E, NonNullable<A>>

Added in v2.12.0

fromOption

Signature

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

Added in v2.0.0

fromTask

Signature

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

Added in v2.7.0

fromTaskOption

Signature

export declare const fromTaskOption: <E>(onNone: LazyArg<E>) => <A>(fa: TaskOption<A>) => TaskEither<E, A>

Added in v2.11.0

toUnion

Signature

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

Added in v2.10.0

do notation

Do

Signature

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

Added in v2.9.0

apS

Signature

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

Added in v2.8.0

apSW

Less strict version of apS.

The W suffix (short for Widening) means that the error types will be merged.

Signature

export declare const apSW: <A, N extends string, E2, B>(
  name: Exclude<N, keyof A>,
  fb: TaskEither<E2, B>
) => <E1>(fa: TaskEither<E1, A>) => TaskEither<E2 | E1, { readonly [K in N | keyof A]: K extends keyof A ? A[K] : B }>

Added in v2.8.0

bind

Signature

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

Added in v2.8.0

bindTo

Signature

export declare const bindTo: <N>(name: N) => <E, A>(fa: TaskEither<E, A>) => TaskEither<E, { readonly [K in N]: A }>

Added in v2.8.0

bindW

The W suffix (short for Widening) means that the error types will be merged.

Signature

export declare const bindW: <N extends string, A, E2, B>(
  name: Exclude<N, keyof A>,
  f: (a: A) => TaskEither<E2, B>
) => <E1>(fa: TaskEither<E1, A>) => TaskEither<E2 | E1, { readonly [K in N | keyof A]: K extends keyof A ? A[K] : B }>

Added in v2.8.0

let

Signature

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

Added in v2.13.0

error handling

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.

altW

Less strict version of alt.

The W suffix (short for Widening) means that the error and the return types will be merged.

Signature

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

Added in v2.9.0

getAltTaskValidation

The default Alt instance returns the last error, if you want to get all errors you need to provide a way to concatenate them via a Semigroup.

See getAltValidation.

Signature

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

Added in v2.7.0

getApplicativeTaskValidation

The default ApplicativePar instance returns the first error, if you want to get all errors you need to provide a way to concatenate them via a Semigroup.

Signature

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

Example

import * as E from 'fp-ts/Either'
import { pipe } from 'fp-ts/function'
import * as RA from 'fp-ts/ReadonlyArray'
import * as S from 'fp-ts/Semigroup'
import * as string from 'fp-ts/string'
import * as T from 'fp-ts/Task'
import * as TE from 'fp-ts/TaskEither'

interface User {
  readonly id: string
  readonly name: string
}

const remoteDatabase: ReadonlyArray<User> = [
  { id: 'id1', name: 'John' },
  { id: 'id2', name: 'Mary' },
  { id: 'id3', name: 'Joey' },
]

const fetchUser = (id: string): TE.TaskEither<string, User> =>
  pipe(
    remoteDatabase,
    RA.findFirst((user) => user.id === id),
    TE.fromOption(() => `${id} not found`)
  )

async function test() {
  assert.deepStrictEqual(
    await pipe(['id4', 'id5'], RA.traverse(TE.ApplicativePar)(fetchUser))(),
    E.left('id4 not found') // <= first error
  )

  const Applicative = TE.getApplicativeTaskValidation(T.ApplyPar, pipe(string.Semigroup, S.intercalate(', ')))

  assert.deepStrictEqual(
    await pipe(['id4', 'id5'], RA.traverse(Applicative)(fetchUser))(),
    E.left('id4 not found, id5 not found') // <= all errors
  )
}

test()

Added in v2.7.0

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.

The W suffix (short for Widening) means that the handler return type will be merged.

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

mapBoth

Returns a TaskEither whose failure and success channels have been mapped by the specified pair of functions, f and g.

Signature

export declare const mapBoth: {
  <E, G, A, B>(f: (e: E) => G, g: (a: A) => B): (self: TaskEither<E, A>) => TaskEither<G, B>
  <E, A, G, B>(self: TaskEither<E, A>, f: (e: E) => G, g: (a: A) => B): TaskEither<G, B>
}

Example

import * as TaskEither from 'fp-ts/TaskEither'
import * as Either from 'fp-ts/Either'

const f = (s: string) => new Error(s)
const g = (n: number) => n * 2

async function test() {
  assert.deepStrictEqual(await TaskEither.mapBoth(TaskEither.right(1), f, g)(), Either.right(2))
  assert.deepStrictEqual(await TaskEither.mapBoth(TaskEither.left('err'), f, g)(), Either.left(new Error('err')))
}

test()

Added in v2.16.0

mapError

Returns a TaskEither with its error channel mapped using the specified function.

Signature

export declare const mapError: {
  <E, G>(f: (e: E) => G): <A>(self: TaskEither<E, A>) => TaskEither<G, A>
  <E, A, G>(self: TaskEither<E, A>, f: (e: E) => G): TaskEither<G, A>
}

Example

import * as TaskEither from 'fp-ts/TaskEither'
import * as Either from 'fp-ts/Either'

const f = (s: string) => new Error(s)

async function test() {
  assert.deepStrictEqual(await TaskEither.mapError(TaskEither.right(1), f)(), Either.right(1))
  assert.deepStrictEqual(await TaskEither.mapError(TaskEither.left('err'), f)(), Either.left(new Error('err')))
}

test()

Added in v2.16.0

orElse

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

orElseFirstIOK

Signature

export declare const orElseFirstIOK: <E, B>(onLeft: (e: E) => IO<B>) => <A>(ma: TaskEither<E, A>) => TaskEither<E, A>

Added in v2.12.0

orElseFirstTaskK

Signature

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

Added in v2.12.0

orElseW

Less strict version of orElse.

The W suffix (short for Widening) means that the return types will be merged.

Signature

export declare const orElseW: <E1, E2, B>(
  onLeft: (e: E1) => TaskEither<E2, B>
) => <A>(ma: TaskEither<E1, A>) => TaskEither<E2, B | A>

Added in v2.10.0

orLeft

Signature

export declare const orLeft: <E1, E2>(onLeft: (e: E1) => T.Task<E2>) => <A>(fa: TaskEither<E1, A>) => TaskEither<E2, A>

Added in v2.11.0

tapError

Returns an effect that effectfully “peeks” at the failure of this effect.

Signature

export declare const tapError: {
  <E1, E2, _>(onLeft: (e: E1) => TaskEither<E2, _>): <A>(self: TaskEither<E1, A>) => TaskEither<E1 | E2, A>
  <E1, A, E2, _>(self: TaskEither<E1, A>, onLeft: (e: E1) => TaskEither<E2, _>): TaskEither<E1 | E2, A>
}

Added in v2.15.0

filtering

filterOrElse

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): <B extends A>(mb: TaskEither<E, B>) => TaskEither<E, B>
  <E, A>(predicate: Predicate<A>, onFalse: (a: A) => E): (ma: TaskEither<E, A>) => TaskEither<E, A>
}

Added in v2.0.0

filterOrElseW

Less strict version of filterOrElse.

The W suffix (short for Widening) means that the error types will be merged.

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, B extends A>(
    mb: TaskEither<E1, B>
  ) => 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

getCompactable

Signature

export declare const getCompactable: <E>(M: Monoid<E>) => Compactable2C<'TaskEither', E>

Added in v2.10.0

getFilterable

Signature

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

Added in v2.1.0

instances

Alt

Signature

export declare const Alt: Alt2<'TaskEither'>

Added in v2.7.0

ApplicativePar

Runs computations in parallel.

Signature

export declare const ApplicativePar: Applicative2<'TaskEither'>

Added in v2.7.0

ApplicativeSeq

Runs computations sequentially.

Signature

export declare const ApplicativeSeq: Applicative2<'TaskEither'>

Added in v2.7.0

ApplyPar

Runs computations in parallel.

Signature

export declare const ApplyPar: Apply2<'TaskEither'>

Added in v2.10.0

ApplySeq

Runs computations sequentially.

Signature

export declare const ApplySeq: Apply2<'TaskEither'>

Added in v2.10.0

Bifunctor

Signature

export declare const Bifunctor: Bifunctor2<'TaskEither'>

Added in v2.7.0

Chain

Signature

export declare const Chain: chainable.Chain2<'TaskEither'>

Added in v2.10.0

FromEither

Signature

export declare const FromEither: FromEither2<'TaskEither'>

Added in v2.10.0

FromIO

Signature

export declare const FromIO: FromIO2<'TaskEither'>

Added in v2.10.0

FromTask

Signature

export declare const FromTask: FromTask2<'TaskEither'>

Added in v2.10.0

Functor

Signature

export declare const Functor: Functor2<'TaskEither'>

Added in v2.7.0

Monad

Signature

export declare const Monad: Monad2<'TaskEither'>

Added in v2.10.0

MonadIO

Signature

export declare const MonadIO: MonadIO2<'TaskEither'>

Added in v2.10.0

MonadTask

Signature

export declare const MonadTask: MonadTask2<'TaskEither'>

Added in v2.10.0

MonadThrow

Signature

export declare const MonadThrow: MonadThrow2<'TaskEither'>

Added in v2.10.0

Pointed

Signature

export declare const Pointed: Pointed2<'TaskEither'>

Added in v2.10.0

interop

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

tryCatch

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

tryCatchK

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

Signature

export declare const tryCatchK: <E, A extends readonly unknown[], B>(
  f: (...a: A) => Promise<B>,
  onRejected: (reason: unknown) => E
) => (...a: A) => TaskEither<E, B>

Added in v2.5.0

legacy

bimap

Alias of mapBoth.

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

chain

Alias of flatMap.

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

chainEitherK

Alias of flatMapEither.

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

Alias of flatMapEither.

Signature

export declare const chainEitherKW: <E2, A, B>(
  f: (a: A) => E.Either<E2, B>
) => <E1>(ma: TaskEither<E1, A>) => TaskEither<E2 | E1, B>

Added in v2.6.1

chainFirst

Alias of tap.

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

chainFirstEitherK

Alias of tapEither.

Signature

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

Added in v2.12.0

chainFirstEitherKW

Alias of tapEither.

Less strict version of chainFirstEitherK.

The W suffix (short for Widening) means that the error types will be merged.

Signature

export declare const chainFirstEitherKW: <A, E2, B>(
  f: (a: A) => E.Either<E2, B>
) => <E1>(ma: TaskEither<E1, A>) => TaskEither<E2 | E1, A>

Added in v2.12.0

chainFirstIOK

Alias of tapIO.

Signature

export declare const chainFirstIOK: <A, B>(f: (a: A) => IO<B>) => <E>(first: TaskEither<E, A>) => TaskEither<E, A>

Added in v2.10.0

chainFirstTaskK

Alias of tapTask.

Signature

export declare const chainFirstTaskK: <A, B>(f: (a: A) => T.Task<B>) => <E>(first: TaskEither<E, A>) => TaskEither<E, A>

Added in v2.10.0

chainFirstW

Alias of tap.

Signature

export declare const chainFirstW: <E2, A, B>(
  f: (a: A) => TaskEither<E2, B>
) => <E1>(ma: TaskEither<E1, A>) => TaskEither<E2 | E1, A>

Added in v2.8.0

chainIOEitherK

Alias of flatMapIOEither.

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

Alias of flatMapIOEither.

Less strict version of chainIOEitherK.

The W suffix (short for Widening) means that the error types will be merged.

Signature

export declare const chainIOEitherKW: <E2, A, B>(
  f: (a: A) => IOEither<E2, B>
) => <E1>(ma: TaskEither<E1, A>) => TaskEither<E2 | E1, B>

Added in v2.6.1

chainIOK

Alias of flatMapIO.

Signature

export declare const chainIOK: <A, B>(f: (a: A) => IO<B>) => <E>(first: TaskEither<E, A>) => TaskEither<E, B>

Added in v2.10.0

chainNullableK

Use flatMapNullable.

Signature

export declare const chainNullableK: <E>(
  e: E
) => <A, B>(f: (a: A) => B | null | undefined) => (ma: TaskEither<E, A>) => TaskEither<E, NonNullable<B>>

Added in v2.12.0

chainOptionK

Use flatMapOption.

Signature

export declare const chainOptionK: <E>(
  onNone: LazyArg<E>
) => <A, B>(f: (a: A) => Option<B>) => (ma: TaskEither<E, A>) => TaskEither<E, B>

Added in v2.10.0

chainOptionKW

Use flatMapOption.

Signature

export declare const chainOptionKW: <E2>(
  onNone: LazyArg<E2>
) => <A, B>(f: (a: A) => Option<B>) => <E1>(ma: TaskEither<E1, A>) => TaskEither<E2 | E1, B>

Added in v2.13.2

chainTaskK

Alias of flatMapTask.

Signature

export declare const chainTaskK: <A, B>(f: (a: A) => T.Task<B>) => <E>(first: TaskEither<E, A>) => TaskEither<E, B>

Added in v2.10.0

chainTaskOptionK

Use flatMapTaskOption.

Signature

export declare const chainTaskOptionK: <E>(
  onNone: LazyArg<E>
) => <A, B>(f: (a: A) => TaskOption<B>) => (ma: TaskEither<E, A>) => TaskEither<E, B>

Added in v2.11.0

chainTaskOptionKW

Use flatMapTaskOption.

The W suffix (short for Widening) means that the error types will be merged.

Signature

export declare const chainTaskOptionKW: <E2>(
  onNone: LazyArg<E2>
) => <A, B>(f: (a: A) => TaskOption<B>) => <E1>(ma: TaskEither<E1, A>) => TaskEither<E2 | E1, B>

Added in v2.12.3

chainW

Alias of flatMap.

Signature

export declare const chainW: <E2, A, B>(
  f: (a: A) => TaskEither<E2, B>
) => <E1>(ma: TaskEither<E1, A>) => TaskEither<E2 | E1, B>

Added in v2.6.0

fromNullableK

Use liftNullable.

Signature

export declare const fromNullableK: <E>(
  e: E
) => <A extends readonly unknown[], B>(
  f: (...a: A) => B | null | undefined
) => (...a: A) => TaskEither<E, NonNullable<B>>

Added in v2.12.0

fromOptionK

Use liftOption.

Signature

export declare const fromOptionK: <E>(
  onNone: LazyArg<E>
) => <A extends readonly unknown[], B>(f: (...a: A) => Option<B>) => (...a: A) => TaskEither<E, B>

Added in v2.10.0

mapLeft

Alias of mapError.

Signature

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

Added in v2.0.0

orElseFirst

Alias of tapError.

Signature

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

Added in v2.11.0

orElseFirstW

Alias of tapError.

Signature

export declare const orElseFirstW: <E1, E2, B>(
  onLeft: (e: E1) => TaskEither<E2, B>
) => <A>(ma: TaskEither<E1, A>) => TaskEither<E1 | E2, A>

Added in v2.11.0

lifting

fromEitherK

Signature

export declare const fromEitherK: <E, A extends readonly unknown[], B>(
  f: (...a: A) => E.Either<E, B>
) => (...a: A) => TaskEither<E, B>

Added in v2.4.0

fromIOEitherK

Signature

export declare const fromIOEitherK: <E, A extends readonly unknown[], B>(
  f: (...a: A) => IOEither<E, B>
) => (...a: A) => TaskEither<E, B>

Added in v2.4.0

fromIOK

Signature

export declare const fromIOK: <A extends readonly unknown[], B>(
  f: (...a: A) => IO<B>
) => <E = never>(...a: A) => TaskEither<E, B>

Added in v2.10.0

fromPredicate

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): <B extends A>(b: B) => TaskEither<E, B>
  <E, A>(predicate: Predicate<A>, onFalse: (a: A) => E): (a: A) => TaskEither<E, A>
}

Added in v2.0.0

fromTaskK

Signature

export declare const fromTaskK: <A extends readonly unknown[], B>(
  f: (...a: A) => T.Task<B>
) => <E = never>(...a: A) => TaskEither<E, B>

Added in v2.10.0

fromTaskOptionK

Signature

export declare const fromTaskOptionK: <E>(
  onNone: LazyArg<E>
) => <A extends readonly unknown[], B>(f: (...a: A) => TaskOption<B>) => (...a: A) => TaskEither<E, B>

Added in v2.11.0

liftNullable

Signature

export declare const liftNullable: <A extends readonly unknown[], B, E>(
  f: (...a: A) => B | null | undefined,
  onNullable: (...a: A) => E
) => (...a: A) => TaskEither<E, NonNullable<B>>

Added in v2.15.0

liftOption

Signature

export declare const liftOption: <A extends readonly unknown[], B, E>(
  f: (...a: A) => Option<B>,
  onNone: (...a: A) => E
) => (...a: A) => TaskEither<E, B>

Added in v2.15.0

mapping

as

Maps the Right value of this TaskEither to the specified constant value.

Signature

export declare const as: {
  <A>(a: A): <E, _>(self: TaskEither<E, _>) => TaskEither<E, A>
  <E, _, A>(self: TaskEither<E, _>, a: A): TaskEither<E, A>
}

Added in v2.16.0

asUnit

Maps the Right value of this TaskEither to the void constant value.

Signature

export declare const asUnit: <E, _>(self: TaskEither<E, _>) => TaskEither<E, void>

Added in v2.16.0

flap

Signature

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

Added in v2.10.0

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

model

TaskEither (interface)

Signature

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

Added in v2.0.0

pattern matching

fold

Alias of matchE.

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

foldW

Alias of matchEW.

Signature

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

Added in v2.10.0

match

Signature

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

Added in v2.10.0

matchE

The E suffix (short for Effect) means that the handlers return an effect (Task).

Signature

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

Added in v2.10.0

matchEW

Less strict version of matchE.

The W suffix (short for Widening) means that the handler return types will be merged.

Signature

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

Added in v2.10.0

matchW

Less strict version of match.

The W suffix (short for Widening) means that the handler return types will be merged.

Signature

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

Added in v2.10.0

sequencing

flatMap

Signature

export declare const flatMap: {
  <A, E2, B>(f: (a: A) => TaskEither<E2, B>): <E1>(ma: TaskEither<E1, A>) => TaskEither<E2 | E1, B>
  <E1, A, E2, B>(ma: TaskEither<E1, A>, f: (a: A) => TaskEither<E2, B>): TaskEither<E1 | E2, B>
}

Added in v2.14.0

flatMapEither

Signature

export declare const flatMapEither: {
  <A, E2, B>(f: (a: A) => E.Either<E2, B>): <E1>(self: TaskEither<E1, A>) => TaskEither<E2 | E1, B>
  <E1, A, E2, B>(self: TaskEither<E1, A>, f: (a: A) => E.Either<E2, B>): TaskEither<E1 | E2, B>
}

Added in v2.15.0

flatMapIO

Signature

export declare const flatMapIO: {
  <A, B>(f: (a: A) => IO<B>): <E>(self: TaskEither<E, A>) => TaskEither<E, B>
  <E, A, B>(self: TaskEither<E, A>, f: (a: A) => IO<B>): TaskEither<E, B>
}

Added in v2.15.0

flatMapIOEither

Signature

export declare const flatMapIOEither: {
  <A, E2, B>(f: (a: A) => IOEither<E2, B>): <E1>(self: TaskEither<E1, A>) => TaskEither<E2 | E1, B>
  <E1, A, E2, B>(self: TaskEither<E1, A>, f: (a: A) => IOEither<E2, B>): TaskEither<E1 | E2, B>
}

Added in v2.16.0

flatMapNullable

Signature

export declare const flatMapNullable: {
  <A, B, E2>(f: (a: A) => B | null | undefined, onNullable: (a: A) => E2): <E1>(
    self: TaskEither<E1, A>
  ) => TaskEither<E2 | E1, NonNullable<B>>
  <E1, A, B, E2>(self: TaskEither<E1, A>, f: (a: A) => B | null | undefined, onNullable: (a: A) => E2): TaskEither<
    E1 | E2,
    NonNullable<B>
  >
}

Added in v2.15.0

flatMapOption

Signature

export declare const flatMapOption: {
  <A, B, E2>(f: (a: A) => Option<B>, onNone: (a: A) => E2): <E1>(self: TaskEither<E1, A>) => TaskEither<E2 | E1, B>
  <E1, A, B, E2>(self: TaskEither<E1, A>, f: (a: A) => Option<B>, onNone: (a: A) => E2): TaskEither<E1 | E2, B>
}

Added in v2.15.0

flatMapTask

Signature

export declare const flatMapTask: {
  <A, B>(f: (a: A) => T.Task<B>): <E>(self: TaskEither<E, A>) => TaskEither<E, B>
  <E, A, B>(self: TaskEither<E, A>, f: (a: A) => T.Task<B>): TaskEither<E, B>
}

Added in v2.16.0

flatMapTaskOption

Signature

export declare const flatMapTaskOption: {
  <A, E2, B>(f: (a: A) => TaskOption<B>, onNone: (a: A) => E2): <E1>(self: TaskEither<E1, A>) => TaskEither<E2 | E1, B>
  <E1, A, E2, B>(self: TaskEither<E1, A>, f: (a: A) => TaskOption<B>, onNone: (a: A) => E2): TaskEither<E1 | E2, B>
}

Added in v2.16.0

flatten

Signature

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

Added in v2.0.0

flattenW

Less strict version of flatten.

The W suffix (short for Widening) means that the error types will be merged.

Signature

export declare const flattenW: <E1, E2, A>(mma: TaskEither<E1, TaskEither<E2, A>>) => TaskEither<E1 | E2, A>

Added in v2.11.0

traversing

sequenceArray

Equivalent to ReadonlyArray#sequence(Applicative).

Signature

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

Added in v2.9.0

sequenceSeqArray

Equivalent to ReadonlyArray#sequence(ApplicativeSeq).

Signature

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

Added in v2.9.0

traverseArray

Equivalent to ReadonlyArray#traverse(Applicative).

Signature

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

Added in v2.9.0

traverseArrayWithIndex

Equivalent to ReadonlyArray#traverseWithIndex(Applicative).

Signature

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

Added in v2.9.0

traverseReadonlyArrayWithIndex

Equivalent to ReadonlyArray#traverseWithIndex(ApplicativePar).

Signature

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

Added in v2.11.0

traverseReadonlyArrayWithIndexSeq

Equivalent to ReadonlyArray#traverseWithIndex(ApplicativeSeq).

Signature

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

Added in v2.11.0

traverseReadonlyNonEmptyArrayWithIndex

Equivalent to ReadonlyNonEmptyArray#traverseWithIndex(ApplicativePar).

Signature

export declare const traverseReadonlyNonEmptyArrayWithIndex: <A, E, B>(
  f: (index: number, a: A) => TaskEither<E, B>
) => (as: ReadonlyNonEmptyArray<A>) => TaskEither<E, ReadonlyNonEmptyArray<B>>

Added in v2.11.0

traverseReadonlyNonEmptyArrayWithIndexSeq

Equivalent to ReadonlyArray#traverseWithIndex(ApplicativeSeq).

Signature

export declare const traverseReadonlyNonEmptyArrayWithIndexSeq: <A, E, B>(
  f: (index: number, a: A) => TaskEither<E, B>
) => (as: ReadonlyNonEmptyArray<A>) => TaskEither<E, ReadonlyNonEmptyArray<B>>

Added in v2.11.0

traverseSeqArray

Equivalent to ReadonlyArray#traverse(ApplicativeSeq).

Signature

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

Added in v2.9.0

traverseSeqArrayWithIndex

Equivalent to ReadonlyArray#traverseWithIndex(ApplicativeSeq).

Signature

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

Added in v2.9.0

type lambdas

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

utils

ApT

Signature

export declare const ApT: TaskEither<never, readonly []>

Added in v2.11.0

ap

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

apFirst

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

Signature

export declare const apFirst: <E, B>(second: TaskEither<E, B>) => <A>(first: TaskEither<E, A>) => TaskEither<E, A>

Added in v2.0.0

apFirstW

Less strict version of apFirst.

The W suffix (short for Widening) means that the error types will be merged.

Signature

export declare const apFirstW: <E2, B>(
  second: TaskEither<E2, B>
) => <E1, A>(first: TaskEither<E1, A>) => TaskEither<E2 | E1, A>

Added in v2.12.0

apSecond

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

Signature

export declare const apSecond: <E, B>(second: TaskEither<E, B>) => <A>(first: TaskEither<E, A>) => TaskEither<E, B>

Added in v2.0.0

apSecondW

Less strict version of apSecond.

The W suffix (short for Widening) means that the error types will be merged.

Signature

export declare const apSecondW: <E2, B>(
  second: TaskEither<E2, B>
) => <E1, A>(first: TaskEither<E1, A>) => TaskEither<E2 | E1, B>

Added in v2.12.0

apW

Less strict version of ap.

The W suffix (short for Widening) means that the error types will be merged.

Signature

export declare const apW: <E2, A>(
  fa: TaskEither<E2, A>
) => <E1, B>(fab: TaskEither<E1, (a: A) => B>) => TaskEither<E2 | E1, 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

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

bracketW

Less strict version of bracket.

The W suffix (short for Widening) means that the error types will be merged.

Signature

export declare const bracketW: <E1, A, E2, B, E3>(
  acquire: TaskEither<E1, A>,
  use: (a: A) => TaskEither<E2, B>,
  release: (a: A, e: E.Either<E2, B>) => TaskEither<E3, void>
) => TaskEither<E1 | E2 | E3, B>

Added in v2.12.0

swap

Signature

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

Added in v2.0.0

throwError

Signature

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

Added in v2.7.0

zone of death

getApplyMonoid

Use getApplicativeMonoid instead.

Signature

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

Added in v2.0.0

getApplySemigroup

Use getApplySemigroup instead.

Signature

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

Added in v2.0.0

getSemigroup

Use getApplySemigroup instead.

Signature

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

Added in v2.0.0

getTaskValidation

Use getApplicativeTaskValidation and getAltTaskValidation instead.

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

taskEitherSeq

This instance is deprecated, use small, specific instances instead. For example if a function needs a Functor instance, pass TE.Functor instead of TE.taskEitherSeq (where TE is from import TE from 'fp-ts/TaskEither')

Signature

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

Added in v2.0.0

taskEither

This instance is deprecated, use small, specific instances instead. For example if a function needs a Functor instance, pass TE.Functor instead of TE.taskEither (where TE is from import TE from 'fp-ts/TaskEither')

Signature

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

Added in v2.0.0

TaskEither overview

Table of contents

combinators

tap

tapEither

tapIO

tapTask

constructors

left

leftIO

leftTask

of

right

rightIO

rightTask

conversions

fromEither

fromIO

fromIOEither

fromNullable

fromOption

fromTask

fromTaskOption

toUnion

do notation

Do

apS

apSW

bind

bindTo

bindW

let

error handling

alt

altW

getAltTaskValidation

getApplicativeTaskValidation

getOrElse

getOrElseW

mapBoth

mapError

orElse

orElseFirstIOK

orElseFirstTaskK

orElseW

orLeft

tapError

filtering

filterOrElse

filterOrElseW

getCompactable

getFilterable

instances

Alt

ApplicativePar

ApplicativeSeq

ApplyPar

ApplySeq

Bifunctor

Chain

FromEither

FromIO

FromTask

Functor

Monad

MonadIO

MonadTask

MonadThrow

Pointed

interop

taskify

tryCatch

tryCatchK

legacy

bimap

chain

chainEitherK

chainEitherKW

chainFirst

chainFirstEitherK