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
- Applicative
- Apply
- Bifunctor
- Functor
- Monad
- MonadIO
- MonadTask
- combinators
- constructors
- destructors
- instances
- model
- utils
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
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
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 Right
s 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 Right
s 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