IO overview 

interface IO<A> {
  (): A
}

IO<A> represents a non-deterministic synchronous computation that can cause side effects, yields a value of type A and never fails.

If you want to represent a synchronous computation that may fail, please see IOEither. If you want to represent a synchronous computation that may yield nothing, please see IOOption.

Added in v2.0.0

Table of contents

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: {
  <A, _>(self: IO<A>, f: (a: A) => IO<_>): IO<A>
  <A, _>(f: (a: A) => IO<_>): (self: IO<A>) => IO<A>
}

Added in v2.15.0

constructors

of

Signature

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

Added in v2.0.0

do notation

Do

Signature

export declare const Do: IO<{}>

Added in v2.9.0

apS

Signature

export declare const apS: <N, A, B>(
  name: Exclude<N, keyof A>,
  fb: IO<B>
) => (fa: IO<A>) => IO<{ 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, B>(
  name: Exclude<N, keyof A>,
  f: (a: A) => IO<B>
) => (ma: IO<A>) => IO<{ 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) => <A>(fa: IO<A>) => IO<{ readonly [K in N]: A }>

Added in v2.8.0

let

Signature

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

Added in v2.13.0

instances

Applicative

Signature

export declare const Applicative: Applicative1<'IO'>

Added in v2.7.0

Apply

Signature

export declare const Apply: Apply1<'IO'>

Added in v2.10.0

Chain

Signature

export declare const Chain: chainable.Chain1<'IO'>

Added in v2.10.0

ChainRec

Signature

export declare const ChainRec: ChainRec1<'IO'>

Added in v2.7.0

FromIO

Signature

export declare const FromIO: FromIO1<'IO'>

Added in v2.10.0

Functor

Signature

export declare const Functor: Functor1<'IO'>

Added in v2.7.0

Monad

Signature

export declare const Monad: Monad1<'IO'>

Added in v2.7.0

MonadIO

Signature

export declare const MonadIO: MonadIO1<'IO'>

Added in v2.7.0

Pointed

Signature

export declare const Pointed: Pointed1<'IO'>

Added in v2.10.0

legacy

chain

Alias of flatMap.

Signature

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

Added in v2.0.0

chainFirst

Alias of tap.

Signature

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

Added in v2.0.0

mapping

as

Maps the value to the specified constant value.

Signature

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

Added in v2.16.0

asUnit

Maps the value to the void constant value.

Signature

export declare const asUnit: <_>(self: IO<_>) => IO<void>

Added in v2.16.0

flap

Signature

export declare const flap: <A>(a: A) => <B>(fab: IO<(a: A) => B>) => IO<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) => (fa: IO<A>) => IO<B>

Added in v2.0.0

model

IO (interface)

Signature

export interface IO<A> {
  (): A
}

Added in v2.0.0

sequencing

flatMap

Signature

export declare const flatMap: {
  <A, B>(f: (a: A) => IO<B>): (ma: IO<A>) => IO<B>
  <A, B>(ma: IO<A>, f: (a: A) => IO<B>): IO<B>
}

Added in v2.14.0

flatten

Signature

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

Added in v2.0.0

traversing

sequenceArray

Equivalent to ReadonlyArray#sequence(Applicative).

Signature

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

Added in v2.9.0

traverseArray

Equivalent to ReadonlyArray#traverse(Applicative).

Signature

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

Added in v2.9.0

traverseArrayWithIndex

Equivalent to ReadonlyArray#traverseWithIndex(Applicative).

Signature

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

Added in v2.9.0

traverseReadonlyArrayWithIndex

Equivalent to ReadonlyArray#traverseWithIndex(Applicative).

Signature

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

Added in v2.11.0

traverseReadonlyNonEmptyArrayWithIndex

Equivalent to ReadonlyNonEmptyArray#traverseWithIndex(Applicative).

Signature

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

Added in v2.11.0

type lambdas

URI

Signature

export declare const URI: 'IO'

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: IO<readonly []>

Added in v2.11.0

ap

Signature

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

Added in v2.0.0

apFirst

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

Signature

export declare const apFirst: <B>(second: IO<B>) => <A>(first: IO<A>) => IO<A>

Added in v2.0.0

apSecond

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

Signature

export declare const apSecond: <B>(second: IO<B>) => <A>(first: IO<A>) => IO<B>

Added in v2.0.0

zone of death

fromIO

Signature

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

Added in v2.7.0

getMonoid

Use getApplicativeMonoid instead.

Signature

export declare const getMonoid: <A>(M: Monoid<A>) => Monoid<IO<A>>

Added in v2.0.0

getSemigroup

Use getApplySemigroup instead.

Signature

export declare const getSemigroup: <A>(S: Semigroup<A>) => Semigroup<IO<A>>

Added in v2.0.0

io

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

Signature

export declare const io: Monad1<'IO'> & MonadIO1<'IO'> & ChainRec1<'IO'>

Added in v2.0.0