## 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`.

# 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>
}
``````

# constructors

## of

Signature

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

# do notation

## Do

Signature

``````export declare const Do: IO<{}>
``````

## 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 }>
``````

## 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 }>
``````

## bindTo

Signature

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

## 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 }>
``````

# instances

## Applicative

Signature

``````export declare const Applicative: Applicative1<'IO'>
``````

## Apply

Signature

``````export declare const Apply: Apply1<'IO'>
``````

## Chain

Signature

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

## ChainRec

Signature

``````export declare const ChainRec: ChainRec1<'IO'>
``````

## FromIO

Signature

``````export declare const FromIO: FromIO1<'IO'>
``````

## Functor

Signature

``````export declare const Functor: Functor1<'IO'>
``````

Signature

``````export declare const Monad: Monad1<'IO'>
``````

Signature

``````export declare const MonadIO: MonadIO1<'IO'>
``````

## Pointed

Signature

``````export declare const Pointed: Pointed1<'IO'>
``````

# legacy

## chain

Alias of `flatMap`.

Signature

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

## chainFirst

Alias of `tap`.

Signature

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

# 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> }
``````

## asUnit

Maps the value to the void constant value.

Signature

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

## flap

Signature

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

## 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>
``````

# model

## IO (interface)

Signature

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

# 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>
}
``````

## flatten

Signature

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

# traversing

## sequenceArray

Equivalent to `ReadonlyArray#sequence(Applicative)`.

Signature

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

## traverseArray

Equivalent to `ReadonlyArray#traverse(Applicative)`.

Signature

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

## traverseArrayWithIndex

Equivalent to `ReadonlyArray#traverseWithIndex(Applicative)`.

Signature

``````export declare const traverseArrayWithIndex: <A, B>(
f: (index: number, a: A) => IO<B>
``````

Equivalent to `ReadonlyArray#traverseWithIndex(Applicative)`.

Signature

``````export declare const traverseReadonlyArrayWithIndex: <A, B>(
f: (index: number, a: A) => IO<B>
``````

Equivalent to `ReadonlyNonEmptyArray#traverseWithIndex(Applicative)`.

Signature

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

# type lambdas

## URI

Signature

``````export declare const URI: 'IO'
``````

## URI (type alias)

Signature

``````export type URI = typeof URI
``````

# utils

## ApT

Signature

``````export declare const ApT: IO<readonly []>
``````

## ap

Signature

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

## 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>
``````

## 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>
``````

# zone of death

## fromIO

Signature

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

## getMonoid

Use `getApplicativeMonoid` instead.

Signature

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

## getSemigroup

Use `getApplySemigroup` instead.

Signature

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

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