## These overview

A data structure providing “inclusive-or” as opposed to `Either`’s “exclusive-or”.

If you interpret `Either<E, A>` as suggesting the computation may either fail or succeed (exclusively), then `These<E, A>` may fail, succeed, or do both at the same time.

There are a few ways to interpret the both case:

• You can think of a computation that has a non-fatal error.
• You can think of a computation that went as far as it could before erroring.
• You can think of a computation that keeps track of errors as it completes.

Another way you can think of `These<E, A>` is saying that we want to handle `E` kind of data, `A` kind of data, or both `E` and `A` kind of data at the same time. This is particularly useful when it comes to displaying UI’s.

# constructors

## both

Signature

``````export declare function both<E, A>(left: E, right: A): These<E, A>
``````

## left

Signature

``````export declare function left<E = never, A = never>(left: E): These<E, A>
``````

## leftOrBoth

Signature

``````export declare function leftOrBoth<E>(e: E): <A>(ma: Option<A>) => These<E, A>
``````

Example

``````import { leftOrBoth, left, both } from 'fp-ts/These'
import { none, some } from 'fp-ts/Option'

assert.deepStrictEqual(leftOrBoth('a')(none), left('a'))
assert.deepStrictEqual(leftOrBoth('a')(some(1)), both('a', 1))
``````

## of

Signature

``````export declare const of: <E = never, A = never>(right: A) => These<E, A>
``````

Signature

``````export declare function right<E = never, A = never>(right: A): These<E, A>
``````

## rightOrBoth

Signature

``````export declare function rightOrBoth<A>(a: A): <E>(me: Option<E>) => These<E, A>
``````

Example

``````import { rightOrBoth, right, both } from 'fp-ts/These'
import { none, some } from 'fp-ts/Option'

assert.deepStrictEqual(rightOrBoth(1)(none), right(1))
assert.deepStrictEqual(rightOrBoth(1)(some('a')), both('a', 1))
``````

# conversions

## fromOption

Signature

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

## fromOptions

Takes a pair of `Option`s and attempts to create a `These` from them

Signature

``````export declare const fromOptions: <E, A>(fe: Option<E>, fa: Option<A>) => Option<These<E, A>>
``````

Example

``````import { fromOptions, left, right, both } from 'fp-ts/These'
import { none, some } from 'fp-ts/Option'

assert.deepStrictEqual(fromOptions(none, none), none)
assert.deepStrictEqual(fromOptions(some('a'), none), some(left('a')))
assert.deepStrictEqual(fromOptions(none, some(1)), some(right(1)))
assert.deepStrictEqual(fromOptions(some('a'), some(1)), some(both('a', 1)))
``````

## getLeft

Returns an `E` value if possible

Signature

``````export declare function getLeft<E, A>(fa: These<E, A>): Option<E>
``````

Example

``````import { getLeft, left, right, both } from 'fp-ts/These'
import { none, some } from 'fp-ts/Option'

assert.deepStrictEqual(getLeft(left('a')), some('a'))
assert.deepStrictEqual(getLeft(right(1)), none)
assert.deepStrictEqual(getLeft(both('a', 1)), some('a'))
``````

## getLeftOnly

Returns the `E` value if and only if the value is constructed with `Left`

Signature

``````export declare function getLeftOnly<E, A>(fa: These<E, A>): Option<E>
``````

Example

``````import { getLeftOnly, left, right, both } from 'fp-ts/These'
import { none, some } from 'fp-ts/Option'

assert.deepStrictEqual(getLeftOnly(left('a')), some('a'))
assert.deepStrictEqual(getLeftOnly(right(1)), none)
assert.deepStrictEqual(getLeftOnly(both('a', 1)), none)
``````

## getRight

Returns an `A` value if possible

Signature

``````export declare function getRight<E, A>(fa: These<E, A>): Option<A>
``````

Example

``````import { getRight, left, right, both } from 'fp-ts/These'
import { none, some } from 'fp-ts/Option'

assert.deepStrictEqual(getRight(left('a')), none)
assert.deepStrictEqual(getRight(right(1)), some(1))
assert.deepStrictEqual(getRight(both('a', 1)), some(1))
``````

## getRightOnly

Returns the `A` value if and only if the value is constructed with `Right`

Signature

``````export declare function getRightOnly<E, A>(fa: These<E, A>): Option<A>
``````

Example

``````import { getRightOnly, left, right, both } from 'fp-ts/These'
import { none, some } from 'fp-ts/Option'

assert.deepStrictEqual(getRightOnly(left('a')), none)
assert.deepStrictEqual(getRightOnly(right(1)), some(1))
assert.deepStrictEqual(getRightOnly(both('a', 1)), none)
``````

## toTuple2

Signature

``````export declare const toTuple2: <E, A>(e: LazyArg<E>, a: LazyArg<A>) => (fa: These<E, A>) => readonly [E, A]
``````

Example

``````import { toTuple2, left, right, both } from 'fp-ts/These'

assert.deepStrictEqual(
toTuple2(
() => 'a',
() => 1
)(left('b')),
['b', 1]
)
assert.deepStrictEqual(
toTuple2(
() => 'a',
() => 1
)(right(2)),
['a', 2]
)
assert.deepStrictEqual(
toTuple2(
() => 'a',
() => 1
)(both('b', 2)),
['b', 2]
)
``````

# error handling

## 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: These<E, A>) => These<G, A>
``````

# folding

## foldMap

Signature

``````export declare const foldMap: <M>(M: Monoid<M>) => <A>(f: (a: A) => M) => <E>(fa: These<E, A>) => M
``````

## reduce

Signature

``````export declare const reduce: <A, B>(b: B, f: (b: B, a: A) => B) => <E>(fa: These<E, A>) => B
``````

## reduceRight

Signature

``````export declare const reduceRight: <A, B>(b: B, f: (a: A, b: B) => B) => <E>(fa: These<E, A>) => B
``````

# instances

## Bifunctor

Signature

``````export declare const Bifunctor: Bifunctor2<'These'>
``````

## Foldable

Signature

``````export declare const Foldable: Foldable2<'These'>
``````

## FromEither

Signature

``````export declare const FromEither: FromEither2<'These'>
``````

## FromThese

Signature

``````export declare const FromThese: FromThese2<'These'>
``````

## Functor

Signature

``````export declare const Functor: Functor2<'These'>
``````

## Pointed

Signature

``````export declare const Pointed: Pointed2<'These'>
``````

## Traversable

Signature

``````export declare const Traversable: Traversable2<'These'>
``````

## getApplicative

Signature

``````export declare function getApplicative<E>(S: Semigroup<E>): Applicative2C<URI, E>
``````

## getApply

Signature

``````export declare const getApply: <E>(S: Semigroup<E>) => Apply2C<'These', E>
``````

## getChain

Signature

``````export declare function getChain<E>(S: Semigroup<E>): Chain2C<URI, E>
``````

## getEq

Signature

``````export declare function getEq<E, A>(EE: Eq<E>, EA: Eq<A>): Eq<These<E, A>>
``````

Signature

``````export declare function getMonad<E>(S: Semigroup<E>): Monad2C<URI, E> & MonadThrow2C<URI, E>
``````

## getSemigroup

Signature

``````export declare function getSemigroup<E, A>(SE: Semigroup<E>, SA: Semigroup<A>): Semigroup<These<E, A>>
``````

## getShow

Signature

``````export declare function getShow<E, A>(SE: Show<E>, SA: Show<A>): Show<These<E, A>>
``````

# lifting

## fromOptionK

Signature

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

## fromPredicate

Signature

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

# mapping

## 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: These<E, A>) => These<G, B>
``````

## flap

Signature

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

# model

## Both (interface)

Signature

``````export interface Both<E, A> {
}
``````

## These (type alias)

Signature

``````export type These<E, A> = Either<E, A> | Both<E, A>
``````

# pattern matching

## fold

Alias of `match`.

Signature

``````export declare const fold: <E, A, B>(
onLeft: (e: E) => B,
onRight: (a: A) => B,
onBoth: (e: E, a: A) => B
) => (fa: These<E, A>) => B
``````

## foldW

Alias of `matchW`.

Signature

``````export declare const foldW: <E, B, A, C, D>(
onLeft: (e: E) => B,
onRight: (a: A) => C,
onBoth: (e: E, a: A) => D
) => (fa: These<E, A>) => B | C | D
``````

## match

Signature

``````export declare const match: <E, A, B>(
onLeft: (e: E) => B,
onRight: (a: A) => B,
onBoth: (e: E, a: A) => B
) => (fa: These<E, A>) => B
``````

## 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, D>(
onLeft: (e: E) => B,
onRight: (a: A) => C,
onBoth: (e: E, a: A) => D
) => (fa: These<E, A>) => B | C | D
``````

# refinements

## isBoth

Returns `true` if the these is an instance of `Both`, `false` otherwise

Signature

``````export declare function isBoth<E, A>(fa: These<E, A>): fa is Both<E, A>
``````

## isLeft

Returns `true` if the these is an instance of `Left`, `false` otherwise

Signature

``````export declare const isLeft: <E>(fa: These<E, unknown>) => fa is Left<E>
``````

## isRight

Returns `true` if the these is an instance of `Right`, `false` otherwise

Signature

``````export declare const isRight: <A>(fa: These<unknown, A>) => fa is Right<A>
``````

# traversing

## sequence

Signature

``````export declare const sequence: Sequence2<'These'>
``````

## traverse

Signature

``````export declare const traverse: PipeableTraverse2<'These'>
``````

Equivalent to `ReadonlyArray#traverseWithIndex(getApplicative(S))`.

Signature

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

Equivalent to `ReadonlyNonEmptyArray#traverseWithIndex(getApplicative(S))`.

Signature

``````export declare const traverseReadonlyNonEmptyArrayWithIndex: <E>(
S: Semigroup<E>
) => <A, B>(
f: (index: number, a: A) => These<E, B>
``````

# type lambdas

## URI

Signature

``````export declare const URI: 'These'
``````

## URI (type alias)

Signature

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

# utils

## ApT

Signature

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

## elem

Signature

``````export declare const elem: <A>(E: Eq<A>) => (a: A) => <E>(ma: These<E, A>) => boolean
``````

## exists

Signature

``````export declare const exists: <A>(predicate: Predicate<A>) => (ma: These<unknown, A>) => boolean
``````

## swap

Signature

``````export declare const swap: <E, A>(fa: These<E, A>) => These<A, E>
``````

# zone of death

## these

This instance is deprecated, use small, specific instances instead. For example if a function needs a `Functor` instance, pass `T.Functor` instead of `T.these` (where `T` is from `import T from 'fp-ts/These'`)

Signature

``````export declare const these: Functor2<'These'> & Bifunctor2<'These'> & Foldable2<'These'> & Traversable2<'These'>
``````

Use `toTuple2` instead.
``````export declare const toTuple: <E, A>(e: E, a: A) => (fa: These<E, A>) => [E, A]