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.

(description adapted from https://package.elm-lang.org/packages/joneshf/elm-these)

Adapted from https://github.com/purescript-contrib/purescript-these

Added in v2.0.0

Table of contents

constructors

both

Signature

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

Added in v2.0.0

left

Signature

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

Added in v2.0.0

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))

Added in v2.0.0

of

Signature

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

Added in v2.0.0

Signature

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

Added in v2.0.0

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))

Added in v2.0.0

conversions

fromOption

Signature

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

Added in v2.10.0

fromOptions

Takes a pair of Options 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)))

Added in v2.0.0

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'))

Added in v2.0.0

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)

Added in v2.0.0

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))

Added in v2.0.0

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)

Added in v2.0.0

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]
)

Added in v2.10.0

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>

Added in v2.0.0

folding

foldMap

Signature

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

Added in v2.0.0

reduce

Signature

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

Added in v2.0.0

reduceRight

Signature

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

Added in v2.0.0

instances

Bifunctor

Signature

export declare const Bifunctor: Bifunctor2<'These'>

Added in v2.7.0

Foldable

Signature

export declare const Foldable: Foldable2<'These'>

Added in v2.7.0

FromEither

Signature

export declare const FromEither: FromEither2<'These'>

Added in v2.10.0

FromThese

Signature

export declare const FromThese: FromThese2<'These'>

Added in v2.11.0

Functor

Signature

export declare const Functor: Functor2<'These'>

Added in v2.7.0

Pointed

Signature

export declare const Pointed: Pointed2<'These'>

Added in v2.10.0

Traversable

Signature

export declare const Traversable: Traversable2<'These'>

Added in v2.7.0

getApplicative

Signature

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

Added in v2.7.0

getApply

Signature

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

Added in v2.10.0

getChain

Signature

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

Added in v2.10.0

getEq

Signature

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

Added in v2.0.0

getMonad

Signature

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

Added in v2.0.0

getSemigroup

Signature

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

Added in v2.0.0

getShow

Signature

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

Added in v2.0.0

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>

Added in v2.10.0

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>
}

Added in v2.13.0

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>

Added in v2.0.0

flap

Signature

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

Added in v2.0.0

model

Both (interface)

Signature

export interface Both<E, A> {
  readonly _tag: 'Both'
  readonly left: E
  readonly right: A
}

Added in v2.0.0

These (type alias)

Signature

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

Added in v2.0.0

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

Added in v2.0.0

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

Added in v2.10.0

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

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

Added in v2.10.0

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>

Added in v2.0.0

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>

Added in v2.0.0

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>

Added in v2.0.0

traversing

sequence

Signature

export declare const sequence: Sequence2<'These'>

Added in v2.6.3

traverse

Signature

export declare const traverse: PipeableTraverse2<'These'>

Added in v2.6.3

traverseReadonlyArrayWithIndex

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[]>

Added in v2.11.0

traverseReadonlyNonEmptyArrayWithIndex

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>
) => (as: ReadonlyNonEmptyArray<A>) => These<E, ReadonlyNonEmptyArray<B>>

Added in v2.11.0

type lambdas

URI

Signature

export declare const URI: 'These'

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: These<never, readonly []>

Added in v2.11.0

elem

Signature

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

Added in v2.11.0

exists

Signature

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

Added in v2.11.0

swap

Signature

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

Added in v2.4.0

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'>

Added in v2.0.0

toTuple

Use toTuple2 instead.

Signature

export declare const toTuple: <E, A>(e: E, a: A) => (fa: These<E, A>) => [E, A]

Added in v2.0.0