fp-ts

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Overview

The Choice class extends Profunctor with combinators for working with sum types.

left and right lift values in a Profunctor to act on the Left and Right components of a sum, respectively.

Looking at Choice through the intuition of inputs and outputs yields the following type signature:

left ::  forall input output a. p input output -> p (Either input a) (Either output a)
right :: forall input output a. p input output -> p (Either a input) (Either a output)

If we specialize the profunctor p to the function arrow, we get the following type signatures:

left ::  forall input output a. (input -> output) -> (Either input a) -> (Either output a)
right :: forall input output a. (input -> output) -> (Either a input) -> (Either a output)

When the profunctor is Function application, left allows you to map a function over the left side of an Either, and right maps it over the right side (same as map would do).

Adapted from https://github.com/purescript/purescript-profunctor/blob/master/src/Data/Profunctor/Choice.purs

Table of contents

Choice (interface)

Signature

export interface Choice<F> extends Profunctor<F> {
  readonly left: <A, B, C>(pab: HKT2<F, A, B>) => HKT2<F, Either<A, C>, Either<B, C>>
  readonly right: <A, B, C>(pbc: HKT2<F, B, C>) => HKT2<F, Either<A, B>, Either<A, C>>
}

Added in v1.11.0

Choice2 (interface)

Signature

export interface Choice2<F extends URIS2> extends Profunctor2<F> {
  readonly left: <A, B, C>(pab: Type2<F, A, B>) => Type2<F, Either<A, C>, Either<B, C>>
  readonly right: <A, B, C>(pbc: Type2<F, B, C>) => Type2<F, Either<A, B>, Either<A, C>>
}

Choice3 (interface)

Signature

export interface Choice3<F extends URIS3> extends Profunctor3<F> {
  readonly left: <U, A, B, C>(pab: Type3<F, U, A, B>) => Type3<F, U, Either<A, C>, Either<B, C>>
  readonly right: <U, A, B, C>(pbc: Type3<F, U, B, C>) => Type3<F, U, Either<A, B>, Either<A, C>>
}

Choice4 (interface)

Signature

export interface Choice4<F extends URIS4> extends Profunctor4<F> {
  readonly left: <X, U, A, B, C>(pab: Type4<F, X, U, A, B>) => Type4<F, X, U, Either<A, C>, Either<B, C>>
  readonly right: <X, U, A, B, C>(pbc: Type4<F, X, U, B, C>) => Type4<F, X, U, Either<A, B>, Either<A, C>>
}

fanin (function)

Compose a value which eliminates a sum from two values, each eliminating one side of the sum.

This combinator is useful when assembling values from smaller components, because it provides a way to support two different types of input.

Specializing (|||) to function application would look like this:

(|||) :: forall a b c d. (a -> c) -> (b -> c) -> Either a b -> c

We take two functions, f and g, which both return the same type c and we transform them into a single function which takes an Either value with the parameter type of f on the left side and the parameter type of g on the right side. The function then runs either f or g, depending on whether the Either value is a Left or a Right. This allows us to bundle two different computations which both have the same result type into one function which will run the approriate computation based on the parameter supplied in the Either value.

Signature

export function fanin<F extends URIS3>(
  F: Category3<F> & Choice3<F>
): <U, A, B, C>(pac: Type3<F, U, A, C>, pbc: Type3<F, U, B, C>) => Type3<F, U, Either<A, B>, C>
export function fanin<F extends URIS2>(
  F: Category2<F> & Choice2<F>
): <A, B, C>(pac: Type2<F, A, C>, pbc: Type2<F, B, C>) => Type2<F, Either<A, B>, C>
export function fanin<F>(
  F: Category<F> & Choice<F>
): <A, B, C>(pac: HKT2<F, A, C>, pbc: HKT2<F, B, C>) => HKT2<F, Either<A, B>, C> { ... }

Added in v1.11.0

splitChoice (function)

Compose a value acting on a sum from two values, each acting on one of the components of the sum.

Specializing (+++) to function application would look like this:

(+++) :: forall a b c d. (a -> b) -> (c -> d) -> (Either a c) -> (Either b d)

We take two functions, f and g, and we transform them into a single function which takes an Eitherand maps f over the left side and g over the right side. Just like bi-map would do for the bi-functor instance of Either.

Signature

export function splitChoice<F extends URIS3>(
  F: Category3<F> & Choice3<F>
): <U, A, B, C, D>(pab: Type3<F, U, A, B>, pcd: Type3<F, U, C, D>) => Type3<F, U, Either<A, C>, Either<B, D>>
export function splitChoice<F extends URIS2>(
  F: Category2<F> & Choice2<F>
): <A, B, C, D>(pab: Type2<F, A, B>, pcd: Type2<F, C, D>) => Type2<F, Either<A, C>, Either<B, D>>
export function splitChoice<F>(
  F: Category<F> & Choice<F>
): <A, B, C, D>(pab: HKT2<F, A, B>, pcd: HKT2<F, C, D>) => HKT2<F, Either<A, C>, Either<B, D>> { ... }

Added in v1.11.0