Profunctor instances for UpStar and DownStar

profunctors.ts

import { Functor } from 'fp-ts/lib/Functor';
import { HKT, Kind3, URIS3 } from 'fp-ts/lib/HKT';

export type Fn<A, B> = (a: A) => B;

export interface Profunctor<F extends URIS3, G> {
  dimap: <A, B>(ab: Fn<A, B>) => <C, D>(cd: Fn<C, D>) => (fbc: Kind3<F, G, B, C>) => Kind3<F, G, A, D>;
}

export type UpStar<F, A, B> = (a: A) => HKT<F, B>;
export const UpStarURI = 'UpStar';
export type UpStarURI = typeof UpStarURI;

export type DownStar<F, A, B> = (fa: HKT<F, A>) => B;
export const DownStarURI = 'DownStar';
export type DownStarURI = typeof DownStarURI;

export const UpStar = <F, A, B>(afb: (a: A) => HKT<F, B>): UpStar<F, A, B> => afb;

declare module 'fp-ts/lib/HKT' {
  interface URItoKind3<R, E, A> {
    UpStar: UpStar<R, E, A>;
    DownStar: DownStar<R, E, A>;
  }
}

export const getUpStarProfunctor = <F>(F: Functor<F>): Profunctor<UpStarURI, F> => ({
  dimap: (ab) => (cd) => (fbc) => (a) => F.map(fbc(ab(a)), cd),
});

export const getDownStarProfunctor = <F>(F: Functor<F>): Profunctor<DownStarURI, F> => ({
  dimap: (ab) => (cd) => (fbc) => (fa) => cd(fbc(F.map(fa, ab))),
});