lightweight-hkt.ts

class HKT<F, A> {}

interface Functor<F> {
  map<A, B>(f: (a: A) => B, fa: HKT<F, A>): HKT<F, B>
}

type Seq<A> = HKT<'Seq', A>
type Abc<A> = HKT<'Abc', A>

type seq<A> = Array<A>

const seqMap = <A, B>(f: (a: A) => B, fa: seq<A>): seq<B> => fa.map(f)

const inj = <A>(a: seq<A>): Seq<A> => ((a as any) as Seq<A>)

const prj = <A>(a: Seq<A>): seq<A> => ((a as any) as seq<A>)

const map = <A, B>(f: (a: A) => B, fa: Seq<A>): Seq<B> =>
  inj(seqMap(f, prj(fa)))

const double = <F, A>(fa: HKT<F, A>, f: Functor<F>): HKT<F, [A, A]> =>
  f.map((a: A) => [a, a], fa)

const x: seq<number> = [ 1, 2, 3, 4, 5 ]

// Here is a bug Seq<[number, number]> != Seq<number>
const y: Seq<number> = double<'Seq', number>(x, {map})

console.log(y)