May someone help me out here with Contravariant 🙂 . The Prelude looks like this:

class ForOp private constructor() {
  companion object
}
typealias OpOf<A, X> = arrow.Kind2<ForOp, A, X>
typealias OpPartialOf<A> = arrow.Kind<ForEither, A>

@Suppress("UNCHECKED_CAST", "NOTHING_TO_INLINE")
inline fun <A, X> OpOf<A, X>.fix(): Op<A, X> =
  this as Op<A, X>


@higherkind
data class Op<A, X>(val f: (X) -> A) {
  fun <B> compose(other: Op<B, A>): Op<B, X> =
    Op(other.f compose f)

  fun <B> compose(g: (A) -> B): Op<B, X> =
    compose(Op(g))
}

inline fun <A, X> Op<A, X>.k(): Kind2<ForOp, A, X> = TODO() // does that make sense?

now I want to define:

interface OpContravariant<C> : Contravariant<Conested<ForOp, C>>

How does that look like. I tried it, but I could not find something tangible. cc @Ariostonj

Since

Op<A, X>

looks like a wrapper like

Function1<A, X>

it should have a similar contravariant instance. The instance is here: https://github.com/arrow-kt/arrow/blob/52bc681a41d44a3f14a726a125dff1c5c7e29a46/modules/core/arrow-core/src/main/kotlin/arrow/core/extensions/function1.kt#L56 And contramap is just compose

Thanks @Jannis is there a way to upcast to Hkt. I forgot how to do that

Btw the reason coming up with

Contravariant<Conested<ForOp>, X>

is hard, is that

Op<A, X>

is a wrapper for

(X) -> A

and functions are contravariant in their input and covariant in their output.

Contravariant<Conested<ForOp>, X>

is trying to implement contravariance in the output

yes

is there a way to upcast to Hkt. I forgot how to do that

That is never necessary in arrow because we inherit from

Kind<F, A>

with our datatypes so inheritance takes care of that.

fix()

exists because we also need to cast the other direction

😅 Oh man, I need to fresh up my Arrow in my holidays. Thanks bud.