May someone help me out here with Contravariant :s...

# arrow-contributors

[Imran/Malic]

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

[Jannis]

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

[Jannis]

Actually nvm the parameters are reversed so the contravariant instance is slightly different I think (could probably be andThen instead)

[Jannis]

interface OpContravariant<I> : Contravariant<OpPartialOf<I>> {
  override fun <A, B> Kind<OpPartialOf<I>, A>.contramap(f: (B) -> A): Kind<OpPartialOf<I>, B> =
    Op(fix().f compose f)
}

Isn't this equal to the haskell version? Not sure how exactly that looked but I think this was it

[Jannis]

Yup: https://hackage.haskell.org/package/base-4.12.0.0/docs/src/Data.Functor.Contravariant.html#line-261

[Imran/Malic]

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

[Jannis]

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

[Imran/Malic]

yes

[Imran/Malic]

I was so confused by that signature. Yours make total sense

[Jannis]

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

[Jannis]

So data class Op<A, X>(val f: (X) -> A): OpOf<A, X>

[Imran/Malic]

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