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recursion schemes for cats; to iterate is human, to recurse, divine

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package higherkindness.droste.derivation

import shapeless3.deriving.*
import scala.annotation.*
import scala.compiletime.*

/*
Credit to the `kittens` project. This is a direct copy/paste from there

https://github.com/typelevel/kittens/blob/5b3b7ca168b9636f340b23bc95a4fd1506a26707/core/src/main/scala-3/cats/derived/derived.scala
*/

opaque type Derived[A] = A
object Derived:
  def apply[A](instance: A): Derived[A] = instance
  extension [A](derived: Derived[A]) def instance: A = derived
  given [A]: Conversion[A, Derived[A]] = apply

  type Or0[F[_]] = [x] =>> Or[F[x]]
  type Or1[F[_[_]]] = [x[_]] =>> Or[F[x]]
  type Or11[F[_[_[_]]]] = [x[_[_]]] =>> Or[F[x]]
  type Or2[F[_[_, _]]] = [x[_, _]] =>> Or[F[x]]

  opaque type Or[A] = A
  object Or extends OrInstances:
    def apply[A](instance: A): Or[A] = instance
    extension [A](or: Or[A]) def unify: A = or
    extension [I[f[_], t] <: K0.Instances[f, t], F[_], T]
      (inst: I[Or0[F], T]) @targetName("unifyK0") def unify: I[F, T] = inst
    extension [I[f[_[_]], t[_]] <: K1.Instances[f, t], F[_[_]], T[_]]
      (inst: I[Or1[F], T]) @targetName("unifyK1") def unify: I[F, T] = inst
    extension [I[f[_[_[_]]], t[_[_]]] <: K11.Instances[f, t], F[_[_[_]]], T[_[_]]]
      (inst: I[Or11[F], T]) @targetName("unifyK11") def unify: I[F, T] = inst
    extension [I[f[_[_, _]], t[_, _]] <: K2.Instances[f, t], F[_[_, _]], T[_, _]]
      (inst: I[Or2[F], T]) @targetName("unifyK2") def unify: I[F, T] = inst

sealed abstract class OrInstances:
  inline given [A]: Derived.Or[A] = summonFrom {
    case instance: A => Derived.Or(instance)
    case derived: Derived[A] => Derived.Or(derived.instance)
  }