fixt.scala

trait FixT {
  type T[_[_]]
  def subst[F[_[_[_]]]](kt: F[T]): F[λ[k[_] => k[T[k]]]]
  def unsubst[F[_[_[_]]]](kt: F[λ[k[_] => k[T[k]]]]): F[T]
}

package object matryoshka {
  val FixI: FixT = new FixT {
    type T[F[_]] = Fix[F]
    def subst[F[_[_[_]]]](kt: F[T]): F[λ[k[_] => k[T[k]]]] = kt
    def unsubst[F[_[_[_]]]](kt: F[λ[k[_] => k[T[k]]]]): F[T] = kt
    def unwrap[F[_]](f: T[F]): F[T[F]] = subst[λ[τ[_[_]] => τ[F]]](f)
    def wrap[F[_]](f: F[T[F]]): T[F] = unsubst[λ[τ[_[_]] => τ[F]]](f)
  }

  type Fix[F[_]] = FixI.T[F]

}

[error] /Users/edmund/workspace/scala/matryoshka/core/shared/src/main/scala/matryoshka/package.scala:194: type mismatch;
[error]  found   : kt.type (with underlying type F[this.T])
[error]  required: F[[k[_$1]]k[k[matryoshka.package.FixI.T[k]]]]
[error]     def subst[F[_[_[_]]]](kt: F[T]): F[λ[k[_] => k[T[k]]]] = kt
[error]                                                              ^
[error] /Users/edmund/workspace/scala/matryoshka/core/shared/src/main/scala/matryoshka/package.scala:195: type mismatch;
[error]  found   : kt.type (with underlying type F[[k[_$2]]k[k[matryoshka.package.FixI.T[k]]]])
[error]  required: F[this.T]
[error]     def unsubst[F[_[_[_]]]](kt: F[λ[k[_] => k[T[k]]]]): F[T] = kt

[error]                       ^
[error] /Users/edmund/workspace/scala/matryoshka/core/shared/src/main/scala/matryoshka/package.scala:194: type mismatch;
[error]  found   : kt.type (with underlying type F[matryoshka.package.FixI.T])
[error]  required: F[[k[_$1]]k[matryoshka.package.FixI.T[k]]]
[error]     def subst[F[_[_[_]]]](kt: F[FixI.T]): F[λ[k[_] => k[FixI.T[k]]]] = kt
[error]                                                                        ^
[error] /Users/edmund/workspace/scala/matryoshka/core/shared/src/main/scala/matryoshka/package.scala:195: type mismatch;
[error]  found   : kt.type (with underlying type F[[k[_$2]]k[matryoshka.package.FixI.T[k]]])
[error]  required: F[matryoshka.package.FixI.T]
[error]     def unsubst[F[_[_[_]]]](kt: F[λ[k[_] => k[FixI.T[k]]]]): F[FixI.T] = kt