Higher Kinded type declarations in scala

higherkinds.scala

  trait Y

  trait T[X]
  trait TSub[X <: Y] extends T[X] // bounded

  trait TCo[+X]
  trait TCoSub[+X <: Y] extends TCo[X] // bounded

  trait TContra[-X]
  trait TContraSub[-X <: Y] extends TContra[X] // bounded


  def m1[T0[_] <: T[_]] = ???
  m1[T]
  //  m1[TSub] -> not accepted because of stricter bounds

  def m1Co[T0[_] <: TCo[_]] = ???
  m1Co[TCo]
  //  m1Co[TCoSub] -> not accepted because of stricter bounds

  def m1Contra[T0[_] <: TContra[_]] = ???
  m1Contra[TContra]
  //  m1Contra[TContraSub] -> not accepted because of stricter bounds


  def m2[T0[A0] <: T[A0]] = ???
  m2[T]
  //  m2[TSub] -> not accepted because of stricter bounds

  def m2Co[T0[A0] <: TCo[A0]] = ???
  m2Co[TCo]
  //  m2Co[TCoSub] -> not accepted because of stricter bounds

  def m2Contra[T0[A0] <: T[A0]] = ???
  //  m2Contra[TContra] -> contravariant type is not accepted because for any A0 T0[A0] >: T[A0] instead of <:
  //  m2Contra[TContraSub] -> not accepted because of stricter bounds



  def m3[A, T0[_] <: T[A]] = ???
  // m3[Y, T] -> doesn't conform, T0[X] should be a subtype of T[Y] for every X.
  // m3[Y, TSub] -> not accepted because of stricter bounds

  def m3Co[A, T0[_] <: TCo[A]] = ???
  // m3Co[Y, TCo] -> doesn't conform, T0[X] should be a subtype of TCo[Y] for every X.
  // m3Co[Y, TCoSub] -> not accepted because of stricter bounds

  def m3Contra[A, T0[_] <: TContra[A]] = ???
  // m3Contra[Y, TContra] -> doesn't conform, T0[X] should be a subtype of TContra[Y] for every X.
  // m3Contra[Y, TContraSub]-> not accepted because of stricter bounds


  def m4[A, T0[_ <: A] <: T[_]] = ???
  m4[Y, T] // Ok, wider bound
  m4[Y, TSub] // Ok, wider bound

  def m4Co[A, T0[_ <: A] <: TCo[_]] = ???
  m4Co[Y, TCo] // Ok, wider bound
  m4Co[Y, TCoSub] // Ok, wider bound

  def m4Contra[A, T0[_ <: A] <: TContra[_]] = ???
  m4Contra[Y, TContra] // Ok, wider bound
  m4Contra[Y, TContraSub] // Ok, wider bound


  def m5[A, T0[_ <: A] <: T[A]] = ???
  // m5[Y, T] -> Wider bound but doesn't conform: T0[X <: Y] should be a subtype of T[Y] for every X but T0 is invariant
  // m5[Y, TSub] -> Same bound but doesn't conform: T0[X <: Y] should be a subtype of T[Y] for every X but T0 is invariant

  def m5Co[A, T0[_ <: A] <: TCo[A]] = ???
  // m5Co[Y, TCo] -> doesn't conform: T0[X] should be a subtype of Tco[Y] for every X but T0 bound is larger
  m5Co[Y, TCoSub] // Same bound/conforms because being T covariant T0[X] is a subtype of Tco[Y] for every X  because X is bounded <: Y

  def m5Contra[A, T0[_ <: A] <: TContra[A]] = ???
  // m5Contra[Y, TContra] -> Wider bound but doesn't conform: T0[X] should be a subtype of T[Y] for every X, but X is contravariant
  // m5Contra[Y, TContraSub] -> Same bound but doesn't conform: T0[X] should be a subtype of T[Y] for every X, but X is contravariant


  def m6[A, T0[A0 <: A] <: T[A0]] = ???
  m6[Y, T] // bound ok, conforms because T0[X <: Y] is a subclass of T0[X] for every X (it's actually the same) 
  m6[Y, TSub] // bound ok, conforms because T0[X <: Y] is a subclass of T0[X] for every X (it's actually the same)

  def m6Co[A, T0[A0 <: A] <: TCo[A0]] = ???
  m6Co[Y, TCo] // bound ok, conforms because T0[X <: Y] is a subclass of T0[X] for every X (it's actually the same)
  m6Co[Y, TCoSub] // bound ok, conforms because T0[X <: Y] is a subclass of T0[X] for every X (it's actually the same)

  def m6Contra[A, T0[A0 <: A] <: TContra[A0]] = ???
  m6Contra[Y, TContra] // bound ok, conforms because T0[X <: Y] is a subclass of T0[X] for every X (it's actually the same)
  m6Contra[Y, TContraSub] // bound ok, conforms because T0[X <: Y] is a subclass of T0[X] for every X (it's actually the same)