alias using generic types

[pbrusco]

Is it possible to define generic aliases?

I would like to have something like alias AB(T) = T | Tuple(AB(T), AB(T)) to represent Haskell like, binary trees.

[pbrusco]

Example I want to generalize:

# alias AB(T) = T | Tuple(AB(T), AB(T))
alias ABInt = Int32 | Tuple(ABInt, ABInt)

def fold(e : ABInt, fLeaf : Int32 -> T, fRec : Proc(T, T, T)) : T
  case e
  when Int32
    fLeaf.call(e)
  when Tuple(ABInt, ABInt)
    rec1 = fold(e.fst, fLeaf, fRec)
    rec2 = fold(e.snd, fLeaf, fRec)
    fRec.call(rec1, rec2)
  else
    raise "type Error"
  end
end

struct Tuple
  def fst
    self[0].not_nil!
  end

  def snd
    self[1].not_nil!
  end
end

puts fold( {1, {2, 3}}, ->(n : Int32){n}, ->(reci : Int32, recd : Int32){reci + recd } )

[asterite]

Duplicate of #2803 :-)