# No compila, por que?
alias AbBool = Nil |
               Bool |
               Tuple(AbBool, AbBool)

def fold(e : AbBool, fNil : -> T, fBool : Bool -> T, fTup : Proc(T, T, T))
  case e
  when Nil
    fNil.call()
  when Bool
    fBool.call(e)
  when Tuple(AbBool, AbBool)
    rec1 = fold(e.fst, fNil, fBool, fTup)
    rec2 = fold(e.snd, fNil, fBool, fTup)
    fTup.call(rec1, rec2)
  end
end

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

  def snd
    self[1].not_nil!
  end
end

puts fold( {nil, {true, false}}, ->{0}, ->(b : Bool){b ? 1 : 0}, ->(reci : Int32, recd : Int32){reci + recd } )