OCamlの型
型宣言
mltype t = int;;
type x = X of int;;
type 'a option = None | Some of 'a;;
type 'a pt = Point of 'a * 'a;;型変数が前にあるのキモいな
' をつけるのは慣習？必須 #??Haskellで書くと
hstype T = Integer
data x = X int
data Maybe a = Nothing | Just a
data Pt a = Point a a