Playing around with typing parametric polymorphic vairants

Parametric Polymorphic Variants

type ('b , 'a) foo = [ `Foo of 'a
                     | `Bar of 'b
                     ]

type ('a, 'c) ani = [ (('a, 'c) foo)
              | `Cat of 'a
              | `Dog of 'a
              ]

let foo_to_str = function
  | `Foo f -> "Foo " ^ f
  | `Bar b -> "Bar " ^ b

let ani_to_str = function
  | #foo as f -> foo_to_str f
  | `Cat c -> "Cat " ^ c
  | `Dog d -> "Dog " ^ d

let swap_foo = function
  | `Foo f -> `Bar f
  | `Bar b -> `Foo b

let swap_ani = function
  | #foo as f -> swap_foo f
  | `Cat c -> `Dog c
  | `Dog d -> `Cat d

let swap_ani2 = function
  | `Foo f -> `Bar f
  | `Bar b -> `Foo b
  | `Cat c -> `Dog c
  | `Dog d -> `Cat d