Unifying polymorphic variants

foo_broken.ml

type foo = [`Hello | `World | `Other]
type bar = [`Yet | `Another | `Other]

type baz = [foo | bar]

type qux = Foo of foo | Bar of bar | Quux

let transform = function
  | Foo x -> x
  | Bar x -> x
  | Quux -> `Other

foo_compiling_but_ugly.ml

type foo = [`Hello | `World | `Other]
type bar = [`Yet | `Another | `Other]

type baz = [foo | bar]

type qux = Foo of foo | Bar of bar | Quux

let transform = function
  | Foo `Hello -> `Hello
  | Foo `World -> `World
  | Foo `Other -> `Other
  | Bar `Yet -> `Yet
  | Bar `Another -> `Another
  | Bar `Other -> `Other
  | Quux -> `Other

foo_fixed.ml

type foo = [`Hello | `World | `Other]
type bar = [`Yet | `Another | `Other]

type baz = [foo | bar]

type qux = Foo of foo | Bar of bar | Quux

let transform = function
  | Foo x -> (x :> baz)
  | Bar x -> (x :> baz)
  | Quux -> `Other

(* thanks octachron *)