Sum types + exhaustiveness checking already do this, right? For this example:

  type animal = Cat | Dog
  type action = Sound | Eat | Attack

  let cat_sound () = printf "meow"
  let cat_attack () = printf "scratch"
  let dog_sound () = printf "woof"

  let act animal action = match animal,action with
  | Cat, Sound  -> cat_sound ()
  | Cat, Attack -> cat_attack ()
  | Dog, Sound  -> dog_sound ()

Compiling this yields:

  Warning 8: this pattern-matching is not exhaustive.
  Here is an example of a value that is not matched:
  (Dog, (Eat|Attack))

just as we'd like.