Hi

I don't know if it is theoretically sound or possible, but
it would be helpful, if the typing of match-with statements with polymorphic
variants was able to remove one or more variants from the "catch-all" cases

In the following code, it would mean that three_is_a_lot would be typed like
three_is_a_lot_2 without having to write all the cases.


# let how_much = function
  | 1 -> `One
  | 2 -> `Two
  | 3 -> `Three
  | n -> `A_lot;;

        val how_much : int -> [> `A_lot | `One | `Three | `Two ] = <fun>

# let three_is_a_lot x =
  match how_much x with
  | `Three -> `A_lot
  | any -> any;;

      val three_is_a_lot : int -> [> `A_lot | `One | `Three | `Two ] = <fun>

# let three_is_a_lot_2 x =
  match how_much x with
  | `Three -> `A_lot
  | `One | `Two | `A_lot as any -> any;;

      val three_is_a_lot_2 : int -> [> `A_lot | `One | `Two ] = <fun>


The practical use-case for us is that we use polymorphic variants to encode
"semantic" errors in an Error/Result monad.

What does the list think?

Cheers,
Sebastien