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Comment by mrkgnao | Hacker News Reader

Haskell can, in fact, express the CardValue example using DataKinds. (I haven't written it out, though.)

If I understand it correctly, it should work in OCaml too with polymorphic variants.

Yes:

    type card_value = [`Jack | `Queen | `King | `Ace | `Num of int]

    (* not really necessary, but nice to have *)
    type royal_card = [`Jack | `Queen | `King]

    let royal_value (card: royal_card): int =
        match card with `Jack | `Queen | `King -> 1

When trying to abuse this, you get a static type error at compile time:

    royal_value `Ace;;
    Error: This expression has type [> `Ace ]
    but an expression was expected of type royal_card
    The second variant type does not allow tag(s) `Ace

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