Re: [Caml-list] Potential Feature Request: "Remove poly-variants from match-with statements"

[Sebastien Mondet <sebastien.mondet@gmail.com>]

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>
>
> This has been asked for repeatedly.
>

Sorry, I swear I have searched through Mantis for this :-/


> I admit this could be useful in some cases but there are difficulties.
>
> One first question is, what are you really asking for?
>
> If you just want to be able to omit the pattern before "as",
> then it is probably doable (not easy, as in your example one has
> to detect that while the return type of how_much is open, other cases
> cannot happen), but would not add any expressivity.
>
>
As there is already a nice form of  "at least" exhaustiveness check, I had
the
impression that enough information is available at some point (?)


let three_is_a_lot_3 x =
  match how_much x with
  | `Three -> `A_lot
  | `Two | `A_lot as any -> any;;
      ^^^^^^
Error: This pattern matches values of type [< `A_lot | `Three | `Two ]
       but a pattern was expected which matches values of type
         [> `A_lot | `One | `Three | `Two ]
       The first variant type does not allow tag(s) `One



If you want the real thing, i.e. the ability to have functions that "skim"
> a variant
> type, then this gets much more difficult.
>
>
After a few years trying to follow this list, I start to have an idea of
what it
means when J. Garrigue says that something is difficult  :)

So, no, the "real thing" is really not worth the complexity.



>  let remove_Three = function
>      `Three -> failwith "Three"
>   | any -> any
>
> What kind of type should we give to this function?
> Probably something like:
>
>  [`Three | 'a]  -> [ 'a ]
>
> Note here that 'a is a new kind of type variable, that can only
> be used as a polymorphic variant row variable.
> Such type systems have been studied in the literature, and I think
> there is even an Haskell extension doing that, but this can easily
> get complicated.
>
> OCaml avoids introducing such new type variables by requiring
> that if two types share the same row variable they must be identical.
> In particular this rules out the above type. Removing this restriction
> would be a major design change.
>
>        Jacques Garrigue

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