Re: Question about GADT

Ok. Thank you very much for your help.

On 04.12.2015 08:36, Gabriel Scherer [email protected] 
[ocaml_beginners] wrote:
> This is possible, but when your data is of type (t), you have to
> consider two cases: it may be (a data) for any type (a), (int) or
> (string). If you had a (int data) or a (string data), you would know
> the type of the content, but (t) can be any of those.
>
> let () =
> match test with
> | Wrap (Int n) -> print_int n
> | Wrap (String s) -> print_string s
>
> On Fri, Dec 4, 2015 at 12:15 AM, Stephane Legrand [email protected]
> [ocaml_beginners] <[email protected]> wrote:
>  > I'm trying to unwrap the data built with "data_int" or "data_string". Ie
>  > i'm trying to retrieve the original value "v" used in "data_int" or
>  > "data_string" but only by using the "to_ocaml_value" function. I don't
>  > even know if it's possible?
>  >
>  > On 03.12.2015 23:42, Gabriel Scherer [email protected]
>  > [ocaml_beginners] wrote:
>  >> The program has a type error, as (v) in the last sentence could be
>  >> either (Int n) or (String s). The compiler is right to complain. What
>  >> are you trying to do?
>  >>
>  >> On Thu, Dec 3, 2015 at 11:34 PM, Stephane Legrand [email protected]
>  >> [ocaml_beginners] <[email protected]> wrote:
>  >> > Hello,
>  >> >
>  >> > I'm trying to run the code below. The compiler gives me an error
> for the
>  >> > "print_endline x" line: "This expression has type ex#0 but an
> expression
>  >> > was expected of type string". Is there a way to fix this error? If
> this
>  >> > kind of thing is doable with GADT? Thank you.
>  >> >
>  >> > type _ data =
>  >> > Int : int -> int data
>  >> > | String : string -> string data
>  >> >
>  >> > type t =
>  >> > Wrap : _ data -> t
>  >> >
>  >> > let data_int v = Wrap(Int v)
>  >> > let data_string v = Wrap(String v)
>  >> >
>  >> > let to_ocaml_value : type a. a data -> a = function
>  >> > | Int v -> v
>  >> > | String v -> v
>  >> >
>  >> > let test = data_string "test"
>  >> >
>  >> > let () =
>  >> > match test with
>  >> > | Wrap v ->
>  >> > let x = to_ocaml_value v in
>  >> > print_endline x
>  >> >
>  >> >
>  >> > Regards,
>  >> > Stéphane.
>  >> >
>  >> >