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From: octachron <octa@polychoron.fr>
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Date: Sat, 4 Nov 2017 20:16:17 +0100
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Subject: Re: [Caml-list] What if exn was not an open type?

On 04/11/2017 19:44, Richard W.M. Jones wrote:
> The problem is that the return statement could be called from many
> contexts, all with different types.  The compiler expects to unify all
> these types (as the same type 'b) which is not possible.
This issue can be solved by making the type of return more precise, 
capturing the fact that return always raises:

     type 'a return = { return: 'b. 'a -> 'b } [@@unboxed]
     let with_return (type b) f =
       let exception Return of b in
       try f {return = (fun y -> raise (Return y))} with Return y -> y;;

It becomes then possible to write

     let f () =
       with_return (fun {return} ->
         if false then return "error";
         let a =
           match Some "abc" with
           | None -> return "another error"
           | Some a -> a in
         a
     )

— octachron.