Re: [Caml-list] What if exn was not an open type?

["Richard W.M. Jones" <rich@annexia.org>]

On Sat, Nov 04, 2017 at 08:16:17PM +0100, octachron wrote:
> 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
>     )

This one works great, thanks.

Rich.

-- 
Richard Jones