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

[Yaron Minsky <yminsky@janestreet.com>]

FWIW, this idiom is supported by Base.

https://github.com/janestreet/base/blob/master/src/with_return.mli

I notice we're not yet using the unboxed attribute, though, so we
should fix that...

y

On Sun, Nov 5, 2017 at 12:41 PM, Richard W.M. Jones <rich@annexia.org> wrote:
> 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
>
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