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

[Gabriel Scherer <gabriel.scherer@gmail.com>]

We also have return-by-local-exceptions in Batteries (the BatReturn
module), but reports from js_of_ocaml users (namely, Clément
Pit-Claudel) is that this style gets translated to extremely slow
Javascript code -- functions from Batteries using this style (string
search functions in particular) were a performance bottleneck on the
javascript backend. I fixed the issue by rewriting all uses of
BatReturn in Batteries libraries themselves.
Long-term, it would be better to have Javascript backends produce
better code on this, but I think it is rather difficult -- it is easy
to compile efficiently local exceptions that are raised in the same
basic block, but with_return takes a callback so you have to both
inline and specialize before you are within reach of the optimization.

On Sun, Nov 5, 2017 at 7:39 PM, Yaron Minsky <yminsky@janestreet.com> wrote:
> 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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