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

[Yaron Minsky <yminsky@janestreet.com>]

Yeah, this is a general problem with exception-catching idioms. We
rand into effectively the same issue with respect to the Base's Map
and Set implementations, which are rather slow on JavaScript. We
haven't rewritten those to a more efficient style yet, though.

y

On Sun, Nov 5, 2017 at 3:49 PM, Gabriel Scherer
<gabriel.scherer@gmail.com> wrote:
> 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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