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

[Philippe Veber <philippe.veber@gmail.com>]

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2017-10-25 10:35 GMT+02:00 Richard W.M. Jones <rich@annexia.org>:

> On Tue, Oct 24, 2017 at 09:00:05PM +0200, Petter A. Urkedal wrote:
> > On 24 October 2017 at 15:30, Richard W.M. Jones <rich@annexia.org>
> wrote:
> > > Since I first used OCaml I have wished for a simple (and type safe)
> > > return statement.
> >
> > It's possible to wrap a bit type (and exception) safety about
> > exceptions used for return:
> >
> >   val with_return : (('a -> 'b) -> 'a) -> 'a
> >
> >   let with_return (type b) f =
> >     let exception Return of b in
> >     try f (fun y -> raise (Return y)) with Return y -> y
> >
> > E.g.
> >
> >   with_return (fun return -> List.iter (function 0 -> () | i -> return
> > i) [0; 0; 24; 0]; -1);;
>
> Thanks, that's interesting.
>
>         --
>
> As mine was a bit of a "Hit and run" comment, let me expand on
> why a return statement is useful for the kind of dull code that
> I write.
>
> I often have to write functions of the form:
>
>   let f () =
>     if some_problem then (
>       printf "sorry, can't do that\n";
>       (* return *)
>     )
>     else if some_other_problem then (
>       printf "sorry, can't do that either\n";
>       (* return *)
>     )
>     else (
>       match something with
>       | None ->
>          (* return *)
>       | Some v ->
>          (* finally we get to do some work! *)
>          ...
>     )
>
>
Hi Richard,

isn't that a context where error monads do a pretty decent job?

check_some_problem () >>= fun () ->
check_some_other_problem () >>= fun () ->
expect_something () >>= fun something ->
finally_do something

Cheers,
  Philippe.

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