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

["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! *)
         ...
    )

Real code is often heavily indented, or we have to put the work into
awkward nested functions.  This file contains a few real examples:
https://github.com/libguestfs/libguestfs/blob/master/daemon/inspect_fs_unix_fstab.ml

This would work a lot better with a return statement rather than
workarounds.

Having said all that I was writing a little ML language last
year and I tried to implement a return statement, but it was very
awkward to work out how to map that to my lambda calculus, so
I understand how return statements are rather difficult to implement
in practice.

Rich.

-- 
Richard Jones