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

["Petter A. Urkedal" <paurkedal@gmail.com>]

On 25 October 2017 at 10:35, Richard W.M. Jones <rich@annexia.org> wrote:
> On Tue, Oct 24, 2017 at 09:00:05PM +0200, Petter A. Urkedal wrote:
>>   let with_return (type b) f =
>>     let exception Return of b in
>>     try f (fun y -> raise (Return y)) with Return y -> y

> 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! *)
>          ...
>     )

Using the above trick, this might be written in more C-like style as:

let f () =
  with_return @@ fun return ->

  if some_problem then (
    printf "sorry, can't do that\n";
    return ()
  );

  if some_other_problem then (
    printf "sorry, can't do that either\n";
    return ()
  );

  let v =
    match something with
    | None -> return ()
    | Some v -> v in

  (* finally we get to do some work! *)
  ...

However, I would have written it:

let f () =
  if some_problem then
    printf "sorry, can't do that\n" else

  if some_other_problem then
    printf "sorry, can't do that either\n" else

  (match something with
   | None ->
     (* return *)
   | Some v ->
     (* finally we get to do some work! *)
     ...)

That is, treating the if ... else more like let ... in.  However any ;
would chop of the bodies of the conditionals, so this style only works
when semicolons are parenthesised or rewritten as let () = ... in.
Especially, the latter must be used if one wants to skip indenting an
else block, but insert a debug statement:

let f () =
  if some_problem then
    printf "sorry, can't do that\n" else

  if some_other_problem then
    printf "sorry, can't do that either\n" else

  let () = printf "no problems\n" in

  (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.

Given the change of formatting above, also changes the perception: The
issue now is not that the return is missing, but that one cannot *not*
return from a conditional.

Petter