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

[Nathan Moreau <nathan.moreau@m4x.org>]

Exceptions being an open type allows you to classify failures.

from Unix.ml:

let rec waitpid_non_intr pid =
try waitpid [] pid
with Unix_error (EINTR, _, _) -> waitpid_non_intr pid

With a closed type, there is not much you can do to ensure that you
catch the right type of exception here (matching on strings? seems
much worse).

Note that, as there is no break keyword in the language, you tend to
use exceptions for control flow to avoid rewriting your for loops to
while loops.

You can write some APIs without exposing exceptions, the same way that
you can write an API without exposing a global hash-table.

Exceptions are just a tool, that allow you to work around some
limitations / performance issues / make your life easier. Removing
them is impossible at this point for compatibility reasons, so all you
can do is avoid using them if it makes you sad (the same way you avoid
using some parts of the language when you program in c++ / ecmascript
/ whatever language du jour).

Nathan


On 20 October 2017 at 11:56, Malcolm Matalka <mmatalka@gmail.com> wrote:
> I have a question in two parts:
>
> 1. Would this be a good idea? Why? (I'll describe why I think it is)
>
> 2. If it were a good idea, is it feasible to do?
>
> Full question:
>
> Despite exceptions being a part of the language, there is a trend in
> many libraries to try to avoid using them.  While I cannot find it, I
> recall someone (Daniel maybe?) saying that the standard API advice is
> that exceptions should not cross API boundaries.
>
> The short reason for why people seem to want to avoid exceptions (which
> I agree with) is that they side step the type system for helping you
> understand if your code is correct and handles all situations the code
> might experience.  Since the exn type is open, it means that one can add
> any exception they want so it's not even known what exceptions you might
> get ahead of time.
>
> Another aspect of exceptions, which might be more of my personal
> experience, is that exceptions tend to be pretty useless after the
> fact.  For example, forgetting to handle a Not_found exception is an
> exercise in pain.  Maybe I'm just bad at this, but many exceptions just
> aren't that useful.  End_of_file is another one that, IMO, makes the
> program flow pretty awkward and if you have multiple files you're
> reading from at the same time quite ugly.  I tend to use wrappers that
> give me an option based API.  Maybe I just bad at solving these problems
> though and I'm the problem.
>
> The consequence of this is that even though I put a lot of effort in my
> code trying to avoid exceptions, I can never actually know that I have
> succeeded unless I'm very defensive and wrap all foreign calls in some
> exception handling code.  There are APIs for this, but if I mess up then
> I'm in a bad spot.
>
> My proposal is that exceptions becomes a closed type and they reflect
> what Java calls "errors", which are things your program logic should
> generally not handle but can choose to if it wants to (I think we call
> these failures in Ocaml).  The two specific exceptions I can think if
> that should exist are: Assertion_failure and Out of Memory.  Another one
> that I think might be nice but is open for debate is a
> Not_implemented_failure, I use something like this often while building
> a system.  I'm sure there are a few more that people can think of are
> meaningful, but the point is these represent pretty bad situations that
> the program logic shouldn't handle except in special situations.
>
> Thanks for reading,
> /Malcolm
>
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