Re: [Caml-list] assert caught by try with _

[Jacques Garrigue <garrigue@kurims.kyoto-u.ac.jp>]

From: Chris Hecker <checker@d6.com>

> >You could tell the same for stack overflow and some other kinds of
> >exceptions. OCaml uniformly treat failures as exceptions, and that's the
> >right thing to do. But catching exceptions with _ , without even printing
> >them, is not the "right thing" and is definitly huge pain and error prone.
> 
> I totally disagree about assert being the same as a stack overflow in 
> nature, but more importantly, you're ignoring the lessons learned from C++ 
> and Java on the exception specification front.  You just don't know what 
> exceptions code you call in a real program will throw in general.  There 
> are plenty of times when you want to try something at runtime and just bail 
> if it doesn't work, and you don't care about the specifics of why it didn't 
> work.  A single "with _" will mask any assertions living below it, which 
> seems to me to be a bad thing since by definition an assert is a 
> development debugging tool.  Assert should just blow up, like in C (at 
> least as an option).  You shouldn't have to remember to put a "with" clause 
> in for it.

But you might really want to catch assert failures!
For instance if you're building a debugging tool.
So the problem rather stems from the fact exn is a flat type. You have
no way to specify "categories" of exceptions, like you would do in
Java.
Also you could view the behaviour of [try ... with _ -> ...] as
dangerous, and prefer a different behaviour. Unfortunately, _ is
expected to mean any exception, and it would be difficult to make it
mean something else.

Note that it's easy enough to define a class of exceptions as fatal:

let check_fatal = function
    Assert_failure _ | Stack_overflow | Match_failure _ as exn -> raise exn
  | _ -> ();;

Then you just have to write

  try ... with exn -> check_fatal exn; ...

You can certainly use camlp4 to do this automatically.

Alternatively, one could argue that this is similar to the ctrl-break
problem: [Sys.catch_break] allows you to decide whether you want it to
be catchable or not. Unfortunately this would imply some extra cost
for [raise].

    Jacques

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