Re: [Caml-list] Simple exception - different behaviour between toplevel and compiled

[Arthur Wendling <art.wendling@gmail.com>]

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> exception A of int * int

This is an exception with two arguments.

> exception B of (int*int)

This is an exception with a single argument, which is a product of two
values. So the parenthesis actually matters:

# fun x -> A x ;;
Error: The constructor A expects 2 argument(s),
       but is applied here to 1 argument(s)
# fun x -> B x ;;
- : int * int -> exn = <fun>

I agree that the syntax is a bit surprising, since we are used to ignore
extra parenthesis.

Anyway, now, regarding the output of the interpreter/compiler:
- A of int * int is printable by both, because the runtime can guess that
the two arguments are integers (because they aren't pointers.)
- B of (int * int) is only printed correctly by the interpreter, because it
keeps the source information. The compiler erases all that, so the runtime
only sees a single argument, that contains two ints, but it doesn't know
how to display it: it could be something else than a tuple from the point
of view of the user (like a record { x : int ; y : int }, but the names x,y
have been forgotten, so the runtime representation is identical to (int *
int)).


On Wed, Aug 19, 2015 at 1:32 AM, David Allsopp <dra-news@metastack.com>
wrote:

> > On 19 Aug 2015, at 00:41, Oliver Bandel <oliver@first.in-berlin.de>
> wrote:
> >
> > Hello,
> >
> >
> > using the attached files (executing testexc,bash)
> > I got different results between toplevel and compiled:
> >
> > =====================================================
> > Testcase A
> > exception A of int * int
> > let _ = raise ( A(3,4) )
> > Exception: A (3, 4).
> > Fatal error: exception Exca.A(3, 4)
> > Fatal error: exception Exca.A(3, 4)
> > Testcase B
> > exception B of (int*int)
> > let _ = raise ( B(3,4) )
> > Exception: B (3, 4).
> > Fatal error: exception Excb.B(_)
> > Fatal error: exception Excb.B(_)
> > =====================================================
> >
> > So just adding parantheses in a definition of an exception
> > yields in these differing results, with not-shown exception-values.
> >
> > IMHO looks like a case for the bugtracker...
>
> There's no requirement for the toplevel and the compilers to behave the
> same way for reporting the exception. The output, for example, also differs
> in the way the exception message is formatted, there's no 'Fatal error:'
> and so on - do you want that to be a bug too?
>
> The toplevel has access to typing information which is not available to
> ocamlc/ocamlopt (the runtime only knows the name of the exception - beyond
> that, it's just presented with a standard variant block). I haven't got a
> compiler to hand, but I think you'll also see differences if you use a
> variant instead of numbers (ocaml will display the constructor name, the
> compilers will display its constructor number) and I think you'll also see
> the same output as the compilers if you compile excb.cmo and then #load it
> in the toplevel.
>
> It's not normal to want to terminate your compiled program with an
> uncaught exception, hence the simpler default exception printer in compiled
> code. If you really want the exception printed accurately, you can register
> a printer (see Printexc.register_printer).
>
>
> David
>
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