RE: __DIE__ and exception hooks

["Redford, John" <John.Redford@fmr.com>]

There is also a much better option. Better is the sense of being a more
general solution to a broader set of problems.

Make it possible for objects and other reference types to define their own
string representations.

eval {
  my $e = new Exception("Something broke", [1, 2, 3], CRITICAL);
  die $e;
};

Both of the following then possible & valid:

if ( $@ eq "Something broke" ) { ... }
if ( ref $@ and $@->isa('Exception' <mailto:$@->isa('Exception'> ) and
$@->severity <mailto:$@->severity>  eq CRITICAL) { ... }

I don't really care how this is done, it should be obvious how useful and
sorely lacking it is. One idea is as follows:

For non-blessed references, pick a few more Perl special-variables & let
people store code-refs in them, and use them for conversion of simple
references to strings. Perhaps just use one variable & force the author to
figure out if a ref is SCALAR, HASH, or what.

For objects, define 'PRINT' as a special function which Perl calls to get a
printable string representation of an object, and define (internally)
PRINT_ADDR as a method of all objects to get the string that is returned
presently.

	sub PRINT { $_[0]->PRINT_ADDR }	# emulate old behavior.

	sub PRINT { $_[0]->{StringRep} }	# Return a pre-calculated
string


		-----Original Message-----
		From:	Pete Jordan [mailto:pjordan1@email.mot.com]
		Sent:	Wednesday, January 19, 2000 6:09 AM
		To:	Perl5 Porters
		Subject:	Re: __DIE__ and exception hooks

		Tom Christiansen wrote:

		> In logic so simple as this, nobody but *nobody* else had
jolly well
		> better be folding, spindling, or mutilating that
exception, for to
		> do so is an abhorrent notion that tosses all dependability
and
		> determinism out the window and then proceeds to forget
just what
		> window was so abused.  This bug has broken the code of
many careful
		> and professional programmers through unforeseeable
interactions
		> with wicked modules.

		So what it comes down to is:

		1. Perl provides an (experimental) facility to pass
references into
		C<die> that will, with a horrible inevitability, find their
way into
		C<$@> when C<eval> is in use.

		2. If anyone makes use of this facility in a module (outside
of the sort
		of closed system in which I'm currently operating), they
will break not
		only conventional usage of C<$@> but also Perl's own default
C<die>
		processing. Though the latter of course can be overridden.
Probably
		simultaneously in a variety of mutually incompatible ways by
different
		module authors.

		There are three possible solutions:

		A. Abandon structured exceptions in Perl. Not an acceptable
option.

		B. Force the lusers to rewrite all their evals.
Muharharharhar... etc.

		C. Define a Standard Perl Minimal Exception Object and
integrate it such
		that, at the very least, scalar fetch access to C<$@>
returns an error
		text string regardless of whether text or object were passed
to C<die>.

		It's a pity that a tied scalar can't be used to get at
arbitrary methods
		in the class it's tied to. Tying C<$@> would mean that its
		exception-ness wouldn't be assignable directly, but adding
an extra
		method that returned an untied object reference wouldn't be
a hardship.

		A slight variation on what I use here:

		  use Exception qw(-stacktrace);

		  eval {
		    ...
		    die 'foo'; # mapped to Exception::raise using
$SIG{__DIE__} >;)
		    ...
		    raise Exception 'bar';
		    ...
		    raise Exception -class=>'Fatal', -text=>'baz',
-exitcode=>23;
		    ...
		  };

		  if ($@) {
		    # Can't have both the following two lines working as
things stand
		    my $text1=$@->text;		# assign error text
		    my $text2=$@;		# same as above using tie

		    my $error=$@->exception;	# assign error object
reference

		    # aside from the first line, this is pretty much my
		    # Exception::croak method:
		    print STDERR "$@\n";
		    my $stack=$@->stack;

		    if ($stack) {
		      print STDERR "Stack trace:\n";

		      print STDERR map {
	
"\t$_->{PACKAGE}\t$_->{FILE}\t$_->{LINE}\t$_->{SUBNAME}\n"
		      } @$stack;
		    }

		    exit $@->exitcode;
		  }

		Pete
		-- 
		use Disclaimer::Standard;	# Motorola GSM Software
Factory
		my $phone='+44 1793 564450';	# "'Not twisted,' Salzy once
said of
		my $fax='+44 1793 566918';	#  her own passion, 'it is
helical.
		my $mobile='+44 7973 725120';	#  That sounds better.'"

---

Follow-Ups from:

Martyn Pearce <m.pearce@inpharmatica.co.uk>
Pete Jordan <pjordan1@email.mot.com>

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