Glenn Linderman wrote:
>
> Tony Olekshy wrote:
> >
> > Glenn Linderman wrote:
> > >
> > > actually wrapping a bunch of code inside a try block
> > > affects how that code reacts to die, thus affecting the
> > > behavior of the program that previously used die to mean
> > > terminate the program.
> >
> > Hang on, this can't be true. To quote from RFC 88:
> >
> > try { may_throw_1 }
> > catch may_throw_2 => { may_throw_3 }
> > finally { may_throw_4 }
> >
> > is exactly the same as (assuming traditional Perl semantics):
> >
> > eval { may_throw_1 };
> > my $exception = $@;
> > if ($exception) {
> > my $test = eval { may_throw_2 };
> > $@ and $exception = $@;
> > if ( ! $@ and $test ) {
> > eval { may_throw_3 };
> > $exception = $@;
> > }
> > }
> > eval { may_throw_4 };
> > ($exception ||= $@) and die $exception;
> >
> > How can this affect how code reacts to die? It reacts the same
> > as with eval, whether wrapped in a try or not. Say I had:
> >
> > do_something();
> >
> > now I wrap it in a try:
> >
> > try { do_something(); }
> >
> > this is equivalent to:
> >
> > eval { do_something(); };
> > my $exception = $@;
> > $exception and die $exception;
> >
> > Which is *exactly* the same as bare do_something(); as per Perl 5.
>
> OK, this example clearly demonstrates something.
>
> 99% of the catch clauses in the world should be conditional,
> handling only those specific exceptions that it knows about, and
> knows it can handle. The "catch-all" (unconditional catch of
> anything), is error prone; [...] a "catch-all" might bite off
> _lots_ more than it should chew. Maybe "catch-all"s that do
> something and then re-throw would be relatively safe.
Yes!
Only when one knows an awful lot about what's going on in the body
of a try is a catch-all that does much more than set a state
variable and re-throw possibly safe, because by definition once an
exception has raised there are a bunch of things one shouldn't make
too many assumptions about.
Here is one possibly safe example:
try { $y = ( $a * $x * $x + $b * $x + $c ) / $d; }
catch { $y = undef; }
but this would be even safer:
try { $y = ( $a * $x * $x + $b * $x + $c ) / $d; }
catch Exception::CORE::Math => { $y = undef; }
The only case where catch really should be unconditional (and not
just re-throw) is when top-level (some sort of main.pl) code wraps
the whole shebang in a try with this type of explicit purpose:
try { the_whole_shebang() }
catch { pass_to_client($@->show(label => 1, trace => 1)) }
Where pass_to_client may be print + exit;
Yours, &c, Tony Olekshy
