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Re: $var = do { ... }?

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Rainer Weikusat <rweikusat@mssgmbh.com> writes:
> Ben Morrow <ben@morrow.me.uk> writes:
>> Quoth tmcd@panix.com:
>>> In article <877gz2errh.fsf@sapphire.mobileactivedefense.com>,
>>> Rainer Weikusat  <rweikusat@mssgmbh.com> wrote:
>>> >    [an eval here]
>>> >    $@ && do {
>>> >        syslog('ERR', "$@");
>>> >        return;
>>> >    };
>>> 
>>> Is there a reason to prefer that over
>>>     if ($@) {
>>>         syslog('ERR', "$@");
>>>         return;
>>>     }
>>> ?  I don't see a reason, so I prefer the "if" version.
>>
>> You should always prefer Try::Tiny over an explicit eval: there are,
>> unfortunately, rather a lot of nasty corner cases in perl's handling of
>> $@, and Try::Tiny deals with as many as it can.
>
> It is a lot better to state what these corner-cases are than to make
> nebulous scare-mongering statements about them.

To state this in a clearer way: The two cases this module is supposed
to deal with are

-------------
package a;

sub DESTROY
{
    eval { 3 + 1; };
}

package main;

eval {
    my $a;
    bless(\$a, 'a');

    die("gruesome error!");
};

$@ and print("$@\n");
-------------

Upon exiting the eval scope, the a::DESTROY routine will be executed
automatically and the eval in there cause $@ to be cleared. The
solution to this problem is to add a

local $@ if $@

to the beginning of any destructor which does something none-trivial
which might either invoke die or eval. Since a destructor can be
executed automatically after some other code died but before the
unknowing caller had a chance to look at $@, it certainly shouldn't
change an existing value of $@.

The other is

--------------
sub complex_task
{
    eval { 3 + 1; };
}

eval {
    die("gruesome error!");
};

complex_task();

$@ and print("$@\n");
---------------

This time, the caller is at fault: If some non-trivial action needs to
be performed before looking at $@, the value of $@ immediately after
the eval needs to be save in a 'non-global' variable until it is going
to be used. This is especially true because there is, as the
'BACKGROUND' text of the Try::Tiny documentation aptly explains, no
way the called routine can easily do this.

IMO, both of these 'nasty corner-cases' are actually fairly trivial
programming errors and the general solution to these is to teach
people how to avoid them, not to try to write code which enables them
to remain blissfully unaware of the problem.

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$var = do { ... }? tmcd@panix.com (Tim McDaniel) - 2012-03-02 17:15 +0000
  Re: $var = do { ... }? merlyn@stonehenge.com (Randal L. Schwartz) - 2012-03-02 09:52 -0800
  Re: $var = do { ... }? Shmuel (Seymour J.) Metz <spamtrap@library.lspace.org.invalid> - 2012-03-02 13:42 -0500
  Re: $var = do { ... }? Ben Morrow <ben@morrow.me.uk> - 2012-03-02 18:44 +0000
    Re: $var = do { ... }? tmcd@panix.com (Tim McDaniel) - 2012-03-02 19:00 +0000
      Re: $var = do { ... }? merlyn@stonehenge.com (Randal L. Schwartz) - 2012-03-02 11:45 -0800
      Re: $var = do { ... }? Ben Morrow <ben@morrow.me.uk> - 2012-03-02 20:06 +0000
  Re: $var = do { ... }? Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-03-02 19:18 +0000
    Re: $var = do { ... }? tmcd@panix.com (Tim McDaniel) - 2012-03-02 19:24 +0000
      Re: $var = do { ... }? Ben Morrow <ben@morrow.me.uk> - 2012-03-02 20:10 +0000
        Re: $var = do { ... }? Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-03-02 20:43 +0000
          Re: $var = do { ... }? Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-03-02 22:47 +0000
          Re: $var = do { ... }? Martijn Lievaart <m@rtij.nl.invlalid> - 2012-03-03 15:58 +0100
            Re: $var = do { ... }? Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-03-03 17:11 +0000
              Re: $var = do { ... }? Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-03-03 17:19 +0000
              Re: $var = do { ... }? Martijn Lievaart <m@rtij.nl.invlalid> - 2012-03-03 23:58 +0100
                Re: $var = do { ... }? Ben Morrow <ben@morrow.me.uk> - 2012-03-04 00:29 +0000
                Re: $var = do { ... }? Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-03-04 14:44 +0000
                Re: $var = do { ... }? Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-03-05 20:50 +0000
        Re: $var = do { ... }? "Dr.Ruud" <rvtol+usenet@xs4all.nl> - 2012-03-05 11:35 +0100
          Re: $var = do { ... }? Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-03-05 15:22 +0000
          Re: $var = do { ... }? Ben Morrow <ben@morrow.me.uk> - 2012-03-05 16:38 +0000
      Re: $var = do { ... }? Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-03-02 20:44 +0000
  Re: $var = do { ... }? tmcd@panix.com (Tim McDaniel) - 2012-03-15 17:09 +0000
    Re: $var = do { ... }? Ben Morrow <ben@morrow.me.uk> - 2012-03-16 00:18 +0000
      Re: $var = do { ... }? Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-03-16 14:25 +0000
        Re: $var = do { ... }? tmcd@panix.com (Tim McDaniel) - 2012-03-16 21:01 +0000
          Re: $var = do { ... }? Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-03-16 21:37 +0000
          Re: $var = do { ... }? Ben Morrow <ben@morrow.me.uk> - 2012-03-16 22:56 +0000
            Re: $var = do { ... }? Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-03-16 23:54 +0000
            Re: $var = do { ... }? tmcd@panix.com (Tim McDaniel) - 2012-03-17 18:48 +0000
              Re: $var = do { ... }? Ben Morrow <ben@morrow.me.uk> - 2012-03-17 22:53 +0000
              Re: $var = do { ... }? Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-03-18 15:40 +0000
    Re: $var = do { ... }? "Dr.Ruud" <rvtol+usenet@xs4all.nl> - 2012-03-16 14:08 +0100
      Re: $var = do { ... }? tmcd@panix.com (Tim McDaniel) - 2012-03-16 15:25 +0000
      Re: $var = do { ... }? Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-03-16 17:46 +0000

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