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Groups > comp.lang.perl.misc > #4782

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

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Concerning
    my $result = eval {
        $_ % 2 == 0     and return "even";
        $_ % 3 == 0     and return "divisible by three";
        return "just wrong";
    };

In article <41tc39-jv12.ln1@anubis.morrow.me.uk>,
Ben Morrow  <ben@morrow.me.uk> wrote:
>Quoth tmcd@panix.com:
>> In fact, the eval version is the one that needs the testing, not
>> the sub.  eval traps fatal terminations, and just about anything
>> can die().  In this simple case, I think you'd need
>> "use warnings FATAL => 'numeric';"
>
>Why? eval {} can warn perfectly well, if it wants to: in fact, by
>converting warnings to errors you would end up hiding warnings that
>would otherwise be printed.

I was too elliptical.  Sorry.  Expanding:

Just about anything can die, and eval will swallow the results.  You
might think that this example is so simple that it couldn't possibly
die, and so eval is perfectly harmless in this case.  However, you can
even make this die, though the only way I can think of is if to use

    use warnings FATAL => 'numeric';

and a non-numeric value of $_.

>> can't just replace do{...} with eval{...}; you'd have to add proper
>> error checking and we've recently had a discussion about the edge
>> cases to worry about.
>
>But only if you expect something to die, and only if it's not
>acceptable for the eval to (silently) return an empty list in that
>case.

As I demonstrated, surprisingly simple things can die unexpectedly.
Personally, I don't consider something to be a solution if it works
only if nothing dies, but if it doesn't it SILENTLY SUPPRESSES AN
ERROR MESSAGE.  When coding it, I'll overlook a way it can die, or
someone will not realize that it's fragile and will break the
assumptions.  And if the eval block calls some other function, in
practice it'll be utterly doomed.

In sum: you can't just replace do{...} by eval{...}, and if you try
the result will be fragile, you'll need boilerplate code to propagate
errors, and it will suppress errors or otherwise break if you get it
wrong.

-- 
Tim McDaniel, tmcd@panix.com

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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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