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

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tmcd@panix.com (Tim McDaniel) writes:
> 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.

Read: It is possible to attach hidden semantics to the code above in
various ways in order to turn eval into an unsuitable solution to the
original problem. Yes, of course. Name a solution of your choice. I
will find a way to break that by violating the invariant conditions it
relies on, probably even without resorting to using, say, dynamically
linked C code to wreak havoc onto some internal data structures.

> You might think that this example is so simple that it couldn't
> possibly die, and so eval is perfectly harmless in this case.

I think that eval is 'perfectly harmless' in the sense that it will
behave according to its documentation and that you're changing your
problem specification after the fact in order to fabricate reasons
against a possible solution you don't like for 'other reasons'.

[...]

>>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 the kind of code you are probably envisioning, where this 'other
function' probably consists of 500 - 1500 lines of code whose exact
raison d'etre was lost with some guy who left the company five years
ago, where generations of successive maintenance programmers have
added patches to the patchwork of patches in order to work around this
or that problem, this is very likely true: As soon as the code calls
another function, only God knows what's going to happen (and if we
can't catch all these weird errors, where to can we hook our next
generation of workarounds).

But that's not a universal 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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