Variable name missing in warning

bogbert has asked for the wisdom of the Perl Monks concerning the following question:

Hi guys,

I noticed that if I'm using several uninitialized variables in a string, and if at least one of those variables is a hash, then the name of the hash only appears in the warnings if it's the first uninitialized variable of the string.

For example

#!/usr/bin/perl

use warnings;
my (%a, %b, $c);
my $x = "$a{'foo'} $b{'bar'} $c"
[download]

prints the following warnings:

Use of uninitialized value $a{"foo"} in concatenation (.) or string at
+ main.pl line 5.
Use of uninitialized value in concatenation (.) or string at main.pl l
+ine 5.
Use of uninitialized value $c in concatenation (.) or string at main.p
+l line 5.
[download]

You'll notice that the name of the variables $a{"foo"} and $c appear in the 1st and 3rd warnings, but the name of $b{"bar"} doesn't appear in the 2nd warning.

Is there a good reason for that or is it a defect ?

Comment on Variable name missing in warning Select or Download Code

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Re: Variable name missing in warning by bigj (Monk) on Apr 02, 2014 at 16:48 UTC
Allthough I originally wrote a module like Test::Warn, I wasn't aware of that behaviour. However perldiag describes it as a kind of optimization: Use of uninitialized value%s (W uninitialized) An undefined value was used as if it were already defined. It was interpreted as a "" or a 0, but maybe it was a mistake. To suppress this warning assign a defined value to your variables. To help you figure out what was undefined, perl will try to tell you the name of the variable (if any) that was undefined. In some cases it cannot do this, so it also tells you what operation you used the undefined value in. Note, however, that perl optimizes your program and the operation displayed in the warning may not necessarily appear literally in your program. For example, "that $foo" is usually optimized into ""that " . $foo", and the warning will refer to the "concatenation (.)" operator, even though there is no "." in your program. [download] Greetings, Janek	[reply] [d/l]
Re^2: Variable name missing in warning by LanX (Saint) on Apr 02, 2014 at 21:36 UTC
I checked with B::Deparse and B::Concise and the variable name is clearly visible i.e not optimized away: `> perl -MO=Deparse,-q use warnings; my (%a, %b, $c); my $x = "$a{'foo'} $b{'bar'} $c" __END__ use warnings; my(%a, %b, $c); my $x = $a{'foo'} . ' ' . $b{'bar'} . ' ' . $c; - syntax OK` [download] -q reveals the mentioned optimization of string-interpolations, but the variable name is not affected. <Reveal this spoiler or all in this thread> I thought this is only a negligible bug, such that after fixing `$a{foo}` the following `$b{bar}` would be visible, but that's unfortunately not the case: `> perl -w my (%a, %b, $c); $a{foo}="FIXED"; my $x = "$a{'foo'} $b{'bar'} $c"; __END__ Use of uninitialized value in concatenation (.) or string at - line 3. Use of uninitialized value $c in concatenation (.) or string at - line + 3.` [download] Cheers Rolf ( addicted to the Perl Programming Language)	[reply] [d/l] [select]
Re^3: Variable name missing in warning (source) by tye (Sage) on Apr 03, 2014 at 06:24 UTC
Then why don't you (anybody in this thread) go read (and understand) the near-400-line subroutine of C code that tries to find variable names for undef warnings? See S_find_uninit_var in sv.c. The docs didn't say that sometimes variable names are optimized away (the mention of optimization was about the reported operation). The docs pretty clearly hint (to me, anyway) that finding the variable name isn't close to a trivial task and is known to fail somewhat often for reasons too complicated to usefully summarize in those docs. - tye	[reply]
Re^4: Variable name missing in warning (source) by LanX (Saint) on Apr 03, 2014 at 10:02 UTC
Re^5: Variable name missing in warning (source) by tye (Sage) on Apr 03, 2014 at 20:59 UTC
Some notes below your chosen depth have not been shown here
Re^2: Variable name missing in warning by Anonymous Monk on Apr 02, 2014 at 17:23 UTC
This says that optimizations (compile time optimizations I guess) can explain why perl doesn't know exactly in what operation the variable was used. But this doesn't say why perl would not know the name of the variable.	[reply]
Re: Variable name missing in warning by toolic (Bishop) on Apr 02, 2014 at 16:33 UTC
UPDATE: Nevermind. As bigj states, perldiag has this covered. ~~Looks like a bug to me. I also see it, and I'm on perl version 5.14.2. How recent is your perl? perlbug~~	[reply]
Re^2: Variable name missing in warning by Your Mother (Archbishop) on Apr 02, 2014 at 16:40 UTC
v5.16.1 yields the same.	[reply]
Re^2: Variable name missing in warning by LanX (Saint) on Apr 02, 2014 at 16:44 UTC
same v5.10.0 Cheers Rolf ( addicted to the Perl Programming Language)	[reply]
Re^2: Variable name missing in warning by bogbert (Initiate) on Apr 02, 2014 at 16:39 UTC
I'm using 5.14.2 too.	[reply]