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diagnostics masks "uninitialized value in range (or flip)" warning

by toolic (Bishop)
on Dec 08, 2009 at 15:37 UTC ( #811761=perlmeditation: print w/ replies, xml ) Need Help??


use diagnostics masks the following type of warning when use warnings is in effect:
Use of uninitialized value in range (or flip)


Here, we get the warning message, as desired:
$ perl -w -e 'my $x = 1..3' Use of uninitialized value in range (or flip) at -e line 1. $
Now, add diagnostics so we can get a more verbose explanation of the warning message:
$ perl -w -e 'my $x = 1..3' -Mdiagnostics $
D'oh! Not only did diagnostics not display additional information, but it also made the warning message disappear. WTF?

Furthermore, if diagnostics is used in conjunction with

use warnings FATAL => 'all';
the script will not die because of the warning.


We can't use diagnostics to get more information on this warning, but we can still use splain to do the job:
$ echo 'Use of uninitialized value in range (or flip)' | splain Use of uninitialized value in range (or flip) (#1) (W uninitialized) An undefined value was used as if it were alread +y defined. It was interpreted as a "" or a 0, but maybe it was a mi +stake. To suppress this warning assign a defined value to your variables. To help you figure out what was undefined, perl tells you what ope +ration you used the undefined value in. Note, however, that perl optimiz +es your program and the operation displayed in the warning may not necessa +rily 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.
Well, this message is not all that helpful because it does not seem specific to this case. That's a separate issue for perldiag, which can be dealt with on another day.

Root cause

Here is my hypothesis on why this happens. A quote from Range Operator:
If either operand of scalar ".." is a constant expression, that operand is considered true if it is equal ("==") to the current input line number (the $. variable).
By default, $. is undefined:
$ perl -w -e 'print ">>>$.<<<\n"' Use of uninitialized value in concatenation (.) or string at -e line 1 +. >>><<< $
Unless you use diagnostics:
$ perl -w -e 'print ">>>$.<<<\n"' -Mdiagnostics >>>0<<< $
So, it seems that diagnostics has the side effect of initializing the $. variable. I see some calls to open in the source code. Perhaps diagnostics is opening a file handle which is giving the global $. variable a value, thereby suppressing the warning message.

Possible solution

If I modify by adding the following line to localize $., the problem is fixed:
local $.;

What next

This is a bug. Unless the monks can convince me otherwise, I plan to submit this patch as a bug report via perlbug:
--- 2009-06-15 04:21:42.000000000 -0400 +++ 2009-12-05 10:29:53.096561000 -0500 @@ -222,6 +222,7 @@ local $| = 1; my $_; +local $.; my $standalone; my(%HTML_2_Troff, %HTML_2_Latin_1, %HTML_2_ASCII_7);

Recommended usage

Until this gets fixed, make sure you use diagnostics only when you need it. Do not keep use diagnostics; in your code indefinitely. Unfortunately, I have done this in many of my scripts, and I must now go back and comment them out.

This is similar to tye's advice in (tye)Re: use diagnostics eats memory?.


$ perl -v This is perl, v5.10.0 built for x86_64-linux
The $VERSION of is 1.17. The latest version I can find on CPAN is 1.18, and the issue exists there too.


In trying to unravel a range operator mystery, a Super Search led me to an observation by QM. I could not see the warning message because I had use diagnostics; in my code.

Update: Dec. 11, 2009: submitted bug report:
Update: Apr. 13, 2010: confirmed fix in perl version 5.12.0 ($VERSION of is 1.19)

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Re: diagnostics masks "uninitialized value in range (or flip)" warning
by ikegami (Pope) on Dec 08, 2009 at 15:53 UTC

    I plan to submit this patch as a bug report via perlbug:

    Excellent! It could use a test to make sure the current behaviour isn't reintroduced.

Re: diagnostics masks "uninitialized value in range (or flip)" warning
by Anonymous Monk on Dec 08, 2009 at 16:34 UTC

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