bcrowell2 has asked for the wisdom of the Perl Monks concerning the following question:

In the following minimal example, I get an error message from the Math::Complex package, but no stack trace or indication that the error happened at line 4 of the program. In a situation like this, how do I force a stack trace to be output so that I can find out where the error is happening? Is this package somehow defeating perl's normal error-reporting mechanisms? Is this a bug, or just a bad design choice?

Code: 

use Math::Complex;
use strict;
my $x = 1+1*i;
print $x*undef;

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Output: 
Use of uninitialized value $z2 in multiplication (*) at /usr/share/per
+l/5.14/Math/Complex.pm line 471.
Use of uninitialized value $z2 in multiplication (*) at /usr/share/per
+l/5.14/Math/Complex.pm line 471.
0

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Re: how to force a stack trace
by educated_foo (Vicar) on Mar 13, 2013 at 16:22 UTC
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      Thanks, educated_foo, McA, Athanasius, and Dallaylaen -- that was very helpful! -Ben
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Re: how to force a stack trace
by McA (Priest) on Mar 13, 2013 at 16:25 UTC

    Hi,

    have a look at Carp::Always, Devel::SimpleTrace and perldoc perlvar searching for %SIG.

    Best regards
    McA

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Re: how to force a stack trace
by Athanasius (Archbishop) on Mar 13, 2013 at 17:10 UTC
    Is this package somehow defeating perl's normal error-reporting mechanisms?

    No, the messages are warnings, not errors (i.e. exceptions). They appear because Math::Complex invokes the warnings pragma on line 73 of the source code (version 1.59).

    Carp::Always will display the line number you are seeking. But it won’t produce a stack trace, because no exception is thrown:

     3:06 >perl 573_SoPW.pl
Use of uninitialized value $z2 in multiplication (*) at C:\Perl\lib\pe
+rl5/Math/Complex.pm line 489.
        Math::Complex::_multiply('Math::Complex=HASH(0x186e5e4)', unde
+f, '') called at 573_SoPW.pl line 5
Use of uninitialized value $z2 in multiplication (*) at C:\Perl\lib\pe
+rl5/Math/Complex.pm line 489.
        Math::Complex::_multiply('Math::Complex=HASH(0x186e5e4)', unde
+f, '') called at 573_SoPW.pl line 5
0
 3:06 >

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    Hope that helps,

    Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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Re: how to force a stack trace
by Dallaylaen (Chaplain) on Mar 13, 2013 at 18:18 UTC

    Sometimes I use the following method for debugging hard to track warnings (there's actually a Log4perl-compatible object instead of print there, use whatever fits you):

    use Carp; 
local $SIG{__WARN__} = sub { print( Carp::longmess (shift) ); };

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    This is a "trick" however and should be agreed upon by the rest of the team. See also this node for sigwarn/sigdie explanation.

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