scratchpad kimmel working title: Debugging: Inspecting a variable as the value changes <p>This is a simple tutorial on how to find out when a variable's value changes without needing prior knowledge of the codebase.</p> It is common practice when writing/debugging/refactoring/messing around with a program that you want to know when a certain variable is set or its value changes. For example I was looking at this piece code <code> if ( $show_banner == 1 ) { print STDERR "$app_name $version ($app_www)\n"; print STDERR "$copyright $author, $author_email\n\n"; } </code> and I realized it was never getting executed. I could have just sprinkled in some print $show_banner; statements but this code was written by another programmer so I am not familiar with its structure and flow so I would waste time learning the code to know the best places to put a print statement. <p>There has to be a faster way to track this variable and I know this is a pretty common problem. So what is the Perl answer you ask? CPAN. There I found the Tie::Trace module.</p> <p>Tie::Trace allows you to watch a variable and every time the value changes you get a debug message. Here is how I added it to my existing program.</p> <code> use Tie::Trace qw( watch ); my $show_banner = 1; watch $show_banner; </code> Then I ran my application and got the following output. <code> main:: $show_banner => 0 at ./colordiff.pl line 156. </code> From there I ran the test suite to see if $show_banner changed due to any edge cases. Each test in the test suite produced the same above output. <p>Now I knew where the value was being changed. From there it was a simple change to an if statement and a rerun of the application test suite to verify no regressions were introduced.</p> <p>The watch function provided by Tie::Trace does all the heavy lifting by issuing print Dumper statements on the variable as it changes. This is an invaluable tool for following value changes of variables as a program executes.