How can I tell if a Perl module is actually used in my program?

Question

I have been on a "cleaning spree" lately at work, doing a lot of touch-up stuff that should have been done awhile ago. One thing I have been doing is deleted modules that were imported into files and never used, or they were used at one point but not anymore. To do this I have just been deleting an import and running the program's test file. Which gets really, really tedious.

Is there any programmatic way of doing this? Short of me writing a program myself to do it.

Answer 1

Short answer, you can't.

Longer possibly more useful answer, you won't find a general purpose tool that will tell you with 100% certainty whether the module you're purging will actually be used. But you may be able to build a special purpose tool to help you with the manual search that you're currently doing on your codebase. Maybe try a wrapper around your test suite that removes the use statements for you and ignores any error messages except messages that say Undefined subroutine &__PACKAGE__::foo and other messages that occur when accessing missing features of any module. The wrapper could then automatically perform a dumb source scan on the codebase of the module being purged to see if the missing subroutine foo (or other feature) might be defined in the unwanted module.

You can supplement this with Devel::Cover to determine which parts of your code don't have tests so you can manually inspect those areas and maybe get insight into whether they are using code from the module you're trying to purge.

Due to the halting problem you can't statically determine whether any program, of sufficient complexity, will exit or not. This applies to your problem because the "last" instruction of your program might be the one that uses the module you're purging. And since it is impossible to determine what the last instruction is, or if it will ever be executed, it is impossible to statically determine if that module will be used. Further, in a dynamic language, which can extend the program during it's run, analysis of the source or even the post-compile symbol tables would only tell you what was calling the unwanted module just before run-time (whatever that means).

Because of this you won't find a general purpose tool that works for all programs. However, if you are positive that your code doesn't use certain run-time features of Perl you might be able to write a tool suited to your program that can determine if code from the module you're purging will actually be executed.