Why are Perl source filters bad and when is it OK to use them?

Question

It is \"common knowledge\" that source filters are bad and should not be used in production code.

When answering a a similar, but more specific question I couldn\'t

Answer 1

In theory, a source filter is no more dangerous than any other module, since you could easily write a module that redefines builtins or other constructs in "unexpected" ways. In practice however, it is quite hard to write a source filter in a way where you can prove that its not going to make a mistake. I tried my hand at writing a source filter that implements the perl6 feed operators in perl5 (Perl6::Feeds on cpan). You can take a look at the regular expressions to see the acrobatics required to simply figure out the boundaries of expression scope. While the filter works, and provides a test bed to experiment with feeds, I wouldn't consider using it in a production environment without many many more hours of testing.

Filter::Simple certainly comes in handy by dealing with 'the gory details of parsing quoted constructs', so I would be wary of any source filter that doesn't start there.

In all, it really depends on the filter you are using, and how broad a scope it tries to match against. If it is something simple like a c macro, then its "probably" ok, but if its something complicated then its a judgement call. I personally can't wait to play around with perl6's macro system. Finally lisp wont have anything on perl :-)