Perl Best Practices

18.13. Semi-Automatic Debugging

Consider using "smart comments" when debugging, rather than warn statements.

Serialized warnings work well for manual debugging, but they can be tedious to code correctly^[*]. And, even with the editor macro suggested earlier, the output of a statement like:

^[*] Which is vital. If there's anything less enjoyable than beating your head against a bug for several hours, it's finally discovering that your debugging print statement was itself buggy, and the problem isn't anywhere near where you thought it was. This is presumably a homerbug.

warn 'results: ', Dumper($results);

still leaves something to be desired in terms of readability:

results: $VAR1 = bless( do{\(my $o = undef)}, 'Achievements' )

The Smart::Comments module (previously described under "Automatic Progress Indicators" in Chapter 10) supports a form of smart comment that can help your debugging. For example, instead of:

use Data::Dumper qw( Dumper ); my $results = $scenario->project_outcomes( ); warn '$results: ', Dumper($results);

you could just write:

use Smart::Comments; my $results = $scenario->project_outcomes( );

### $results

which would then output either:

### $results: <opaque Achievements object (blessed scalar)>

or:

### $results: 'Achievements=SCALAR(0x811130)'

depending on whether $results is an actual object reference or merely its stringification.

Smart::Comments also supports comment-based assertions:

### check: @candidates >= @elected

which issue warnings when the specified condition is not met. For example, the previous comment might print:

### @candidates >= @elected was not true at ch18/Ch18.049_Best line 23. ### @candidates was: [ ### 'Smith', ### 'Nguyen', ### 'Ibrahim' ### ] ### @elected was: [ ### 'Smith', ### 'Nguyen', ### 'Ibrahim', ### 'Nixon' ### ]

The module also supports stronger assertions:

### require: @candidates >= @elected

which prints the same warning as the ### check:, but then immediately terminates the program.

Apart from producing more readable debugging messages, the major advantage of this approach is that you can later switch off all these comment-based debugging statements simply by removing (or commenting out) the use Smart::Comments line. When Smart::Comments isn't loaded, those smart comments become regular comments, which means you can leave the actual debugging statements in your source code^[*] without incurring any performance penalty.

^[*] If you needed them once, you'll almost certainly need them again.