How I use unless

9^th January 2013 · 2 min read

unless is a language construct that I think sums up Perl in many ways. In the right hands, such syntactic choices can bring the code closer to the problem domain, aid readability, and allow great freedom of expression. In the wrong hands, it can produce a horrible mess which nobody can understand.

I have some simple heuristics for applying unless that have worked well for me and people who read my code:

1. Use unless when the condition is negative.

   You can avoid a double-negative by using unless with negatively-named variables or functions. For example:

   lock() unless $skip_locking;
   

   …compared with

   lock() if !$skip_locking;
   

2. Use unless to imply success.

   Unlike the neutral if, unless implies that the block is likely to be executed (i.e. the condition is unlikely to occur, just as it does in English). It also implies that we want the block to execute.

   unless ($going_to_throw_up) {
       drink_shot();  # implication: throwing up is unlikely. Cheers!
   }
   

   vs.

   if ( ! $going_to_throw_up ) {
       drink_shot();   # implication: uhm, I'm feeling a little woozy...
   }
   

   Even the notorious double negative can work in this context.

   unless ( ! file_readable($fh) ) {
       # do some shiz with $fh
   }
   

   Contrast with the more neutral if ( file_readable($fh) ).

3. Don’t use unless for complex expressions.

   If the programmer needs to apply DeMorgan’s law to understand the conditional, use if and apply it immediately. For instance, which of these two are more readable?

   unless ( $have_work && !$sunday ) {
       sleep_in();
   }
   
   if ( !$have_work || $sunday ) {
       sleep_in();
   }
   

   Most likely, you had to mentally transform the unless to an if to understand it; in these cases, using if is better.

   Of course, even if you use unless because a conditional is simple, it could get more complicated later. Refactoring the conditional into its own function or method protects you from this.

   unless ( $query->param('queue')
         or $query->param('language')
         or $query->param('domain') ) {
       # Provide default status filter
   }
   
   unless ($query->has_status_filter) {
       # Provide default status filter
   }
   

4. Don’t use unless if you have an elsif or else block.

   unless ($foo) {
       ...
   }
   else {
       ...
   }
   

   Yep.