Perl Style: Learn Precedence

Jan 1, 1998 by Tom Christiansen

• It is a myth that you can just plop in and and or wherever you’d been using the punctuation versions. They have difference precedences. You must learn precedence. And a few parens seldom hurt.

     print FH $data      || die "Can't write to FH: $!";  # NO
     print FH $data      or die "Can't write to FH: $!";  # YES
  
     $a = $b or $c;      # bug: this is wrong
     ($a = $b) or $c;    # really means this
     $a = $b || $c;      # better written this way
  
     @info = stat($file) || die;     # oops, scalar sense of stat!
     @info = stat($file) or die;     # better, now @info gets its due
  

• Careful with parens here:

     $a % 2 ? $a += 10 : $a += 2
  

  Really means this:

     (($a % 2) ? ($a += 10) : $a) += 2
  

  Rather than this:

     ($a % 2) ? ($a += 10) : ($a += 2)
  

Forward to Don’t Overdo `?:’
Back to Embrace && and || for Control and Values
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Copyright © 1998, Tom Christiansen All rights reserved.