Perl - Coding Standard

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Each programmer will, of course, have his or her own preferences in regards to formatting, but there are some general guidelines that will make your programs easier to read, understand, and maintain.

The most important thing is to run your programs under the -w flag at all times. You may turn it off explicitly for particular portions of code via the no warnings pragma or the $^W variable if you must. You should also always run under use strict or know the reason why not. The use sigtrap and even use diagnostics pragmas may also prove useful.

Regarding aesthetics of code lay out, about the only thing Larry cares strongly about is that the closing curly bracket of a multi-line BLOCK should line up with the keyword that started the construct. Beyond that, he has other preferences that aren't so strong −

  • 4-column indent.
  • Opening curly on same line as keyword, if possible, otherwise line up.
  • Space before the opening curly of a multi-line BLOCK.
  • One-line BLOCK may be put on one line, including curlies.
  • No space before the semicolon.
  • Semicolon omitted in "short" one-line BLOCK.
  • Space around most operators.
  • Space around a "complex" subscript (inside brackets).
  • Blank lines between chunks that do different things.
  • Uncuddled elses.
  • No space between function name and its opening parenthesis.
  • Space after each comma.
  • Long lines broken after an operator (except and and or).
  • Space after last parenthesis matching on current line.
  • Line up corresponding items vertically.
  • Omit redundant punctuation as long as clarity doesn't suffer.

Here are some other more substantive style issues to think about: Just because you CAN do something a particular way doesn't mean that you SHOULD do it that way. Perl is designed to give you several ways to do anything, so consider picking the most readable one. For instance −

open(FOO,$foo) || die "Can't open $foo: $!";

Is better than −

die "Can't open $foo: $!" unless open(FOO,$foo);

Because the second way hides the main point of the statement in a modifier. On the other hand,

print "Starting analysis\n" if $verbose;

Is better than −

$verbose && print "Starting analysis\n";

Because the main point isn't whether the user typed -v or not.

Don't go through silly contortions to exit a loop at the top or the bottom, when Perl provides the last operator so you can exit in the middle. Just "outdent" it a little to make it more visible −

LINE:
for (;;) {
   statements;
   last LINE if $foo;
   next LINE if /^#/;
   statements;
}

Let's see few more important points −

  • Don't be afraid to use loop labels--they're there to enhance readability as well as to allow multilevel loop breaks. See the previous example.

  • Avoid using grep() (or map()) or `backticks` in a void context, that is, when you just throw away their return values. Those functions all have return values, so use them. Otherwise use a foreach() loop or the system() function instead.

  • For portability, when using features that may not be implemented on every machine, test the construct in an eval to see if it fails. If you know what version or patchlevel a particular feature was implemented, you can test $] ($PERL_VERSION in English) to see if it will be there. The Config module will also let you interrogate values determined by the Configure program when Perl was installed.

  • Choose mnemonic identifiers. If you can't remember what mnemonic means, you've got a problem.

  • While short identifiers like $gotit are probably ok, use underscores to separate words in longer identifiers. It is generally easier to read $var_names_like_this than $VarNamesLikeThis, especially for non-native speakers of English. It's also a simple rule that works consistently with VAR_NAMES_LIKE_THIS.

  • Package names are sometimes an exception to this rule. Perl informally reserves lowercase module names for "pragma" modules like integer and strict. Other modules should begin with a capital letter and use mixed case, but probably without underscores due to limitations in primitive file systems' representations of module names as files that must fit into a few sparse bytes.

  • If you have a really hairy regular expression, use the /x modifier and put in some whitespace to make it look a little less like line noise. Don't use slash as a delimiter when your regexp has slashes or backslashes.

  • Always check the return codes of system calls. Good error messages should go to STDERR, include which program caused the problem, what the failed system call and arguments were, and (VERY IMPORTANT) should contain the standard system error message for what went wrong. Here's a simple but sufficient example − 

opendir(D, $dir) or die "can't opendir $dir: $!";

  • Think about reusability. Why waste brainpower on a one-shot when you might want to do something like it again? Consider generalizing your code. Consider writing a module or object class. Consider making your code run cleanly with use strict and use warnings (or -w) in effect. Consider giving away your code. Consider changing your whole world view. Consider... oh, never mind.

  • Be consistent.

  • Be nice.

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