PERLSTYLE

# NAME

perlstyle - Perl style guide

# DESCRIPTION

## Style

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.

Regarding aesthetics of code lay out, about the only thing Larry cares strongly about is that the closing curly brace 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 multiline 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 paren.

*
  Space after each comma.

*
  Long lines broken after an operator (except "and" and "or").

*
  Space after last paren matching on current line.

*
  Line up corresponding items vertically.

*
  Omit redundant punctuation as long as clarity doesn't suffer.

Larry has his reasons for each of these things, but he doen't claim that.everyone else's mind works the same as his does.

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";

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

  Similarly, just because an operator lets you assume default arguments doesn't mean that you have to make use of the defaults. The defaults are there for lazy systems programmers writing one-shot programs. If you want your program to be readable, consider supplying the argument.

  Along the same lines, just because you CAN omit parentheses in many places doesn't mean that you ought to:

          return print reverse sort num values %array;
          return print(reverse(sort num (values(%array))));

  When in doubt, parenthesize. At the very least it will let some poor schmuck bounce on the % key in vi.

  Even if you aren't in doubt, consider the mental welfare of the person who has to maintain the code after you, and who will probably put parens in the wrong place.

*
  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;
              }

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

*
  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.

*
  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.

*
  Use the new "and" and "or" operators to avoid having to parenthesize list operators so much, and to reduce the incidence of punctuational operators like `&&` and `||`. Call your subroutines as if they were functions or list operators to avoid excessive ampersands and parens.

*
  Use here documents instead of repeated print() statements.

*
  Line up corresponding things vertically, especially if it'd be too long to fit on one line anyway.

          $IDX = $ST_MTIME;
          $IDX = $ST_ATIME       if $opt_u;
          $IDX = $ST_CTIME       if $opt_c;
          $IDX = $ST_SIZE        if $opt_s;
          mkdir $tmpdir, 0700 or die "can't mkdir $tmpdir: $!";
          chdir($tmpdir)      or die "can't chdir $tmpdir: $!";
          mkdir 'tmp',   0777 or die "can't mkdir $tmpdir/tmp: $!";

*
  Line up your translations when it makes sense:

          tr [abc]
             [xyz];

*
  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 -w in effect. Consider giving away your code. Consider changing your whole world view. Consider... oh, never mind.

*
  Be consistent.

*
  Be nice.

.