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PERLSTYLE(1)	       Perl Programmers Reference Guide		  PERLSTYLE(1)

NAME
       perlstyle - Perl style guide

DESCRIPTION
       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.

       Larry has his reasons for each of these things, but he doesn'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";

	   because 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 parentheses 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 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.

       ·   You may find it helpful to use letter case to indicate the scope or
	   nature of a variable. For example:

	       $ALL_CAPS_HERE	constants only (beware clashes with perl vars!)
	       $Some_Caps_Here	package-wide global/static
	       $no_caps_here	function scope my() or local() variables

	   Function and method names seem to work best as all lowercase.
	   E.g., "$obj->as_string()".

	   You can use a leading underscore to indicate that a variable or
	   function should not be used outside the package that defined it.

       ·   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
	   punctuation operators like "&&" and "||".  Call your subroutines as
	   if they were functions or list operators to avoid excessive
	   ampersands and parentheses.

       ·   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: $!";

       ·   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: $!";

       ·   Line up your transliterations 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 "use
	   warnings" (or -w) in effect.	 Consider giving away your code.
	   Consider changing your whole world view.  Consider... oh, never
	   mind.

       ·   Try to document your code and use Pod formatting in a consistent
	   way. Here are commonly expected conventions:

	   ·   use "C<>" for function, variable and module names (and more
	       generally anything that can be considered part of code, like
	       filehandles or specific values). Note that function names are
	       considered more readable with parentheses after their name,
	       that is "function()".

	   ·   use "B<>" for commands names like cat or grep.

	   ·   use "F<>" or "C<>" for file names. "F<>" should be the only Pod
	       code for file names, but as most Pod formatters render it as
	       italic, Unix and Windows paths with their slashes and
	       backslashes may be less readable, and better rendered with
	       "C<>".

       ·   Be consistent.

       ·   Be nice.

perl v5.10.1			  2009-02-12			  PERLSTYLE(1)
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