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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:

       o   4-column indent.

       o   Opening curly on same line as keyword, if possible, otherwise line up.

       o   Space before the opening curly of a multi-line BLOCK.

       o   One-line BLOCK may be put on one line, including curlies.

       o   No space before the semicolon.

       o   Semicolon omitted in "short" one-line BLOCK.

       o   Space around most operators.

       o   Space around a "complex" subscript (inside brackets).

       o   Blank lines between chunks that do different things.

       o   Uncuddled elses.

       o   No space between function name and its opening parenthesis.

       o   Space after each comma.

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

       o   Space after last parenthesis matching on current line.

       o   Line up corresponding items vertically.

       o   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:

       o   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.

       o   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;
		   }

       o   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.

       o   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.

       o   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.

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

       o   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.

       o   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.

       o   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.

       o   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.

       o   Use here documents instead of repeated "print()" statements.

       o   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: $!";

       o   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: $!";

       o   Line up your transliterations when it makes sense:

	       tr [abc]
		  [xyz];

       o   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.

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

	   o   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()".

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

	   o   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<>".

       o   Be consistent.

       o   Be nice.

perl v5.16.3				    2013-02-26				     PERLSTYLE(1)
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