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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"	or
	   "/xx" modifiers 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 v5.28.3			  2020-05-14			  PERLSTYLE(1)

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