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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 $^W variable if you must. You
should also always run under "use strict" or know the reason why not.
The "use sigtrap" and even "use diagnostics" pragmas may also prove
useful.
Regarding aesthetics of code lay out, about the only thing Larry cares
strongly about is that the closing curly bracket of a multi-line BLOCK
should line up with the keyword that started the construct. Beyond
that, he has other preferences that aren't so strong:
o 4-column indent.
o Opening curly on same line as keyword, if possible, otherwise line
up.
o Space before the opening curly of a multi-line BLOCK.
o One-line BLOCK may be put on one line, including curlies.
o No space before the semicolon.
o Semicolon omitted in "short" one-line BLOCK.
o Space around most operators.
o Space around a "complex" subscript (inside brackets).
o Blank lines between chunks that do different things.
o Uncuddled elses.
o No space between function name and its opening parenthesis.
o Space after each comma.
o Long lines broken after an operator (except "and" and "or").
o Space after last parenthesis matching on current line.
o Line up corresponding items vertically.
o Omit redundant punctuation as long as clarity doesn't suffer.
Larry has his reasons for each of these things, but he doesn't claim
that everyone else's mind works the same as his does.
Here are some other more substantive style issues to think about:
o Just because you CAN do something a particular way doesn't mean
that you SHOULD do it that way. Perl is designed to give you sev-
eral ways to do anything, so consider picking the most readable
one. For instance
open(FOO,$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 argu-
ments 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 sup-
plying 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 proba-
bly 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 read-
ability as well as to allow multilevel loop breaks. See the previ-
ous example.
o Avoid using "grep()" (or "map()") or `backticks` in a void context,
that is, when you just throw away their return values. Those func-
tions all have return values, so use them. Otherwise use a "fore-
ach()" 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 interro-
gate 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 under-
scores to separate words in longer identifiers. It is generally
easier to read $var_names_like_this than $VarNamesLikeThis, espe-
cially 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 infor-
mally 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" modi-
fier 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 parenthe-
size list operators so much, and to reduce the incidence of punctu-
ation 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 IMPOR-
TANT) 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 generaliz-
ing your code. Consider writing a module or object class. Con-
sider making your code run cleanly with "use strict" and "use warn-
ings" (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 back-
slashes may be less readable, and better rendered with "C<>".
o Be consistent.
o Be nice.
perl v5.8.8 2006-01-07 PERLSTYLE(1)
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