PERLSTYLE(1) Perl Programmers Reference Guide PERLSTYLE(1)NAMEperlstyle - Perl style guide
DESCRIPTION
Each programmer will, of course, have his or her own prefer-
ences 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 $^W variable if you must. You should also always run
under "use strict" or know the reason why not. The "use
sigtrap" and even "use diagnostics" pragmas may also prove
useful.
Regarding aesthetics of code lay out, about the only thing
Larry cares strongly about is that the closing curly bracket
of a multi-line BLOCK should line up with the keyword that
started the construct. Beyond that, he has other preferences
that aren't so strong:
+ 4-column indent.
+ Opening curly on same line as keyword, if possible, oth-
erwise line up.
+ Space before the opening curly of a multi-line BLOCK.
+ One-line BLOCK may be put on one line, including cur-
lies.
+ No space before the semicolon.
+ Semicolon omitted in "short" one-line BLOCK.
+ Space around most operators.
+ Space around a "complex" subscript (inside brackets).
+ Blank lines between chunks that do different things.
+ Uncuddled elses.
+ No space between function name and its opening
parenthesis.
+ Space after each comma.
+ Long lines broken after an operator (except "and" and
"or").
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+ Space after last parenthesis matching on current line.
+ Line up corresponding items vertically.
+ Omit redundant punctuation as long as clarity doesn't
suffer.
Larry has his reasons for each of these things, but he
doesn't claim that everyone else's mind works the same as
his does.
Here are some other more substantive style issues to think
about:
+ Just because you CAN do something a particular way
doesn't mean that you SHOULD do it that way. Perl is
designed to give you several ways to do anything, so
consider picking the most readable one. For instance
open(FOO,$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 sys-
tems 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
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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" opera-
tor 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 inter-
rogate 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
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modules should begin with a capital letter and use mixed
case, but probably without underscores due to limita-
tions 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 del-
imiter 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()" state-
ments.
+ 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 pro-
gram 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:
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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 writ-
ing 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 read-
able 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.
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