PERLSTYLE(1) Perl Programmers Reference Guide PERLSTYLE(1)NAMEperlstyle - 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 $^W variable if you
must. You should also always run under use strict or know
the reason why not. The use sigtrap and even use
diagnostics pragmas may also prove useful.
Regarding aesthetics of code lay out, about the only thing
Larry cares strongly about is that the closing curly
bracket of a multi-line BLOCK should line up with the
keyword that started the construct. Beyond that, he has
other preferences that aren't so strong:
o 4-column indent.
o Opening curly on same line as keyword, if possible,
otherwise line up.
o Space before the opening curly of a multi-line BLOCK.
o One-line BLOCK may be put on one line, including
curlies.
o No space before the semicolon.
o Semicolon omitted in "short" one-line BLOCK.
o Space around most operators.
o Space around a "complex" subscript (inside brackets).
o Blank lines between chunks that do different things.
o Uncuddled elses.
o No space between function name and its opening
parenthesis.
o Space after each comma.
o Long lines broken after an operator (except "and" and
"or").
o Space after last parenthesis matching on current line.
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o Line up corresponding items vertically.
o Omit redundant punctuation as long as clarity doesn't
suffer.
Larry has his reasons for each of these things, but he
doesn't claim that everyone else's mind works the same as
his does.
Here are some other more substantive style issues to think
about:
o Just because you CAN do something a particular way
doesn't mean that you SHOULD do it that way. Perl is
designed to give you several ways to do anything, so
consider picking the most readable one. For instance
open(FOO,$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
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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. 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.
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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:
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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 -w in
effect. Consider giving away your code. Consider
changing your whole world view. Consider... oh, never
mind.
o Be consistent.
o Be nice.
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