perlstyle - Perl style guide
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 $^W variable if you must. You should also always run under use strict or know the reason why not. The use sigtrap and even use diagnostics pragmas may also prove useful.
Regarding aesthetics of code lay out, about the only thing Larry cares strongly about is that the closing curly brace of a multi-line BLOCK should line up with the keyword that started the construct. Beyond that, he has other preferences that aren't so strong:
4-column indent.
Larry has his reasons for each of these things, but he doesn't claim that everyone else's mind works the same as his does.
Here are some other more substantive style issues to think about:
Just because you CAN do something a particular way doesn't mean that you SHOULD do it that way. Perl is designed to give you several ways to do anything, so consider picking the most readable one. For instance
open(FOO,$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 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.
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.
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.
$] ($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.
$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.
$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.
/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.
&& and ||. Call your subroutines as if they were functions or list operators to
avoid excessive ampersands and parentheses.
print() statements.
$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 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: $!";
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 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.
If rather than formatting bugs, you encounter substantive content errors in these documents, such as mistakes in the explanations or code, please use the perlbug utility included with the Perl distribution.