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 $^W variable if you must. You should
also always run under "use strict" or know the reason why not. The
"use sigtrap" and even "use diagnostics" pragmas may also
prove useful.
Regarding aesthetics of code lay out, about the only thing Larry cares strongly
about is that the closing curly bracket of a multi-line BLOCK should line up
with the keyword that started the construct. Beyond that, he has other
preferences that aren't so strong:
- •
- 4-column indent.
- •
- Opening curly on same line as keyword, if possible,
otherwise line up.
- •
- Space before the opening curly of a multi-line BLOCK.
- •
- One-line BLOCK may be put on one line, including
curlies.
- •
- No space before the semicolon.
- •
- Semicolon omitted in "short" one-line BLOCK.
- •
- Space around most operators.
- •
- Space around a "complex" subscript (inside
brackets).
- •
- Blank lines between chunks that do different things.
- •
- Uncuddled elses.
- •
- No space between function name and its opening
parenthesis.
- •
- Space after each comma.
- •
- Long lines broken after an operator (except "and"
and "or").
- •
- Space after last parenthesis matching on current line.
- •
- Line up corresponding items vertically.
- •
- Omit redundant punctuation as long as clarity doesn't
suffer.
Larry has his reasons for each of these things, but he doesn't claim that
everyone else's mind works the same as his does.
Here are some other more substantive style issues to think about:
- •
- Just because you CAN do something a particular way
doesn't mean that you SHOULD do it that way. Perl is designed to
give you several ways to do anything, so consider picking the most
readable one. For instance
open(FOO,$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 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.
- •
- 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;
}
- •
- 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
interrogate 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 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.
- •
- 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 delimiter 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()"
statements.
- •
- 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 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 "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 readable 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.