PERLSTYLE(1) PERLSTYLE(1)
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 $^W variable if you must. You should also always run under use
strict or know the reason why not. The use sigtrap and even use
diagnostics pragmas may also prove useful.
Regarding aesthetics of code lay out, about the only thing Larry cares
strongly about is that the closing curly brace of a multi-line BLOCK
should line up with the keyword that started the construct. Beyond that,
he has other preferences that aren't so strong:
o 4-column indent.
o Opening curly on same line as keyword, if possible, otherwise line
up.
o Space before the opening curly of a multi-line BLOCK.
o One-line BLOCK may be put on one line, including curlies.
o No space before the semicolon.
o Semicolon omitted in "short" one-line BLOCK.
o Space around most operators.
o Space around a "complex" subscript (inside brackets).
o Blank lines between chunks that do different things.
o Uncuddled elses.
o No space between function name and its opening parenthesis.
o Space after each comma.
o Long lines broken after an operator (except "and" and "or").
o Space after last parenthesis matching on current line.
o Line up corresponding items vertically.
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o Omit redundant punctuation as long as clarity doesn't suffer.
Larry has his reasons for each of these things, but he doesn't claim that
everyone else's mind works the same as his does.
Here are some other more substantive style issues to think about:
o Just because you CAN do something a particular way doesn't mean that
you SHOULD do it that way. Perl is designed to give you several ways
to do anything, so consider picking the most readable one. For
instance
open(FOO,$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 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:
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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 nonnative
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.
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
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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 translations 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 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.
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o Be consistent.
o Be nice.
PPPPaaaaggggeeee 5555 [ Back ]
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