PERLSTYLE(1) Perl Programmers Reference Guide PERLSTYLE(1)NAMEperlstyle - Perl style guide
DESCRIPTION
Each programmer will, of course, have his or her own pref
erences in regards to formatting, but there are some gen
eral 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 "use warnings" pragma
or the "$^W" variable if you must. You should also always
run under "use strict" or know the reason why not. The
"use sigtrap" and even "use diagnostics" pragmas may also
prove useful.
Regarding aesthetics of code lay out, about the only thing
Larry cares strongly about is that the closing curly
bracket of a multi-line BLOCK should line up with the key
word that started the construct. Beyond that, he has
other preferences that aren't so strong:
4-column indent.
Opening curly on same line as keyword, if possible,
otherwise line up.
Space before the opening curly of a multi-line BLOCK.
One-line BLOCK may be put on one line, including
curlies.
No space before the semicolon.
Semicolon omitted in "short" one-line BLOCK.
Space around most operators.
Space around a "complex" subscript (inside brackets).
Blank lines between chunks that do different things.
Uncuddled elses.
No space between function name and its opening paren
thesis.
Space after each comma.
Long lines broken after an operator (except "and" and
"or").
Space after last parenthesis matching on current line.
Line up corresponding items vertically.
Omit redundant punctuation as long as clarity doesn't
suffer.
Larry has his reasons for each of these things, but he
doesn't claim that everyone else's mind works the same as
his does.
Here are some other more substantive style issues to think
about:
Just because you CAN do something a particular way
doesn't mean that you SHOULD do it that way. Perl is
designed to give you several ways to do anything, so
consider picking the most readable one. For instance
open(FOO,$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 paren
theses 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 wel
fare 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 "out
dent" it a little to make it more visible:
LINE:
for (;;) {
statements;
last LINE if $foo;
next LINE if /^#/;
statements;
}
Don't be afraid to use loop labels--they're there to
enhance readability as well as to allow multilevel
loop breaks. See the previous example.
Avoid using grep() (or map()) or `backticks` in a void
context, that is, when you just throw away their
return values. Those functions all have return val
ues, so use them. Otherwise use a foreach() loop or
the system() function instead.
For portability, when using features that may not be
implemented on every machine, test the construct in an
eval to see if it fails. If you know what version or
patchlevel a particular feature was implemented, you
can test "$]" ("$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. It is generally
easier to read $var_names_like_this than $VarNames
LikeThis, especially for non-native speakers of
English. It's also a simple rule that works consis
tently 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 back
slashes.
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 con
tain 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 warn
ings" (or -w) in effect Consider giving away your
code. Consider changing your whole world view. Con
sider... oh, never mind.
Be consistent.
Be nice.
2001-03-18 perl v5.6.1 PERLSTYLE(1)
