You'll certainly have your own preferences in regard 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
use warnings
pragma. (You can turn off unwanted
warnings with no warnings
.) You should also always
run under use strict
or have a good reason not to.
The use sigtrap
and even the use
diagnostics
pragmas may also prove of benefit.
Regarding aesthetics of code layout, about the only thing Larry
cares strongly about is that the closing brace of a multiline
BLOCK
should be "outdented" to line up with
the keyword that started the construct. Beyond that, he has other
preferences that aren't so strong. Examples in this book (should) all
follow these coding conventions:
Use four-column indents.
An opening brace should be put on the same line as its preceding keyword, if possible; otherwise, line them up vertically.
while ($condition) { # for short ones, align with keywords # do something } # if the condition wraps, line up the braces with each other while ($this_condition and $that_condition and $this_other_long_condition) { # do something }
A short BLOCK
may be put on one
line, including braces.
Omit the semicolon in a short, one-line
BLOCK
.
Surround most operators with space.
Surround a "complex" subscript (inside brackets) with space.
Put blank lines between chunks of code that do different things.
Put a newline between a closing brace and
else
.
Do not put space between a function name and its opening parenthesis.
Do not put space before a semicolon.
Put space after each comma.
Break long lines after an operator (but before
and
and or
, even when
spelled &&
and
||
).
Line up corresponding items vertically.
Omit redundant punctuation as long as clarity doesn't suffer.
Larry has his reasons for each of these things, but he doesn't claim that everyone else's mind works the same as his does (or doesn't).
Here are some other, more substantive style issues to think about:
Just because you can do something a particular way doesn't mean you should do it that way. Perl is designed to give you several ways to do anything, so consider picking the most readable one. For instance:
open(FOO,$foo) or 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 " if $verbose;
is better than:
$verbose and print "Starting analysis ";
since 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 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. Perl provides the
last
operator so you can exit in the middle.
You can optionally "outdent" it 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 example just given.
Avoid using grep
,
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.
For portability, when using features that may not be
implemented on every machine, test the construct in an
eval
to see whether it fails. If you know the
version or patch level of a particular feature, you can test
$]
($PERL_VERSION
in the
English module) to see whether the feature is 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.
Although short identifiers like
$gotit
are probably okay, use underscores to
separate words. It is generally much easier to read
$var_names_like_this
than
$VarNamesLikeThis
, especially for non-native
speakers of English. Besides, the same rule works for
$VAR_NAMES_LIKE_THIS
.
Package names are sometimes an exception to this
rule. Perl informally reserves lowercase module names for
pragmatic modules like integer
and
strict
. Other modules should begin with a
capital letter and use mixed case, but should probably omit
underscores due to name-length limitations on certain primitive
filesystems.
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
For various vague reasons, function and method names seem to
work best as all lowercase. For example,
$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. (Perl does not enforce this; it's just a form of documentation.)
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 regular expression already has too many slashes or backslashes.
Don't use quotes as delimiters when your string contains the
same kind of quote. Use the q//
,
qq//
, or qx//
pseudofunctions instead.
Use the and
and
or
operators to avoid having to parenthesize
list operators so much and to reduce the incidence of
punctuational 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 they're 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: $!";
That which we tell you three times is true:
Always check the return codes of system calls.
Always check the return codes of system calls.
ALWAYS CHECK THE RETURN CODES OF SYSTEM CALLS!
Error messages should go to
STDERR
and should say which program caused the
problem and what the failed call and its arguments were. Most
importantly, for failed syscalls, messages 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 script 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.
Be consistent.
Be nice.