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1995-07-25
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PPPPEEEERRRRLLLLSSSSTTTTYYYYLLLLEEEE((((1111))))UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000))))PPPPEEEERRRRLLLLSSSSTTTTYYYYLLLLEEEE((((1111))))
NNNNAAAAMMMMEEEE
perlstyle - Perl style guide
DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
SSSSttttyyyylllleeee
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.
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 multiline 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 paren.
o+ Space after each comma.
o+ Long lines broken after an operator (except "and" and
"or").
o+ Space after last paren matching on current line.
o+ Line up corresponding items vertically.
o+ Omit redundant punctuation as long as clarity doesn't
suffer.
Page 1 (printed 6/30/95)
PPPPEEEERRRRLLLLSSSSTTTTYYYYLLLLEEEE((((1111))))UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000))))PPPPEEEERRRRLLLLSSSSTTTTYYYYLLLLEEEE((((1111))))
Larry has his reasons for each of these things, but he
doen'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 _C_A_N do something a particular way
doesn't mean that you _S_H_O_U_L_D 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";
since the main point isn't whether the user typed ----vvvv 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 _C_A_N 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 vvvviiii.
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 parens in the wrong place.
Page 2 (printed 6/30/95)
PPPPEEEERRRRLLLLSSSSTTTTYYYYLLLLEEEE((((1111))))UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000))))PPPPEEEERRRRLLLLSSSSTTTTYYYYLLLLEEEE((((1111))))
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:
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 multi-level loop
breaks. See the previous example.
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 CCCCoooonnnnffffiiiigggguuuurrrreeee program when Perl was
installed.
o+ Choose mnemonic identifiers. If you can't remember what
mnemonic means, you've got a problem.
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 punctuational operators like && and ||.
Call your subroutines as if they were functions or list
operators to avoid excessive ampersands and parens.
o+ Use here documents instead of repeated _p_r_i_n_t()
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;
Page 3 (printed 6/30/95)
PPPPEEEERRRRLLLLSSSSTTTTYYYYLLLLEEEE((((1111))))UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000))))PPPPEEEERRRRLLLLSSSSTTTTYYYYLLLLEEEE((((1111))))
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+ 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 ----wwww in effect.
Consider giving away your code. Consider changing your
whole world view. Consider... oh, never mind.
o+ Be consistent.
o+ Be nice.
Page 4 (printed 6/30/95)