1PERLSTYLE(1) Perl Programmers Reference Guide PERLSTYLE(1)
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6 perlstyle - Perl style guide
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9 Each programmer will, of course, have his or her own preferences in
10 regards to formatting, but there are some general guidelines that will
11 make your programs easier to read, understand, and maintain.
12 The most important thing is to use strict and warnings in all your code
14 or know the reason why not to. You may turn them off explicitly for
15 particular portions of code via "no warnings" or "no strict", and this
16 can be limited to the specific warnings or strict features you wish to
17 disable. The -w flag and $^W variable should not be used for this
18 purpose since they can affect code you use but did not write, such as
19 modules from core or CPAN.
20 A concise way to arrange for this is to use the "use VERSION" syntax,
22 requesting a version 5.36 or above, which will enable both the "strict"
23 and "warnings" pragmata (as well as several other useful named
24 features).
25 use v5.36;
27 Regarding aesthetics of code layout, about the only thing Larry cares
29 strongly about is that the closing curly bracket of a multi-line BLOCK
30 should line up with the keyword that started the construct. Beyond
31 that, he has other preferences that aren't so strong:
32 • 4-column indent.
34 • Opening curly on same line as keyword, if possible, otherwise line
36 up.
37 • Space before the opening curly of a multi-line BLOCK.
39 • One-line BLOCK may be put on one line, including curlies.
41 • No space before the semicolon.
43 • Semicolon omitted in "short" one-line BLOCK.
45 • Space around most operators.
47 • Space around a "complex" subscript (inside brackets).
49 • Blank lines between chunks that do different things.
51 • Uncuddled elses.
53 • No space between function name and its opening parenthesis.
55 • Space after each comma.
57 • Long lines broken after an operator (except "and" and "or").
59 • Space after last parenthesis matching on current line.
61 • Line up corresponding items vertically.
63 • Omit redundant punctuation as long as clarity doesn't suffer.
65 Larry has his reasons for each of these things, but he doesn't claim
67 that everyone else's mind works the same as his does.
68 Here are some other more substantive style issues to think about:
70 • Just because you CAN do something a particular way doesn't mean
72 that you SHOULD do it that way. Perl is designed to give you
73 several ways to do anything, so consider picking the most readable
74 one. For instance
75 open(my $fh, '<', $foo) || die "Can't open $foo: $!";
77 is better than
79 die "Can't open $foo: $!" unless open(my $fh, '<', $foo);
81 because the second way hides the main point of the statement in a
83 modifier. On the other hand
84 print "Starting analysis\n" if $verbose;
86 is better than
88 $verbose && print "Starting analysis\n";
90 because the main point isn't whether the user typed -v or not.
92 Similarly, just because an operator lets you assume default
94 arguments doesn't mean that you have to make use of the defaults.
95 The defaults are there for lazy systems programmers writing one-
96 shot programs. If you want your program to be readable, consider
97 supplying the argument.
98 Along the same lines, just because you CAN omit parentheses in many
100 places doesn't mean that you ought to:
101 return print reverse sort num values %array;
103 return print(reverse(sort num (values(%array))));
104 When in doubt, parenthesize. At the very least it will let some
106 poor schmuck bounce on the % key in vi.
107 Even if you aren't in doubt, consider the mental welfare of the
109 person who has to maintain the code after you, and who will
110 probably put parentheses in the wrong place.
111 • Don't go through silly contortions to exit a loop at the top or the
113 bottom, when Perl provides the "last" operator so you can exit in
114 the middle. Just "outdent" it a little to make it more visible:
115 LINE:
117 for (;;) {
118 statements;
119 last LINE if $foo;
120 next LINE if /^#/;
121 statements;
122 }
123 • Don't be afraid to use loop labels--they're there to enhance
125 readability as well as to allow multilevel loop breaks. See the
126 previous example.
127 • Avoid using grep() (or map()) or `backticks` in a void context,
129 that is, when you just throw away their return values. Those
130 functions all have return values, so use them. Otherwise use a
131 foreach() loop or the system() function instead.
132 • For portability, when using features that may not be implemented on
134 every machine, test the construct in an eval to see if it fails.
135 If you know what version or patchlevel a particular feature was
136 implemented, you can test $] ($PERL_VERSION in "English") to see if
137 it will be there. The "Config" module will also let you
138 interrogate values determined by the Configure program when Perl
139 was installed.
140 • Choose mnemonic identifiers. If you can't remember what mnemonic
142 means, you've got a problem.
143 • While short identifiers like $gotit are probably ok, use
145 underscores to separate words in longer identifiers. It is
146 generally easier to read $var_names_like_this than
147 $VarNamesLikeThis, especially for non-native speakers of English.
148 It's also a simple rule that works consistently with
149 "VAR_NAMES_LIKE_THIS".
150 Package names are sometimes an exception to this rule. Perl
152 informally reserves lowercase module names for "pragma" modules
153 like "integer" and "strict". Other modules should begin with a
154 capital letter and use mixed case, but probably without underscores
155 due to limitations in primitive file systems' representations of
156 module names as files that must fit into a few sparse bytes.
157 • You may find it helpful to use letter case to indicate the scope or
159 nature of a variable. For example:
160 $ALL_CAPS_HERE constants only (beware clashes with perl vars!)
162 $Some_Caps_Here package-wide global/static
163 $no_caps_here function scope my() or local() variables
164 Function and method names seem to work best as all lowercase.
166 E.g., "$obj->as_string()".
167 You can use a leading underscore to indicate that a variable or
169 function should not be used outside the package that defined it.
170 • If you have a really hairy regular expression, use the "/x" or
172 "/xx" modifiers and put in some whitespace to make it look a little
173 less like line noise. Don't use slash as a delimiter when your
174 regexp has slashes or backslashes.
175 • Use the "and" and "or" operators to avoid having to parenthesize
177 list operators so much, and to reduce the incidence of punctuation
178 operators like "&&" and "||". Call your subroutines as if they
179 were functions or list operators to avoid excessive ampersands and
180 parentheses.
181 • Use here documents instead of repeated print() statements.
183 • Line up corresponding things vertically, especially if it'd be too
185 long to fit on one line anyway.
186 $IDX = $ST_MTIME;
188 $IDX = $ST_ATIME if $opt_u;
189 $IDX = $ST_CTIME if $opt_c;
190 $IDX = $ST_SIZE if $opt_s;
191 mkdir $tmpdir, 0700 or die "can't mkdir $tmpdir: $!";
193 chdir($tmpdir) or die "can't chdir $tmpdir: $!";
194 mkdir 'tmp', 0777 or die "can't mkdir $tmpdir/tmp: $!";
195 • Always check the return codes of system calls. Good error messages
197 should go to "STDERR", include which program caused the problem,
198 what the failed system call and arguments were, and (VERY
199 IMPORTANT) should contain the standard system error message for
200 what went wrong. Here's a simple but sufficient example:
201 opendir(my $dh, $dir) or die "can't opendir $dir: $!";
203 • Line up your transliterations when it makes sense:
205 tr [abc]
207 [xyz];
208 • Think about reusability. Why waste brainpower on a one-shot when
210 you might want to do something like it again? Consider
211 generalizing your code. Consider writing a module or object class.
212 Consider making your code run cleanly with "use strict" and "use
213 warnings" in effect. Consider giving away your code. Consider
214 changing your whole world view. Consider... oh, never mind.
215 • Try to document your code and use Pod formatting in a consistent
217 way. Here are commonly expected conventions:
218 • use "C<>" for function, variable and module names (and more
220 generally anything that can be considered part of code, like
221 filehandles or specific values). Note that function names are
222 considered more readable with parentheses after their name,
223 that is function().
224 • use "B<>" for commands names like cat or grep.
226 • use "F<>" or "C<>" for file names. "F<>" should be the only Pod
228 code for file names, but as most Pod formatters render it as
229 italic, Unix and Windows paths with their slashes and
230 backslashes may be less readable, and better rendered with
231 "C<>".
232 • Be consistent.
234 • Be nice.
236perl v5.38.2 2023-11-30 PERLSTYLE(1)