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.
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13 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.
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21 Regarding aesthetics of code lay out, about the only thing Larry cares
22 strongly about is that the closing curly bracket of a multi-line BLOCK
23 should line up with the keyword that started the construct. Beyond
24 that, he has other preferences that aren't so strong:
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26 • 4-column indent.
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28 • Opening curly on same line as keyword, if possible, otherwise line
29 up.
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31 • Space before the opening curly of a multi-line BLOCK.
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33 • One-line BLOCK may be put on one line, including curlies.
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35 • No space before the semicolon.
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37 • Semicolon omitted in "short" one-line BLOCK.
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39 • Space around most operators.
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41 • Space around a "complex" subscript (inside brackets).
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43 • Blank lines between chunks that do different things.
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45 • Uncuddled elses.
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47 • No space between function name and its opening parenthesis.
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49 • Space after each comma.
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51 • Long lines broken after an operator (except "and" and "or").
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53 • Space after last parenthesis matching on current line.
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55 • Line up corresponding items vertically.
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57 • Omit redundant punctuation as long as clarity doesn't suffer.
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59 Larry has his reasons for each of these things, but he doesn't claim
60 that everyone else's mind works the same as his does.
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62 Here are some other more substantive style issues to think about:
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64 • Just because you CAN do something a particular way doesn't mean
65 that you SHOULD do it that way. Perl is designed to give you
66 several ways to do anything, so consider picking the most readable
67 one. For instance
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69 open(my $fh, '<', $foo) || die "Can't open $foo: $!";
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71 is better than
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73 die "Can't open $foo: $!" unless open(my $fh, '<', $foo);
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75 because the second way hides the main point of the statement in a
76 modifier. On the other hand
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78 print "Starting analysis\n" if $verbose;
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80 is better than
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82 $verbose && print "Starting analysis\n";
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84 because the main point isn't whether the user typed -v or not.
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86 Similarly, just because an operator lets you assume default
87 arguments doesn't mean that you have to make use of the defaults.
88 The defaults are there for lazy systems programmers writing one-
89 shot programs. If you want your program to be readable, consider
90 supplying the argument.
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92 Along the same lines, just because you CAN omit parentheses in many
93 places doesn't mean that you ought to:
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95 return print reverse sort num values %array;
96 return print(reverse(sort num (values(%array))));
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98 When in doubt, parenthesize. At the very least it will let some
99 poor schmuck bounce on the % key in vi.
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101 Even if you aren't in doubt, consider the mental welfare of the
102 person who has to maintain the code after you, and who will
103 probably put parentheses in the wrong place.
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105 • Don't go through silly contortions to exit a loop at the top or the
106 bottom, when Perl provides the "last" operator so you can exit in
107 the middle. Just "outdent" it a little to make it more visible:
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109 LINE:
110 for (;;) {
111 statements;
112 last LINE if $foo;
113 next LINE if /^#/;
114 statements;
115 }
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117 • Don't be afraid to use loop labels--they're there to enhance
118 readability as well as to allow multilevel loop breaks. See the
119 previous example.
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121 • Avoid using "grep()" (or "map()") or `backticks` in a void context,
122 that is, when you just throw away their return values. Those
123 functions all have return values, so use them. Otherwise use a
124 "foreach()" loop or the "system()" function instead.
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126 • For portability, when using features that may not be implemented on
127 every machine, test the construct in an eval to see if it fails.
128 If you know what version or patchlevel a particular feature was
129 implemented, you can test $] ($PERL_VERSION in "English") to see if
130 it will be there. The "Config" module will also let you
131 interrogate values determined by the Configure program when Perl
132 was installed.
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134 • Choose mnemonic identifiers. If you can't remember what mnemonic
135 means, you've got a problem.
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137 • While short identifiers like $gotit are probably ok, use
138 underscores to separate words in longer identifiers. It is
139 generally easier to read $var_names_like_this than
140 $VarNamesLikeThis, especially for non-native speakers of English.
141 It's also a simple rule that works consistently with
142 "VAR_NAMES_LIKE_THIS".
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144 Package names are sometimes an exception to this rule. Perl
145 informally reserves lowercase module names for "pragma" modules
146 like "integer" and "strict". Other modules should begin with a
147 capital letter and use mixed case, but probably without underscores
148 due to limitations in primitive file systems' representations of
149 module names as files that must fit into a few sparse bytes.
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151 • You may find it helpful to use letter case to indicate the scope or
152 nature of a variable. For example:
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154 $ALL_CAPS_HERE constants only (beware clashes with perl vars!)
155 $Some_Caps_Here package-wide global/static
156 $no_caps_here function scope my() or local() variables
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158 Function and method names seem to work best as all lowercase.
159 E.g., "$obj->as_string()".
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161 You can use a leading underscore to indicate that a variable or
162 function should not be used outside the package that defined it.
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164 • If you have a really hairy regular expression, use the "/x" or
165 "/xx" modifiers and put in some whitespace to make it look a little
166 less like line noise. Don't use slash as a delimiter when your
167 regexp has slashes or backslashes.
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169 • Use the new "and" and "or" operators to avoid having to
170 parenthesize list operators so much, and to reduce the incidence of
171 punctuation operators like "&&" and "||". Call your subroutines as
172 if they were functions or list operators to avoid excessive
173 ampersands and parentheses.
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175 • Use here documents instead of repeated "print()" statements.
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177 • Line up corresponding things vertically, especially if it'd be too
178 long to fit on one line anyway.
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180 $IDX = $ST_MTIME;
181 $IDX = $ST_ATIME if $opt_u;
182 $IDX = $ST_CTIME if $opt_c;
183 $IDX = $ST_SIZE if $opt_s;
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185 mkdir $tmpdir, 0700 or die "can't mkdir $tmpdir: $!";
186 chdir($tmpdir) or die "can't chdir $tmpdir: $!";
187 mkdir 'tmp', 0777 or die "can't mkdir $tmpdir/tmp: $!";
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189 • Always check the return codes of system calls. Good error messages
190 should go to "STDERR", include which program caused the problem,
191 what the failed system call and arguments were, and (VERY
192 IMPORTANT) should contain the standard system error message for
193 what went wrong. Here's a simple but sufficient example:
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195 opendir(my $dh, $dir) or die "can't opendir $dir: $!";
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197 • Line up your transliterations when it makes sense:
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199 tr [abc]
200 [xyz];
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202 • Think about reusability. Why waste brainpower on a one-shot when
203 you might want to do something like it again? Consider
204 generalizing your code. Consider writing a module or object class.
205 Consider making your code run cleanly with "use strict" and "use
206 warnings" in effect. Consider giving away your code. Consider
207 changing your whole world view. Consider... oh, never mind.
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209 • Try to document your code and use Pod formatting in a consistent
210 way. Here are commonly expected conventions:
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212 • use "C<>" for function, variable and module names (and more
213 generally anything that can be considered part of code, like
214 filehandles or specific values). Note that function names are
215 considered more readable with parentheses after their name,
216 that is "function()".
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218 • use "B<>" for commands names like cat or grep.
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220 • use "F<>" or "C<>" for file names. "F<>" should be the only Pod
221 code for file names, but as most Pod formatters render it as
222 italic, Unix and Windows paths with their slashes and
223 backslashes may be less readable, and better rendered with
224 "C<>".
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226 • Be consistent.
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228 • Be nice.
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232perl v5.34.0 2021-10-18 PERLSTYLE(1)