1Perl::Critic::DEVELOPERU(s3e)r Contributed Perl DocumentaPteiroln::Critic::DEVELOPER(3)
2
6 Perl::Critic::DEVELOPER - How to make new Perl::Critic::Policy modules.
7
9 For developers who want to create custom coding standards, the
10 following tells how to create a Policy module for Perl::Critic.
11 Although the Perl::Critic distribution already includes a number of
12 Policies based on Damian Conway's book Perl Best Practices (which will
13 be referred to via "PBP" from here on), Perl::Critic is not limited to
14 his guidelines and can be used to enforce any practice, preference, or
15 style that you want to follow. You can even write Policies to enforce
16 contradictory guidelines. All you need to do is write a corresponding
17 Perl::Critic::Policy subclass, which may require as little as 10 lines
18 of code.
19
21 The heart of Perl::Critic is PPI, a parser and lexer for Perl. PPI
22 transforms Perl source code into a Document Object Model (DOM). Each
23 token in the document is represented by a PPI class, such as
24 PPI::Token::Operator or PPI::Token::Word, and then organized into
25 structure classes, like PPI::Statement::Expression and
26 PPI::Structure::Subroutine. The root node of the hierarchy is the
27 PPI::Document.
28 The Perl::Critic engine traverses each node in the PPI::Document tree
30 and invokes each of the Perl::Critic::Policy subclasses at the
31 appropriate node. The Policy can inspect the node, look at the
32 surrounding nodes, and do whatever else it wants. If the Policy
33 decides that a coding standard has been violated, it returns one or
34 more Perl::Critic::Violation objects. If there are no violations, then
35 the Policy returns nothing.
36 Policies are usually written based on existing policies, so let's look
38 at one to see how it works. The RequireBlockGrep.pm Policy is
39 relatively simple and demonstrates most of the important issues. The
40 goal of this Policy is to enforce that every call to "grep" uses a
41 block for the first argument and not an expression. The reasons for
42 this Policy are discussed in detail in PBP.
43
45 First, the Policy module needs to have a name. Perl::Critic uses
46 Module::Pluggable to automatically discover all modules in the
47 "Perl::Critic::Policy" namespace. Also, we've adopted the convention
48 of grouping Policies into directories according to the chapters of PBP.
49 Since the goal of this Policy is to enforce the use of block arguments
50 to "grep" and it comes from the "Builtin Functions" chapter of PBP, we
51 call it "Perl::Critic::Policy::BuiltinFunctions::RequireBlockGrep".
52 package Perl::Critic::Policy::BuiltinFunctions::RequireBlockGrep;
54 Next, we set some pragmas and load the modules that we'll need. All
56 Policy modules inherit from the Perl::Critic::Policy class, which
57 provides no-op implementations of the basic methods. Our job is to
58 override these methods to make them do something useful.
59 Technically, "use strict" and "use warnings" are optional, but we don't
61 want Perl::Critic to be a hypocrite, now do we?
62 use strict;
64 use warnings;
65 use Readonly;
67 use Perl::Critic::Utils qw{ :severities :classification :ppi };
69 use base 'Perl::Critic::Policy';
70 our $VERSION = '1.05';
72 Next, we'll declare a description and explanation for this Policy. The
74 description is always just a string that basically says "this is what's
75 wrong." The explanation can be either a string with further details,
76 or a reference to an array of integers that correspond to page numbers
77 in PBP. We make them read-only because they never change. (See
78 Perl::Critic::Policy::ValuesAndExpressions::ProhibitConstantPragma for
79 why we don't "use constant".)
80 Readonly::Scalar my $DESC => q{Expression form of "grep"};
82 Readonly::Scalar my $EXPL => [ 169 ];
83 Most policies don't need to override the "initialize_if_enabled()"
85 method provided by Perl::Critic::Policy. However, if your Policy is
86 configurable via .perlcriticrc, you should implement a
87 "supported_parameters()" method and need to implement
88 "initialize_if_enabled()" to examine the $config values. Since this
89 Policy isn't configurable, we'll declare that by providing an
90 implementation of "supported_parameters()" that returns an empty list.
91 sub supported_parameters { return () }
93 Next, we define the "default_severity()" method, which must return an
95 integer indicating the severity of violating this Policy. Severity
96 values range from 1 to 5, where 5 is the "most severe." In general,
97 level 5 is reserved for things that are frequently misused and/or cause
98 bugs. Level 1 is for things that are highly subjective or purely
99 cosmetic. The Perl::Critic::Utils package exports several severity
100 constants that you can use here via the ":severities" tag.
101 sub default_severity { return $SEVERITY_HIGH }
103 Likewise, the "default_themes()" method returns a list of theme names.
105 Themes are intended to be named groups of Policies. All Policies that
106 ship with Perl::Critic have a "core" theme. Since use of "grep"
107 without blocks often leads to bugs, we include a "bugs" theme. And
108 since this Policy comes directly from PBP, this Policy should be a
109 member of the "pbp" theme.
110 sub default_themes { return qw( core bugs pbp ) }
112 As a Policy author, you can assign any themes you want to the Policy.
114 If you're publishing a suite of custom Policies, we suggest that you
115 create a unique theme that covers all the Policies in the distribution.
116 That way, users can easily enable or disable all of your policies at
117 once. For example, Policies in the Perl::Critic::More distribution all
118 have a "more" theme.
119 Next, we indicate what elements of the code this Policy will analyze,
121 like statements or variables or conditionals or POD. These elements
122 are specified as PPI classes such as PPI::Statement,
123 PPI::Token::Symbol, PPI::Structure::Conditional or PPI::Token::Pod
124 respectively. The "applies_to()" method returns a list of PPI package
125 names. (You can get that list of available package names via "perldoc
126 PPI".) As Perl::Critic traverses the document, it will call the
127 "violates()" method from this module whenever it encounters one of the
128 PPI types that are given here. In this case, we just want to test
129 calls to "grep". Since the token "grep" is a PPI::Token::Word, we
130 return that package name from the "applies_to()" method.
131 sub applies_to { return 'PPI::Token::Word' }
133 If your Policy needs to analyze several different types of elements,
135 the "applies_to" method may return the name of several PPI packages.
136 If your Policy needs to examine the file as a whole, then the
137 "applies_to" method should return PPI::Document. Since there is only
138 one PPI::Document element, your Policy would only be invoked once per
139 file.
140 Now comes the interesting part. The "violates()" method does all the
142 work. It is always called with 2 arguments: a reference to the current
143 PPI element that Perl::Critic is traversing, and a reference to the
144 entire PPI document. [And since this is an object method, there will be
145 an additional argument that is a reference to this object ($self), but
146 you already knew that!] Since this Policy does not need access to the
147 document as a whole, we ignore the last parameter by assigning to
148 "undef".
149 sub violates {
151 my ( $self, $elem, undef ) = @_;
152 The "violates()" method then often performs some tests to make sure we
154 have the right "type" of element. In our example, we know that the
155 element will be a PPI::Token::Word because that's what we declared back
156 in the "applies_to()" method. However, we didn't specify exactly which
157 "word" we were looking for. Evaluating a PPI element in a string
158 context returns the literal form of the code. (You can also use the
159 "content()" method.) So we make sure that this "PPI::Token::Word" is,
160 in fact, "grep". If it's not, then we don't need to bother examining
161 it.
162 return if $elem ne 'grep';
164 The "PPI::Token::Word" class is also used for barewords and methods
166 called on object references. It is possible for someone to declare a
167 bareword hash key as "%hash = ( grep => 'foo')". We don't want to test
168 those types of elements because they don't represent function calls to
169 "grep". So we use one of handy utility functions from
170 Perl::Critic::Utils to make sure that this "grep" is actually in the
171 right context. (The "is_function_call()" subroutine is brought in via
172 the ":classification" tag.)
173 return if ! is_function_call($elem);
175 Now that we know this element is a call to the "grep" function, we can
177 look at the nearby elements to see what kind of arguments are being
178 passed to it. In the following paragraphs, we discuss how to do this
179 manually in order to explore PPI; after that, we'll show how this
180 Policy actually uses facilities provided by Perl::Critic::Utils to get
181 this done.
182 Every PPI element is linked to its siblings, parent, and children (if
184 it has any). Since those siblings could just be whitespace, we use the
185 "snext_sibling()" to get the next code-sibling (the "s" in
186 "snext_sibling" stands for "significant").
187 my $sib = $elem->snext_sibling() or return;
189 In Perl, the parenthesis around argument lists are usually optional,
191 and PPI packs the elements into a PPI::Structure::List object when
192 parentheses are used. So if the sibling is a "PPI::Structure::List",
193 we pull out the first (significant) child of that list. This child
194 will be the first argument to "grep". If parentheses were not used,
195 then the sibling itself is the first argument.
196 my $arg = $sib->isa('PPI::Structure::List') ? $sib->schild(0) : $sib;
198 In actuality, this sort of function argument lookup is common, so there
200 is a "first_arg" in Perl::Critic::Utils subroutine available via the
201 ":ppi" tag. So we use that instead.
202 my $arg = first_arg($elem);
204 Finally, we now have a reference to the first argument to "grep". If
206 that argument is a block (i.e. something in curly braces), then it will
207 be a PPI::Structure::Block, in which case our Policy is satisfied and
208 we just return nothing.
209 return if !$arg;
211 return if $arg->isa('PPI::Structure::Block');
212 But if it is not a PPI::Structure::Block, then we know that this call
214 to "grep" must be using the expression form, and that violates our
215 Policy. So we create and return a new Perl::Critic::Violation object
216 via the "violation" in Perl::Critic::Policy method, passing in the
217 description, explanation, and a reference to the PPI element that
218 caused the violation. And that's all there is to it!
219 return $self->violation( $DESC, $EXPL, $elem );
221 }
222 1;
224 One last thing -- people are going to need to understand what is wrong
226 with the code when your Policy finds a problem. It isn't reasonable to
227 include all the details in your violation description or explanation.
228 So please include a DESCRIPTION section in the POD for your Policy. It
229 should succinctly describe the behavior and motivation for your Policy
230 and include a few examples of both good and bad code. Here's an
231 example:
232 =pod
234 =head1 NAME
236 Perl::Critic::Policy::BuiltinFunctions::RequireBlockGrep
238 =head1 DESCRIPTION
241 The expression forms of C<grep> and C<map> are awkward and hard to read.
243 Use the block forms instead.
244 @matches = grep /pattern/, @list; #not ok
246 @matches = grep { /pattern/ } @list; #ok
247 @mapped = map transform($_), @list; #not ok
249 @mapped = map { transform($_) } @list; #ok
250 =cut
252 When your policy has a section like this, users can invoke perlcritic
254 with a "--verbose" parameter of 10 or 11 or with a "%d" escape to see
255 it along with the rest of the output for violations of your policy.
256
258 Perl::Critic takes care of gathering configuration information for your
259 Policy, from whatever source the user specifies. (See "CONFIGURATION"
260 in Perl::Critic for the details of how a user specifies the values
261 you're going to receive.) What your Policy ends up receiving for the
262 value of a parameter is a string with leading and trailing whitespace
263 removed. By default, you will need to handle conversion of that string
264 to a useful form yourself. However, if you provide some metadata about
265 your parameters, the parameter handling will be taken care of for you.
266 (Additionally, tools that deal with Policies themselves can use this
267 information to enhance their functionality. See the perlcritic
268 "--profile-proto" option for an example.)
269 You can look at
271 Perl::Critic::Policy::ControlStructures::ProhibitCascadingIfElse for a
272 simple example of a configurable Policy and
273 Perl::Critic::Policy::Documentation::RequirePodSections for a more
274 complex one.
275 Do It All Yourself
277 The "initialize_if_enabled()" method for a Policy receives one
278 argument: an instance of Perl::Critic::PolicyConfig. This method is
279 only called if the user's configuration has enabled the policy. It
280 returns a boolean stating whether the Policy should continue to be
281 enabled. Generally, the only reason to return $FALSE is when some
282 external requirement is missing. For example,
283 Perl::Critic::Policy::CodeLayout::RequireTidyCode used to disable
284 itself if Perl::Tidy was not installed (that is until we made it no
285 longer optional for the Perl-Critic distribution).
286 A basic, do-nothing implementation of "initialize_if_enabled()" would
288 be:
289 use Perl::Critic::Utils qw< :booleans >;
291 ...
293 sub initialize_if_enabled {
295 my ( $self, $config ) = @_;
296 return $TRUE;
298 }
299 As stated above, what you get in $config are trimmed strings. For
301 example, if the user's .perlcritic contains
302 [Your::Policy]
304 foo = bar baz
305 factor = 5.52
306 selections = 2 78 92
307 then $config will contain the equivalent of
309 my $config = {
311 foo => 'bar baz',
312 factor => '5.52',
313 selections => '2 78 92',
314 };
315 To make this available to the "violates()" method, the values are
317 usually put into $self under the name of the configuration item
318 prefixed with an underscore. E.g.
319 sub initialize_if_enabled {
321 my ( $self, $config ) = @_;
322 $self->{_foo} = $config->get{foo};
324 $self->{_factor} = $config->get{factor};
325 $self->{_selections} = $config->get{selections};
326 return $TRUE;
328 }
329 Often, you'll want to convert the configuration values into something
331 more useful. In this example, "selections" is supposed to be a list of
332 integers. Perl::Critic::Utils contains a number of functions that can
333 help you with this. Assuming that "violates()" wants to have
334 "selections" as an array, you'll want to have something like this:
335 use Perl::Critic::Utils qw{ :booleans :characters :data_conversion };
337 sub initialize_if_enabled {
339 my ( $self, $config ) = @_;
340 $self->{_foo} = $config->get{foo};
342 $self->{_factor} = $config->get{factor};
343 my $selections = $config->get{selections};
345 $selections = defined $selections ? $selections : $EMPTY_STRING;
346 $self->{_selections} = [ words_from_string($selections) ];
347 return $TRUE;
349 }
350 Since "selections" contains numbers, it may be desirable to change the
352 assignment to look like
353 $self->{_selections} = [ map { $_ + 0 } words_from_string($selections) ];
355 If "violates()" needs to quickly determine whether a particular value
357 is in "selections", you would want to use a hash instead of an array,
358 like this:
359 $self->{_selections} = { hashify( words_from_string($selections) ) };
361 For an example of a Policy that has some simple, but non-standard
363 configuration handling, see
364 Perl::Critic::Policy::CodeLayout::RequireTidyCode.
365 Note On Constructors
367 It used to be the case that Policies handled configuration by
368 implementing a constructor. However, there was no requirement to call
369 the base constructor; as long as the Policy ended up being a blessed
370 hash reference, everything was fine. Unfortunately, this meant that
371 Policies would be loaded and their prerequisites would be "use"d, even
372 if the Policy wasn't enabled, slowing things down. Also, this severely
373 restricted the core of Perl::Critic's ability to enhance things. Use
374 of constructors is deprecated and is incompatible with
375 "supported_parameters()" metadata below. Kindly use
376 "initialize_if_enabled()", instead, to do any sort of set up that you
377 need.
378 Providing Basic Configuration Information Via "supported_parameters()"
380 As minimum for a well behaved Policy, you should implement
381 "supported_parameters()" in order to tell the rest of "Perl::Critic"
382 what configuration values the Policy looks for, even if it is only to
383 say that the Policy is not configurable. In the simple form, this
384 function returns a list of the names of the parameters the Policy
385 supports. So, for an non-configurable Policy, as in the
386 "RequireBlockGrep" example above, this looked like
387 sub supported_parameters { return () }
389 For the example being used in the "initialize_if_enabled()" section
391 above, this would be
392 sub supported_parameters { return qw< foo factor selections >; }
394 Given this information, "Perl::Critic" can tell the user when they have
396 specified a parameter for a Policy which isn't valid, e.g. when they've
397 misspelled the name of the parameter, and can emit the parameter as
398 part of a .perlcriticrc prototype.
399 You can provide even more information about your Policy's configuration
401 by giving each parameter a description and a string representation of
402 the default value for the parameter. You do this by having the values
403 in the list returned by "supported_parameters()" be hash references
404 instead of strings, with keys of "name", "description", and
405 "default_string". For example,
406 sub supported_parameters {
408 return (
409 {
410 name => 'allowed_values',
411 description =>
412 'Individual and ranges of values to allow, and/or "all_integers".',
413 default_string => '0 1 2',
414 },
415 {
416 name => 'allowed_types',
417 description => 'Kind of literals to allow.',
418 default_string => 'Float',
419 },
420 );
421 }
422 Note that use of constructors is incompatible with specifying
424 parameters in this way.
425 Using "supported_parameters()" to Get It Done For You
427 The "supported_parameters()" discussion above showed how you could help
428 others with your Policy, but didn't do anything to make your life as a
429 Policy author easier; you still need to implement
430 "initialize_if_enabled()" to access any configuration that the user has
431 specified. To have the configuration automatically handled for you,
432 you need to declare how your parameters act by specifying a value for
433 their "behavior". For example, the following declares that a parameter
434 allows the user to choose from five specific values and that the user
435 can select any combination of them:
436 sub supported_parameters {
438 return (
439 {
440 name => 'allowed_types',
441 description => 'Kind of literals to allow.',
442 default_string => 'Float',
443 behavior => 'enumeration',
444 enumeration_values => [ qw{ Binary Exp Float Hex Octal } ],
445 enumeration_allow_multiple_values => 1,
446 },
447 );
448 }
449 When you specify a behavior, parsing and validation of the user-
451 specified and default values is done for you and your "violates()"
452 method can retrieve the value under the key of the parameter name
453 prefixed with an underscore, e.g., for the above declaration, the
454 parsed and validated value can be accessed via
455 "$self->{_allowed_types}".
456 The behaviors provide additional functionality to "Perl::Critic"; for
458 more on this, see Perl::Critic::PolicyParameter and
459 Perl::Critic::PolicyParameter::Behavior.
460 The following discusses each of the supported behaviors and the options
462 they support. For the full details of a behavior, see the
463 documentation for the implementing class.
464 "string"
466 Implemented in Perl::Critic::PolicyParameter::Behavior::String.
468 The most basic of behaviors, the value of the parameter will be stored
470 in the Policy as a string.
471 This behavior is not configurable.
473 "supported_parameters()" example
475 sub supported_parameters {
477 return (
478 {
479 name => 'a_string',
480 description => 'An example string.',
481 default_string => 'blah blah blah',
482 behavior => 'string',
483 },
484 );
485 }
486 Access example
488 sub violates {
490 my ($self, $element, $document) = @_;
491 ...
493 my $string = $self->{_a_string};
494 ...
495 }
496 "boolean"
498 Implemented in Perl::Critic::PolicyParameter::Behavior::Boolean.
500 The value of the parameter will be either $TRUE or $FALSE.
502 This behavior is not configurable.
504 "supported_parameters()" example
506 sub supported_parameters {
508 return (
509 {
510 name => 'a_boolean',
511 description => 'An example boolean.',
512 default_string => '1',
513 behavior => 'boolean',
514 },
515 );
516 }
517 Access example
519 sub violates {
521 my ($self, $element, $document) = @_;
522 ...
524 my $is_whatever = $self->{_a_boolean};
525 if ($is_whatever) {
526 ...
527 }
528 ...
529 }
530 "integer"
532 Implemented in Perl::Critic::PolicyParameter::Behavior::Integer.
534 The value is validated against "m/ \A [-+]? [1-9] [\d_]* \z /xms" (with
536 an special check for "0"). Notice that this means that underscores are
537 allowed in input values as with Perl numeric literals.
538 This takes two options, "integer_minimum" and "integer_maximum", which
540 specify endpoints of an inclusive range to restrict the value to.
541 Either, neither, or both may be specified.
542 "supported_parameters()" example
544 sub supported_parameters {
546 return (
547 {
548 name => 'an_integer',
549 description => 'An example integer.',
550 default_string => '5',
551 behavior => 'integer',
552 integer_minimum => 0,
553 integer_maximum => 10,
554 },
555 );
556 }
557 Access example
559 sub violates {
561 my ($self, $element, $document) = @_;
562 ...
564 my $integer = $self->{_an_integer};
565 if ($integer > $TURNING_POINT) {
566 ...
567 }
568 ...
569 }
570 "string list"
572 Implemented in Perl::Critic::PolicyParameter::Behavior::StringList.
574 The values will be derived by splitting the input string on blanks.
576 (See "words_from_string" in Perl::Critic::Utils.) The parameter will be
577 stored as a reference to a hash, with the values being the keys.
578 This takes one optional option, "list_always_present_values", of a
580 reference to an array of strings that will always be included in the
581 parameter value, e.g. if the value of this option is "[ qw{ a b c } ]"
582 and the user specifies a value of 'c d e', then the value of the
583 parameter will contain 'a', 'b', 'c', 'd', and 'e'.
584 "supported_parameters()" example
586 sub supported_parameters {
588 return (
589 {
590 name => 'a_string_list',
591 description => 'An example list.',
592 default_string => 'red pink blue',
593 behavior => 'string list',
594 list_always_present_values => [ qw{ green purple} ],
595 },
596 );
597 }
598 Access example
600 sub violates {
602 my ($self, $element, $document) = @_;
603 ...
605 my $list = $self->{_a_string_list};
606 my @list = keys %{$list};
607 ...
608 return if not $list->{ $element->content() };
609 ...
610 }
611 "enumeration"
613 Implemented in Perl::Critic::PolicyParameter::Behavior::Enumeration.
615 The values will be derived by splitting the input string on blanks.
617 (See "words_from_string" in Perl::Critic::Utils.) Depending upon the
618 value of the "enumeration_allow_multiple_values" option, the parameter
619 will be stored as a string or a reference to a hash, with the values
620 being the keys.
621 This behavior takes one required option and one optional one. A value
623 for "enumeration_values" of a reference to an array of valid strings is
624 required. A true value can be specified for
625 "enumeration_allow_multiple_values" to allow the user to pick more than
626 one value, but this defaults to false.
627 "supported_parameters()" example
629 use Perl::Critic::Utils qw{ :characters };
631 sub supported_parameters {
633 return (
634 {
635 name => 'a_single_valued_enumeration',
636 description =>
637 'An example enumeration that can only have a single value.',
638 default_string => $EMPTY,
639 behavior => 'enumeration',
640 enumeration_values => [ qw{ block statement pod operator } ],
641 enumeration_allow_multiple_values => 0,
642 },
643 {
644 name => 'a_multi_valued_enumeration',
645 description =>
646 'An example enumeration that can have multiple values.',
647 default_string => 'fe',
648 behavior => 'enumeration',
649 enumeration_values => [ qw{ fe fi fo fum } ],
650 enumeration_allow_multiple_values => 1,
651 },
652 );
653 }
654 Access example
656 sub violates {
658 my ($self, $element, $document) = @_;
659 ...
661 my $single_value = $self->{_a_single_valued_enumeration};
662 ...
663 my $multi_value = $self->{_a_multi_valued_enumeration};
664 if ( $multi_value->{fum} ) {
665 ...
666 }
667 ...
668 }
669 Using a Custom Parser
671 If none of the behaviors does exactly what you want it to, you can
672 provide your own parser for a parameter. The reason for doing this as
673 opposed to using an implementation of "initialize_if_enabled()" is that
674 it allows you to use a behavior to provide its extra functionality and
675 it provides a means for a "Perl::Critic" configuration program, e.g. an
676 IDE that integrates "Perl::Critic", to validate your parameter as the
677 user modifies its value.
678 The way you declare that you have a custom parser is to include a
680 reference to it in the parameter specification with the "parser" key.
681 For example:
682 sub supported_parameters {
684 return (
685 {
686 name => 'file_name',
687 description => 'A file for to read a list of values from.',
688 default_string => undef,
689 behavior => 'string',
690 parser => \&_parse_file_name,
691 },
692 );
693 }
694 A parser is a method on a subclass of Perl::Critic::Policy that takes
696 two parameters: the Perl::Critic::PolicyParameter that is being
697 specified and the value string provided by the user. The method is
698 responsible for dealing with any default value and for saving the
699 parsed value for later use by the "violates()" method.
700 An example parser (without enough error handling) for the above example
702 declaration:
703 use Path::Tiny;
705 use Perl::Critic::Exception::Configuration::Option::Policy::ParameterValue
707 qw{ throw_policy_value };
708 sub _parse_file_name {
710 my ($self, $parameter, $config_string) = @_;
711 my @thingies;
713 if ($config_string) {
715 if (not -r $config_string) {
716 throw_policy_value
717 policy => $self->get_short_name(),
718 option_name => $parameter->get_name(),
719 option_value => $config_string,
720 message_suffix => 'is not readable.';
721 }
722 @thingies = path($config_string)->slurp;
724 }
725 $self->{_thingies} = \@thingies;
727 return;
729 }
730 Note that, if the value for the parameter is not valid, an instance of
732 Perl::Critic::Exception::Configuration::Option::Policy::ParameterValue
733 is thrown. This allows "Perl::Critic" to include that problem along
734 with any other problems found with the user's configuration in a single
735 error message.
736 Using Both "supported_parameters()" and "initialize_if_enabled()"
738 There are cases where a Policy needs additional initialization beyond
739 configuration or where the way it acts depends upon the combination of
740 multiple parameters. In such situations, you will need to create an
741 implementation of "initialize_if_enabled()". If you want to take
742 advantage of the supplied parameter handling from within implementation
743 of "initialize_if_enabled()", note that the information from
744 "supported_parameters()" will already have been used, with user-
745 supplied parameter values validated and placed into the Policy by the
746 time "initialize_if_enabled()" has been called. It is likely that you
747 will not need to refer the contents of the $config parameter; just pull
748 the information you need out of $self. In fact, any value for the
749 parameter values will be gone.
750 Summary of permitted hash keys in "supported_parameters()".
752 All types
753 - "name" (mandatory)
755 - "description" (optional)
756 - "behavior" (optional)
757 Currently, one of:
758 "boolean"
760 "enumeration"
761 "integer"
762 "string"
763 "string list"
764 - "default_string" (optional)
765 A string representation of the default value of the parameter.
766 - "parser" (optional)
768 A code ref to a custom parser for the parameter.
769 Enumerations
771 - "enumeration_values" (mandatory)
773 A mandatory reference to an array of strings.
774 - "enumeration_allow_multiple_values" (optional)
776 Boolean indicating whether or not the user is restricted to a
777 single value.
778 Integers
780 - "integer_minimum" (optional)
782 Minimum allowed value, inclusive.
783 - "integer_maximum" (optional)
785 Maximum allowed value, inclusive.
786 String lists
788 - "list_always_present_values" (optional)
790 A reference to an array of values that should always be included in
791 the value of the parameter.
792
794 "default_maximum_violations_per_document()"
795 Certain problems that a Policy detects can be endemic to a particular
796 file; if there's one violation, there's likely to be many. A good
797 example of this is
798 Perl::Critic::Policy::TestingAndDebugging::RequireUseStrict; if there's
799 one line before "use strict", there's a good chance that the entire
800 file is missing "use strict". In such cases, it's not much help to the
801 user to report every single violation. If you've got such a policy,
802 you should override default_maximum_violations_per_document() method to
803 provide a limit. The user can override this value with a value for
804 "maximum_violations_per_document" in their .perlcriticrc.
805 See the source code for
807 Perl::Critic::Policy::ValuesAndExpressions::ProhibitMagicNumbers and
808 Perl::Critic::Policy::TestingAndDebugging::RequireUseWarnings for
809 examples.
810 "is_safe()"
812 Most Perl::Critic Policies are purely static. In other words, they
813 never compile or execute any of the source code that they analyze.
814 However it is possible to write dynamic Policies that do compile or
815 execute code, which may result in unsafe operations (see
816 Perl::Critic::Dynamic for an example). So the "is_safe()" method is
817 used to indicate whether a Policy can be trusted to not cause mischief.
818 By default, "is_safe()" returns true. But if you are writing a Policy
819 that will compile or execute any of the source code that it analyzes,
820 then you should override the "is_safe()" method to return false.
821
823 Create a Distribution
824 You need to come up with a name for your set of policies. Sets of add-
825 on policies are generally named "Perl::Critic::something", e.g.
826 Perl::Critic::More.
827 The module representing the distribution will not actually have any
829 functionality; it's just documentation and a name for users to use when
830 installing via CPAN/CPANPLUS. The important part is that this will
831 include a list of the included policies, with descriptions of each.
832 A typical implementation will look like:
834 package Perl::Critic::Example;
836 use strict;
838 use warnings;
839 our $VERSION = '1.000000';
841 1; # Magic true value required at end of module
843 __END__
845 =head1 NAME
847 Perl::Critic::Example - Policies for Perl::Critic that act as an example.
849 =head1 AFFILIATION
851 This module has no functionality, but instead contains documentation
853 for this distribution and acts as a means of pulling other modules
854 into a bundle. All of the Policy modules contained herein will have
855 an "AFFILIATION" section announcing their participation in this
856 grouping.
857 =head1 SYNOPSIS
860 Some L<Perl::Critic|Perl::Critic> policies that will help you keep your
862 code nice and compliant.
863 =head1 DESCRIPTION
866 The included policies are:
868 =over
870 =item L<Perl::Critic::Policy::Documentation::Example|Perl::Critic::Policy::Documentation::Example>
872 Complains about some example documentation issues. [Default severity: 3]
874 =item L<Perl::Critic::Policy::Variables::Example|Perl::Critic::Policy::Variables::Example>
877 All modules must have at least one variable. [Default severity: 3]
879 =back
882 =head1 CONFIGURATION AND ENVIRONMENT
885 All policies included are in the "example" theme. See the
887 L<Perl::Critic|Perl::Critic> documentation for how to make use of this.
888 Themes
890 Users can choose which policies to enable using themes. You should
891 implement "default_themes()" so that users can take advantage of this.
892 In particular, you should use a theme named after your distribution in
893 all your policies; this should match the value listed in the
894 "CONFIGURATION AND ENVIRONMENT" POD section as shown above.
895 default_themes { return qw< example math > }
897 If you're looking for ideas of what themes to use, have a look at the
899 output of "perlcritic --list-themes".
900 Documentation
902 AFFILIATION
903 Since all policies have to go somewhere under the
905 "Perl::Critic::Policy::" namespace, it isn't always clear what
906 distribution a policy came from when browsing through their
907 documentation. For this reason, you should include an "AFFILIATION"
908 section in the POD for all of your policies that state where the policy
909 comes from. For example:
910 =head1 AFFILIATION
912 This policy is part of L<Perl::Critic::Example|Perl::Critic::Example>.
914 CONFIGURATION
916 In order to make it clear what can be done with a policy, you should
918 always include a "CONFIGURATION" section in your POD, even if it's only
919 to say:
920 =head1 CONFIGURATION
922 This Policy is not configurable except for the standard options.
924
926 The Perl::Critic distribution also contains a framework for testing
927 your Policy. See Perl::Critic::TestUtils for the details.
928
930 When you're trying to figure out what PPI is going to hand you for a
931 chunk of code, there is a tools/ppidump program in the Perl::Critic
932 distribution that will help you. For example, when developing the
933 above RequireBlockGrep example, you might want to try
934 tools/ppidump '@matches = grep /pattern/, @list;'
936 and
938 tools/ppidump '@matches = grep { /pattern/ } @list;'
940 to see the differences between the two cases.
942 Alternatively, see the "ppi_dumper" documentation at
944 <http://search.cpan.org/dist/App-PPI-Dumper/script/ppi_dumper> and the
945 "PPI::Tester" documentation at
946 <http://search.cpan.org/dist/PPI-Tester/lib/PPI/Tester.pm>.
947
949 This is part of Perl::Critic version 1.116.
950
952 Chas. Owens has a blog post about developing in-house policies at
953 <http://svok.blogspot.com/2009/09/adding-house-policies-to-perlcritic.html>.
954
956 Jeffrey Ryan Thalhammer <jeff@imaginative-software.com>
957
959 Copyright (c) 2005-2011 Imaginative Software Systems. All rights
960 reserved.
961 This program is free software; you can redistribute it and/or modify it
963 under the same terms as Perl itself. The full text of this license can
964 be found in the LICENSE file included with this module.
965perl v5.28.1 2019-02-02 Perl::Critic::DEVELOPER(3)