=pod

=for stopwords lookup RequireBlockGrep

=head1 NAME

Perl::Critic::DEVELOPER - How to make new Perl::Critic::Policy modules.


=head1 DESCRIPTION

For developers who want to create custom coding standards, the
following tells how to create a Policy module for
L<Perl::Critic|Perl::Critic>.  Although the Perl::Critic distribution
already includes a number of Policies based on Damian Conway's book
I<Perl Best Practices> (which will be referred to via "I<PBP>" from
here on), Perl::Critic is not limited to his guidelines and can be
used to enforce any practice, preference, or style that you want to
follow.  You can even write Policies to enforce contradictory
guidelines.  All you need to do is write a corresponding
L<Perl::Critic::Policy|Perl::Critic::Policy> subclass, which may
require as little as 10 lines of code.


=head1 BACKGROUND

The heart of Perl::Critic is L<PPI|PPI>, a parser and lexer for Perl.
PPI transforms Perl source code into a Document Object Model (DOM).
Each token in the document is represented by a PPI class, such as
L<PPI::Token::Operator|PPI::Token::Operator> or
L<PPI::Token::Word|PPI::Token::Word>, and then organized into
structure classes, like
L<PPI::Statement::Expression|PPI::Statement::Expression> and
L<PPI::Structure::Subroutine|PPI::Structure::Subroutine>. The root
node of the hierarchy is the L<PPI::Document|PPI::Document>.

The L<Perl::Critic|Perl::Critic> engine traverses each node in the
L<PPI::Document|PPI::Document> tree and invokes each of the
L<Perl::Critic::Policy|Perl::Critic::Policy> subclasses at the
appropriate node.  The Policy can inspect the node, look at the
surrounding nodes, and do whatever else it wants.  If the Policy
decides that a coding standard has been violated, it returns one
or more L<Perl::Critic::Violation|Perl::Critic::Violation> objects.
If there are no violations, then the Policy returns nothing.

Policies are usually written based on existing policies, so let's look
at one to see how it works.  The F<RequireBlockGrep.pm> Policy is
relatively simple and demonstrates most of the important issues.  The
goal of this Policy is to enforce that every call to C<grep> uses a
block for the first argument and not an expression.  The reasons for
this Policy are discussed in detail in I<PBP>.


=head1 EXAMPLE POLICY

First, the Policy module needs to have a name.  Perl::Critic uses
L<Module::Pluggable|Module::Pluggable> to automatically discover all
modules in the C<Perl::Critic::Policy> namespace.  Also, we've adopted
the convention of grouping Policies into directories according to the
chapters of I<PBP>.  Since the goal of this Policy is to enforce the
use of block arguments to C<grep> and it comes from the "Builtin
Functions" chapter of I<PBP>, we call it
C<"Perl::Critic::Policy::BuiltinFunctions::RequireBlockGrep">.

    package Perl::Critic::Policy::BuiltinFunctions::RequireBlockGrep;

Next, we set some pragmas and load the modules that we'll need.  All
Policy modules inherit from the
L<Perl::Critic::Policy|Perl::Critic::Policy> class, which provides
no-op implementations of the basic methods.  Our job is to override
these methods to make them do something useful.

Technically, C<use strict> and C<use warnings> are optional, but we
don't want Perl::Critic to be a hypocrite, now do we?

    use strict;
    use warnings;

    use Readonly;

    use Perl::Critic::Utils qw{ :severities :classification :ppi };
    use base 'Perl::Critic::Policy';

    our $VERSION = '1.05';

Next, we'll declare a description and explanation for this Policy.
The description is always just a string that basically says "this is
what's wrong."  The explanation can be either a string with further
details, or a reference to an array of integers that correspond to
page numbers in I<PBP>.  We make them read-only because they never
change.  (See
L<Perl::Critic::Policy::ValuesAndExpressions::ProhibitConstantPragma|Perl::Critic::Policy::ValuesAndExpressions::ProhibitConstantPragma>
for why we don't C<use constant>.)

    Readonly::Scalar my $DESC => q{Expression form of "grep"};
    Readonly::Scalar my $EXPL => [ 169 ];

Most policies don't need to override the C<initialize_if_enabled()>
method provided by L<Perl::Critic::Policy|Perl::Critic::Policy>.
However, if your Policy is configurable via F<.perlcriticrc>, you
should implement a C<supported_parameters()> method and need to
implement C<initialize_if_enabled()> to examine the C<$config> values.
Since this Policy isn't configurable, we'll declare that by providing
an implementation of C<supported_parameters()> that returns an empty
list.

    sub supported_parameters { return ()                  }

Next, we define the C<default_severity()> method, which must return an
integer indicating the severity of violating this Policy.  Severity
values range from 1 to 5, where 5 is the "most severe."  In general,
level 5 is reserved for things that are frequently misused and/or
cause bugs.  Level 1 is for things that are highly subjective or
purely cosmetic.  The L<Perl::Critic::Utils|Perl::Critic::Utils>
package exports several severity constants that you can use here via
the C<:severities> tag.

    sub default_severity     { return $SEVERITY_HIGH      }

Likewise, the C<default_themes()> method returns a list of theme
names.  Themes are intended to be named groups of Policies.  All
Policies that ship with Perl::Critic have a C<"core"> theme.  Since
use of C<grep> without blocks often leads to bugs, we include a
C<"bugs"> theme.  And since this Policy comes directly from I<PBP>,
this Policy should be a member of the C<"pbp"> theme.

    sub default_themes       { return qw( core bugs pbp ) }

As a Policy author, you can assign any themes you want to the Policy.
If you're publishing a suite of custom Policies, we suggest that you
create a unique theme that covers all the Policies in the
distribution.  That way, users can easily enable or disable all of
your policies at once.  For example, Policies in the
L<Perl::Critic::More|Perl::Critic::More> distribution all have a
C<"more"> theme.

Next, we indicate what elements of the code this Policy will analyze,
like statements or variables or conditionals or POD.  These elements
are specified as PPI classes such as L<PPI::Statement|PPI::Statement>,
L<PPI::Token::Symbol|PPI::Token::Symbol>,
L<PPI::Structure::Conditional|PPI::Structure::Conditional> or
L<PPI::Token::Pod|PPI::Token::Pod> respectively.  The C<applies_to()>
method returns a list of PPI package names.  (You can get that list of
available package names via C<perldoc PPI>.)  As Perl::Critic
traverses the document, it will call the C<violates()> method from
this module whenever it encounters one of the PPI types that are given
here.  In this case, we just want to test calls to C<grep>.  Since the
token "grep" is a L<PPI::Token::Word|PPI::Token::Word>, we return that
package name from the C<applies_to()> method.

    sub applies_to           { return 'PPI::Token::Word'  }

If your Policy needs to analyze several different types of elements,
the C<applies_to> method may return the name of several PPI packages.
If your Policy needs to examine the file as a whole, then the
C<applies_to> method should return L<PPI::Document|PPI::Document>.
Since there is only one PPI::Document element, your Policy would only
be invoked once per file.

Now comes the interesting part.  The C<violates()> method does all the
work.  It is always called with 2 arguments: a reference to the
current PPI element that Perl::Critic is traversing, and a reference
to the entire PPI document. [And since this is an object method, there
will be an additional argument that is a reference to this object
(C<$self>), but you already knew that!]  Since this Policy does not
need access to the document as a whole, we ignore the last parameter
by assigning to C<undef>.

    sub violates {
        my ( $self, $elem, undef ) = @_;

The C<violates()> method then often performs some tests to make sure
we have the right "type" of element.  In our example, we know that the
element will be a L<PPI::Token::Word|PPI::Token::Word> because that's
what we declared back in the C<applies_to()> method.  However, we
didn't specify exactly which "word" we were looking for.  Evaluating a
PPI element in a string context returns the literal form of the code.
(You can also use the C<content()> method.)  So we make sure that this
C<PPI::Token::Word> is, in fact, "grep".  If it's not, then we don't
need to bother examining it.

        return if $elem ne 'grep';

The C<PPI::Token::Word> class is also used for barewords and methods
called on object references.  It is possible for someone to declare a
bareword hash key as C<< %hash = ( grep => 'foo') >>.  We don't want
to test those types of elements because they don't represent function
calls to C<grep>.  So we use one of handy utility functions from
L<Perl::Critic::Utils|Perl::Critic::Utils> to make sure that this
"grep" is actually in the right context.  (The C<is_function_call()>
subroutine is brought in via the C<:classification> tag.)

        return if ! is_function_call($elem);

Now that we know this element is a call to the C<grep> function, we
can look at the nearby elements to see what kind of arguments are
being passed to it.  In the following paragraphs, we discuss how to do
this manually in order to explore L<PPI|PPI>; after that, we'll show
how this Policy actually uses facilities provided by
L<Perl::Critic::Utils|Perl::Critic::Utils> to get this done.

Every PPI element is linked to its siblings, parent, and children (if
it has any).  Since those siblings could just be whitespace, we use
the C<snext_sibling()> to get the next code-sibling (the "s" in
C<snext_sibling> stands for "significant").

        my $sib = $elem->snext_sibling() or return;

In Perl, the parenthesis around argument lists are usually optional,
and PPI packs the elements into a
L<PPI::Structure::List|PPI::Structure::List> object when parentheses
are used.  So if the sibling is a C<PPI::Structure::List>, we pull out
the first (significant) child of that list.  This child will be the
first argument to C<grep>.  If parentheses were not used, then the
sibling itself is the first argument.

        my $arg = $sib->isa('PPI::Structure::List') ? $sib->schild(0) : $sib;

In actuality, this sort of function argument lookup is common, so
there is a L<Perl::Critic::Utils/"first_arg"> subroutine available via
the C<:ppi> tag.  So we use that instead.

        my $arg = first_arg($elem);

Finally, we now have a reference to the first argument to C<grep>.  If
that argument is a block (i.e. something in curly braces), then it
will be a L<PPI::Structure::Block|PPI::Structure::Block>, in which
case our Policy is satisfied and we just return nothing.

        return if !$arg;
        return if $arg->isa('PPI::Structure::Block');

But if it is not a L<PPI::Structure::Block|PPI::Structure::Block>,
then we know that this call to C<grep> must be using the expression
form, and that violates our Policy.  So we create and return a new
L<Perl::Critic::Violation|Perl::Critic::Violation> object via the
L<Perl::Critic::Policy/"violation"> method, passing in the
description, explanation, and a reference to the PPI element that
caused the violation.  And that's all there is to it!

        return $self->violation( $DESC, $EXPL, $elem );
    }

    1;

One last thing -- people are going to need to understand what is wrong
with the code when your Policy finds a problem.  It isn't reasonable
to include all the details in your violation description or
explanation.  So please include a DESCRIPTION section in the POD for
your Policy.  It should succinctly describe the behavior and
motivation for your Policy and include a few examples of both good and
bad code.  Here's an example:

    =pod

    =head1 NAME

    Perl::Critic::Policy::BuiltinFunctions::RequireBlockGrep


    =head1 DESCRIPTION

    The expression forms of C<grep> and C<map> are awkward and hard to read.
    Use the block forms instead.

        @matches = grep  /pattern/,    @list;        #not ok
        @matches = grep { /pattern/ }  @list;        #ok

        @mapped = map  transform($_),    @list;      #not ok
        @mapped = map { transform($_) }  @list;      #ok

    =cut

When your policy has a section like this, users can invoke
L<perlcritic|perlcritic> with a C<--verbose> parameter of C<10> or
C<11> or with a "%d" escape to see it along with the rest of the
output for violations of your policy.


=head1 MAKING YOUR POLICY CONFIGURABLE

L<Perl::Critic|Perl::Critic> takes care of gathering configuration
information for your Policy, from whatever source the user specifies.
(See L<Perl::Critic/"CONFIGURATION"> for the details of how a user
specifies the values you're going to receive.)  What your Policy ends
up receiving for the value of a parameter is a string with leading and
trailing whitespace removed.  By default, you will need to handle
conversion of that string to a useful form yourself.  However, if you
provide some metadata about your parameters, the parameter handling
will be taken care of for you.  (Additionally, tools that deal with
Policies themselves can use this information to enhance their
functionality.  See the L<perlcritic|perlcritic> C<--profile-proto>
option for an example.)

You can look at
L<Perl::Critic::Policy::ControlStructures::ProhibitCascadingIfElse|Perl::Critic::Policy::ControlStructures::ProhibitCascadingIfElse>
for a simple example of a configurable Policy and
L<Perl::Critic::Policy::Documentation::RequirePodSections|Perl::Critic::Policy::Documentation::RequirePodSections>
for a more complex one.

=head2 Do It All Yourself

The C<initialize_if_enabled()> method for a Policy receives one
argument: an instance of
L<Perl::Critic::PolicyConfig|Perl::Critic::PolicyConfig>.  This method
is only called if the user's configuration has enabled the policy.  It
returns a boolean stating whether the Policy should continue to be
enabled.  Generally, the only reason to return C<$FALSE> is when some
external requirement is missing.  For example,
L<Perl::Critic::Policy::CodeLayout::RequireTidyCode|Perl::Critic::Policy::CodeLayout::RequireTidyCode>
used to disable itself if L<Perl::Tidy|Perl::Tidy> was not installed
(that is until we made it no longer optional for the Perl-Critic
distribution).

A basic, do-nothing implementation of C<initialize_if_enabled()> would
be:

    use Perl::Critic::Utils qw< :booleans >;

    ...

    sub initialize_if_enabled {
        my ( $self, $config ) = @_;

        return $TRUE;
    }

As stated above, what you get in C<$config> are trimmed strings.  For
example, if the user's F<.perlcritic> contains

    [Your::Policy]
    foo          = bar baz
    factor   =     5.52
    selections =   2 78 92

then C<$config> will contain the equivalent of

    my $config = {
        foo        => 'bar baz',
        factor     => '5.52',
        selections => '2 78 92',
    };

To make this available to the C<violates()> method, the values are
usually put into C<$self> under the name of the configuration item
prefixed with an underscore.  E.g.

    sub initialize_if_enabled {
        my ( $self, $config ) = @_;

        $self->{_foo} = $config->get{foo};
        $self->{_factor} = $config->get{factor};
        $self->{_selections} = $config->get{selections};

        return $TRUE;
    }

Often, you'll want to convert the configuration values into something
more useful.  In this example, C<selections> is supposed to be a list
of integers.  L<Perl::Critic::Utils|Perl::Critic::Utils> contains a
number of functions that can help you with this.  Assuming that
C<violates()> wants to have C<selections> as an array, you'll want to
have something like this:

    use Perl::Critic::Utils qw{ :booleans :characters :data_conversion };

    sub initialize_if_enabled {
        my ( $self, $config ) = @_;

        $self->{_foo} = $config->get{foo};
        $self->{_factor} = $config->get{factor};

        my $selections = $config->get{selections};
        $selections = defined $selections ? $selections : $EMPTY_STRING;
        $self->{_selections} = [ words_from_string($selections) ];

        return $TRUE;
    }

Since C<selections> contains numbers, it may be desirable to change
the assignment to look like

    $self->{_selections} = [ map { $_ + 0 } words_from_string($selections) ];

If C<violates()> needs to quickly determine whether a particular value
is in C<selections>, you would want to use a hash instead of an array,
like this:

    $self->{_selections} = { hashify( words_from_string($selections) ) };

For an example of a Policy that has some simple, but non-standard
configuration handling, see
L<Perl::Critic::Policy::CodeLayout::RequireTidyCode|Perl::Critic::Policy::CodeLayout::RequireTidyCode>.


=head2 Note On Constructors

It used to be the case that Policies handled configuration by
implementing a constructor.  However, there was no requirement to call
the base constructor; as long as the Policy ended up being a blessed
hash reference, everything was fine.  Unfortunately, this meant that
Policies would be loaded and their prerequisites would be C<use>d,
even if the Policy wasn't enabled, slowing things down.  Also, this
severely restricted the core of L<Perl::Critic|Perl::Critic>'s ability
to enhance things.  Use of constructors is deprecated and is
incompatible with C<supported_parameters()> metadata below.  Kindly
use C<initialize_if_enabled()>, instead, to do any sort of set up that
you need.



=head2 Providing Basic Configuration Information Via C<supported_parameters()>

As minimum for a well behaved Policy, you should implement
C<supported_parameters()> in order to tell the rest of C<Perl::Critic>
what configuration values the Policy looks for, even if it is only to
say that the Policy is not configurable.  In the simple form, this
function returns a list of the names of the parameters the Policy
supports.  So, for an non-configurable Policy, as in the
C<RequireBlockGrep> example above, this looked like

    sub supported_parameters { return ()                  }

For the example being used in the C<initialize_if_enabled()> section
above, this would be

    sub supported_parameters { return qw< foo factor selections >; }

Given this information, C<Perl::Critic> can tell the user when they
have specified a parameter for a Policy which isn't valid, e.g. when
they've misspelled the name of the parameter, and can emit the
parameter as part of a F<.perlcriticrc> prototype.

You can provide even more information about your Policy's
configuration by giving each parameter a description and a string
representation of the default value for the parameter.  You do this by
having the values in the list returned by C<supported_parameters()> be
hash references instead of strings, with keys of C<name>,
C<description>, and C<default_string>.  For example,

    sub supported_parameters {
        return (
            {
                name           => 'allowed_values',
                description    =>
                    'Individual and ranges of values to allow, and/or "all_integers".',
                default_string => '0 1 2',
            },
            {
                name           => 'allowed_types',
                description    => 'Kind of literals to allow.',
                default_string => 'Float',
            },
        );
    }

Note that use of constructors is
L<incompatible|/"Note On Constructors"> with specifying parameters in
this way.


=head2 Using C<supported_parameters()> to Get It Done For You

The C<supported_parameters()> discussion above showed how you could
help others with your Policy, but didn't do anything to make your life
as a Policy author easier; you still need to implement
C<initialize_if_enabled()> to access any configuration that the user
has specified.  To have the configuration automatically handled for
you, you need to declare how your parameters act by specifying a value
for their C<behavior>.  For example, the following declares that a
parameter allows the user to choose from five specific values and that
the user can select any combination of them:

    sub supported_parameters {
        return (
            {
                name               => 'allowed_types',
                description        => 'Kind of literals to allow.',
                default_string     => 'Float',
                behavior           => 'enumeration',
                enumeration_values => [ qw{ Binary Exp Float Hex Octal } ],
                enumeration_allow_multiple_values => 1,
            },
        );
    }

When you specify a behavior, parsing and validation of the
user-specified and default values is done for you and your
C<violates()> method can retrieve the value under the key of the
parameter name prefixed with an underscore, e.g., for the above
declaration, the parsed and validated value can be accessed via
C<< $self->{_allowed_types} >>.

The behaviors provide additional functionality to C<Perl::Critic>; for
more on this, see
L<Perl::Critic::PolicyParameter|Perl::Critic::PolicyParameter> and
L<Perl::Critic::PolicyParameter::Behavior|Perl::Critic::PolicyParameter::Behavior>.

The following discusses each of the supported behaviors and the
options they support.  For the full details of a behavior, see the
documentation for the implementing class.


=head3 "string"

Implemented in
L<Perl::Critic::PolicyParameter::Behavior::String|Perl::Critic::PolicyParameter::Behavior::String>.

The most basic of behaviors, the value of the parameter will be stored
in the Policy as a string.

This behavior is not configurable.

=head4 C<supported_parameters()> example

    sub supported_parameters {
        return (
            {
                name           => 'a_string',
                description    => 'An example string.',
                default_string => 'blah blah blah',
                behavior       => 'string',
            },
        );
    }


=head4 Access example

    sub violates {
        my ($self, $element, $document) = @_;

        ...
        my $string = $self->{_a_string};
        ...
    }


=head3 "boolean"

Implemented in
L<Perl::Critic::PolicyParameter::Behavior::Boolean|Perl::Critic::PolicyParameter::Behavior::Boolean>.

The value of the parameter will be either
L<$TRUE|Perl::Critic::Utils/$TRUE> or
L<$FALSE|Perl::Critic::Utils/$FALSE>.

This behavior is not configurable.

=head4 C<supported_parameters()> example

    sub supported_parameters {
        return (
            {
                name           => 'a_boolean',
                description    => 'An example boolean.',
                default_string => '1',
                behavior       => 'boolean',
            },
        );
    }


=head4 Access example

    sub violates {
        my ($self, $element, $document) = @_;

        ...
        my $is_whatever = $self->{_a_boolean};
        if ($is_whatever) {
            ...
        }
        ...
    }


=head3 "integer"

Implemented in
L<Perl::Critic::PolicyParameter::Behavior::Integer|Perl::Critic::PolicyParameter::Behavior::Integer>.

The value is validated against C<m/ \A [-+]? [1-9] [\d_]* \z /xms>
(with an special check for "0").  Notice that this means that
underscores are allowed in input values as with Perl numeric literals.

This takes two options, C<integer_minimum> and C<integer_maximum>,
which specify endpoints of an inclusive range to restrict the value
to.  Either, neither, or both may be specified.

=head4 C<supported_parameters()> example

    sub supported_parameters {
        return (
            {
                name            => 'an_integer',
                description     => 'An example integer.',
                default_string  => '5',
                behavior        => 'integer',
                integer_minimum => 0,
                integer_maximum => 10,
            },
        );
    }


=head4 Access example

    sub violates {
        my ($self, $element, $document) = @_;

        ...
        my $integer = $self->{_an_integer};
        if ($integer > $TURNING_POINT) {
            ...
        }
        ...
    }


=head3 "string list"

Implemented in
L<Perl::Critic::PolicyParameter::Behavior::StringList|Perl::Critic::PolicyParameter::Behavior::StringList>.

The values will be derived by splitting the input string on blanks.
(See L<Perl::Critic::Utils/"words_from_string">.) The parameter will
be stored as a reference to a hash, with the values being the keys.

This takes one optional option, C<list_always_present_values>, of a
reference to an array of strings that will always be included in the
parameter value, e.g. if the value of this option is C<[ qw{ a b c }
]> and the user specifies a value of C<'c d e'>, then the value of the
parameter will contain C<'a'>, C<'b'>, C<'c'>, C<'d'>, and C<'e'>.

=head4 C<supported_parameters()> example

    sub supported_parameters {
        return (
            {
                name                  => 'a_string_list',
                description           => 'An example list.',
                default_string        => 'red pink blue',
                behavior              => 'string list',
                list_always_present_values => [ qw{ green purple} ],
            },
        );
    }


=head4 Access example

    sub violates {
        my ($self, $element, $document) = @_;

        ...
        my $list = $self->{_a_string_list};
        my @list = keys %{$list};
        ...
        return if not $list->{ $element->content() };
        ...
    }


=head3 "enumeration"

Implemented in
L<Perl::Critic::PolicyParameter::Behavior::Enumeration|Perl::Critic::PolicyParameter::Behavior::Enumeration>.

The values will be derived by splitting the input string on blanks.
(See L<Perl::Critic::Utils/"words_from_string">.)  Depending upon the
value of the C<enumeration_allow_multiple_values> option, the
parameter will be stored as a string or a reference to a hash, with
the values being the keys.

This behavior takes one required option and one optional one.  A value
for C<enumeration_values> of a reference to an array of valid strings
is required.  A true value can be specified for
C<enumeration_allow_multiple_values> to allow the user to pick more
than one value, but this defaults to false.

=head4 C<supported_parameters()> example

    use Perl::Critic::Utils qw{ :characters };

    sub supported_parameters {
        return (
            {
                name               => 'a_single_valued_enumeration',
                description        =>
                    'An example enumeration that can only have a single value.',
                default_string     => $EMPTY,
                behavior           => 'enumeration',
                enumeration_values => [ qw{ block statement pod operator } ],
                enumeration_allow_multiple_values => 0,
            },
            {
                name               => 'a_multi_valued_enumeration',
                description        =>
                    'An example enumeration that can have multiple values.',
                default_string     => 'fe',
                behavior           => 'enumeration',
                enumeration_values => [ qw{ fe fi fo fum } ],
                enumeration_allow_multiple_values => 1,
            },
        );
    }


=head4 Access example

    sub violates {
        my ($self, $element, $document) = @_;

        ...
        my $single_value = $self->{_a_single_valued_enumeration};
        ...
        my $multi_value = $self->{_a_multi_valued_enumeration};
        if ( $multi_value->{fum} ) {
            ...
        }
        ...
    }


=head2 Using a Custom Parser

If none of the behaviors does exactly what you want it to, you can
provide your own parser for a parameter.  The reason for doing this as
opposed to using an implementation of C<initialize_if_enabled()> is
that it allows you to use a behavior to provide its extra
functionality and it provides a means for a C<Perl::Critic>
configuration program, e.g. an IDE that integrates C<Perl::Critic>, to
validate your parameter as the user modifies its value.

The way you declare that you have a custom parser is to include a
reference to it in the parameter specification with the C<parser> key.
For example:

    sub supported_parameters {
        return (
            {
                name           => 'file_name',
                description    => 'A file for to read a list of values from.',
                default_string => undef,
                behavior       => 'string',
                parser         => \&_parse_file_name,
            },
        );
    }

A parser is a method on a subclass of
L<Perl::Critic::Policy|Perl::Critic::Policy> that takes two
parameters: the
L<Perl::Critic::PolicyParameter|Perl::Critic::PolicyParameter> that is
being specified and the value string provided by the user.  The method
is responsible for dealing with any default value and for saving the
parsed value for later use by the C<violates()> method.

An example parser (without enough error handling) for the above
example declaration:

    use Path::Tiny;

    use Perl::Critic::Exception::Configuration::Option::Policy::ParameterValue
        qw{ throw_policy_value };

    sub _parse_file_name {
        my ($self, $parameter, $config_string) = @_;

        my @thingies;

        if ($config_string) {
            if (not -r $config_string) {
                throw_policy_value
                    policy         => $self->get_short_name(),
                    option_name    => $parameter->get_name(),
                    option_value   => $config_string,
                    message_suffix => 'is not readable.';
            }

            @thingies = path($config_string)->slurp;
        }

        $self->{_thingies} = \@thingies;

        return;
    }

Note that, if the value for the parameter is not valid, an instance of
L<Perl::Critic::Exception::Configuration::Option::Policy::ParameterValue|Perl::Critic::Exception::Configuration::Option::Policy::ParameterValue>
is thrown.  This allows C<Perl::Critic> to include that problem along
with any other problems found with the user's configuration in a
single error message.


=head2 Using Both C<supported_parameters()> and C<initialize_if_enabled()>

There are cases where a Policy needs additional initialization beyond
configuration or where the way it acts depends upon the combination of
multiple parameters.  In such situations, you will need to create an
implementation of C<initialize_if_enabled()>.  If you want to take
advantage of the supplied parameter handling from within
implementation of C<initialize_if_enabled()>, note that the
information from C<supported_parameters()> will already have been
used, with user-supplied parameter values validated and placed into
the Policy by the time C<initialize_if_enabled()> has been called.  It
is likely that you will not need to refer the contents of the
C<$config> parameter; just pull the information you need out of
C<$self>.  In fact, any value for the parameter values will be gone.



=head2 Summary of permitted hash keys in C<supported_parameters()>.


=head3 All types


=over

=item - "name" (mandatory)

=item - "description" (optional)

=item - "behavior" (optional)

Currently, one of:

=over

=item "boolean"

=item "enumeration"

=item "integer"

=item "string"

=item "string list"

=back

=item - "default_string" (optional)

A string representation of the default value of the parameter.

=item - "parser" (optional)

A code ref to a custom parser for the parameter.

=back

=head3 Enumerations

=over

=item - "enumeration_values" (mandatory)

A mandatory reference to an array of strings.

=item - "enumeration_allow_multiple_values" (optional)

Boolean indicating whether or not the user is restricted to a single
value.

=back

=head3 Integers

=over

=item - "integer_minimum" (optional)

Minimum allowed value, inclusive.

=item - "integer_maximum" (optional)

Maximum allowed value, inclusive.

=back

=head3 String lists

=over

=item - "list_always_present_values" (optional)

A reference to an array of values that should always be included in
the value of the parameter.

=back


=head1 ADDITIONAL FEATURES

=head2 C<default_maximum_violations_per_document()>

Certain problems that a Policy detects can be endemic to a particular
file; if there's one violation, there's likely to be many.  A good
example of this is
L<Perl::Critic::Policy::TestingAndDebugging::RequireUseStrict|Perl::Critic::Policy::TestingAndDebugging::RequireUseStrict>;
if there's one line before C<use strict>, there's a good chance that
the entire file is missing C<use strict>.  In such cases, it's not
much help to the user to report every single violation.  If you've got
such a policy, you should override
L<default_maximum_violations_per_document()|Perl::Critic::Policy/"default_maximum_violations_per_document()">
method to provide a limit.  The user can override this value with a
value for "maximum_violations_per_document" in their F<.perlcriticrc>.

See the source code for
L<Perl::Critic::Policy::ValuesAndExpressions::ProhibitMagicNumbers|Perl::Critic::Policy::ValuesAndExpressions::ProhibitMagicNumbers>
and
L<Perl::Critic::Policy::TestingAndDebugging::RequireUseWarnings|Perl::Critic::Policy::TestingAndDebugging::RequireUseWarnings>
for examples.


=head2 C<is_safe()>

Most L<Perl::Critic> Policies are purely I<static>.  In other words,
they never compile or execute any of the source code that they
analyze.  However it is possible to write I<dynamic> Policies that do
compile or execute code, which may result in unsafe operations (see
L<Perl::Critic::Dynamic|Perl::Critic::Dynamic> for an example).  So
the C<is_safe()> method is used to indicate whether a Policy can be
trusted to not cause mischief.  By default, C<is_safe()> returns true.
But if you are writing a Policy that will compile or execute any of
the source code that it analyzes, then you should override the
C<is_safe()> method to return false.


=head1 DISTRIBUTING YOUR POLICIES

=head2 Create a Distribution

You need to come up with a name for your set of policies.  Sets of
add-on policies are generally named C<Perl::Critic::I<something>>,
e.g. L<Perl::Critic::More|Perl::Critic::More>.

The module representing the distribution will not actually have any
functionality; it's just documentation and a name for users to use
when installing via L<CPAN|CPAN>/L<CPANPLUS|CPANPLUS>.  The important
part is that this will include a list of the included policies, with
descriptions of each.

A typical implementation will look like:

    package Perl::Critic::Example;

    use strict;
    use warnings;

    our $VERSION = '1.000000';

    1; # Magic true value required at end of module

    __END__

    =head1 NAME

    Perl::Critic::Example - Policies for Perl::Critic that act as an example.

    =head1 AFFILIATION

    This module has no functionality, but instead contains documentation
    for this distribution and acts as a means of pulling other modules
    into a bundle.  All of the Policy modules contained herein will have
    an "AFFILIATION" section announcing their participation in this
    grouping.


    =head1 SYNOPSIS

    Some L<Perl::Critic|Perl::Critic> policies that will help you keep your
    code nice and compliant.


    =head1 DESCRIPTION

    The included policies are:

    =over

    =item L<Perl::Critic::Policy::Documentation::Example|Perl::Critic::Policy::Documentation::Example>

    Complains about some example documentation issues.  [Default severity: 3]


    =item L<Perl::Critic::Policy::Variables::Example|Perl::Critic::Policy::Variables::Example>

    All modules must have at least one variable.  [Default severity: 3]


    =back


    =head1 CONFIGURATION AND ENVIRONMENT

    All policies included are in the "example" theme.  See the
    L<Perl::Critic|Perl::Critic> documentation for how to make use of this.


=head2 Themes

Users can choose which policies to enable using themes.  You should
implement C<default_themes()> so that users can take advantage of
this.  In particular, you should use a theme named after your
distribution in all your policies; this should match the value listed
in the C<CONFIGURATION AND ENVIRONMENT> POD section as shown above.

    default_themes { return qw< example math > }

If you're looking for ideas of what themes to use, have a look at the
output of C<perlcritic --list-themes>.


=head2 Documentation

=head3 AFFILIATION

Since all policies have to go somewhere under the
C<Perl::Critic::Policy::> namespace, it isn't always clear what
distribution a policy came from when browsing through their
documentation.  For this reason, you should include an C<AFFILIATION>
section in the POD for all of your policies that state where the
policy comes from.  For example:

    =head1 AFFILIATION

    This policy is part of L<Perl::Critic::Example|Perl::Critic::Example>.


=head3 CONFIGURATION

In order to make it clear what can be done with a policy, you should
always include a C<CONFIGURATION> section in your POD, even if it's
only to say:

    =head1 CONFIGURATION

    This Policy is not configurable except for the standard options.

=head1 TESTING YOUR POLICY

The L<Perl::Critic> distribution also contains a framework for testing your
Policy.  See L<Perl::Critic::TestUtils> for the details.

=head1 HINT

When you're trying to figure out what L<PPI|PPI> is going to hand you
for a chunk of code, there is a F<tools/ppidump> program in the
L<Perl::Critic|Perl::Critic> distribution that will help you.  For
example, when developing the above RequireBlockGrep example, you might
want to try

    tools/ppidump '@matches = grep /pattern/, @list;'

and

    tools/ppidump '@matches = grep { /pattern/ } @list;'

to see the differences between the two cases.

Alternatively, see the C<ppi_dumper> documentation at
L<http://search.cpan.org/dist/App-PPI-Dumper/script/ppi_dumper>
and the C<PPI::Tester> documentation at
L<http://search.cpan.org/dist/PPI-Tester/lib/PPI/Tester.pm>.


=head1 VERSION

This is part of L<Perl::Critic|Perl::Critic> version 1.116.


=head1 SEE ALSO

Chas. Owens has a blog post about developing in-house policies at
L<http://svok.blogspot.com/2009/09/adding-house-policies-to-perlcritic.html>.


=head1 AUTHOR

Jeffrey Ryan Thalhammer <jeff@imaginative-software.com>


=head1 COPYRIGHT

Copyright (c) 2005-2011 Imaginative Software Systems.  All rights reserved.

This program is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.  The full text of this license
can be found in the LICENSE file included with this module.

=cut

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