NAME

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

DESCRIPTION

For developers who want to create custom coding standards, the following tells how to create a Policy module for Perl::Critic. Although the Perl::Critic distribution already includes a number of Policies based on Damian Conway's book Perl Best Practices (which will be referred to via "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 Perl::Critic::Policy subclass, which may require as little as 10 lines of code.

BACKGROUND

The heart of Perl::Critic is 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 PPI::Token::Operator or PPI::Token::Word, and then organized into structure classes, like PPI::Statement::Expression and PPI::Structure::Subroutine. The root node of the hierarchy is the PPI::Document.

The Perl::Critic engine traverses each node in the PPI::Document tree and invokes each of the 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 that a coding standard has been violated, it returns one or more 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 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 grep uses a block for the first argument and not an expression. The reasons for this Policy are discussed in detail in PBP.

EXAMPLE POLICY

First, the Policy module needs to have a name. Perl::Critic uses Module::Pluggable to automatically discover all modules in the Perl::Critic::Policy namespace. Also, we've adopted the convention of grouping Policies into directories according to the chapters of PBP. Since the goal of this Policy is to enforce the use of block arguments to grep and it comes from the "Builtin Functions" chapter of PBP, we call it "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 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, use strict and 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 PBP. We make them read-only because they never change. (See "ProhibitConstantPragma" in Perl::Critic::ValuesAndExpressions for why we don't use constant.)

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

Most policies don't need to override the new() method provided by Perl::Critic::Policy. However, if your Policy is configurable via .perlcriticrc you should implement a supported_parameters() method and need to override the constructor to examine the %config values. (To future-proof your policy, make sure that your constructor invokes the super-constructor.) Since this Policy isn't configurable, we'll declare that by providing an implementation of supported_parameters() that returns an empty list.

  sub supported_parameters { return ()                  }

Next, we define the 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 Perl::Critic::Utils package exports several severity constants that you can use here via the :severities tag.

  sub default_severity     { return $SEVERITY_HIGH      }

Likewise, the 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 "core" theme. Since use of grep without blocks often leads to bugs, we include a "bugs" theme. And since this Policy comes directly from PBP, this Policy should be a member of the "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 Perl::Critic::More distribution all have a "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 PPI::Statement, PPI::Token::Symbol, PPI::Structure::Conditional or PPI::Token::Pod respectively. The applies_to() method returns a list of PPI package names. (You can get that list of available package names via perldoc PPI.) As Perl::Critic traverses the document, it will call the 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 grep. Since the token "grep" is a PPI::Token::Word, we return that package name from the applies_to() method.

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

If your Policy needs to analyze several different types of elements, the applies_to method may return the name of several PPI packages. If your Policy needs to examine the file as a whole, then the applies_to method should return 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 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 ($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 undef.

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

The 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 PPI::Token::Word because that's what we declared back in the 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. So we make sure that this 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 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 <%hash = ( grep = 'foo' )>>. We don't want to test those types of elements because they don't represent function calls to grep. So we use one of handy utility functions from Perl::Critic::Utils to make sure that this "grep" is actually in the right context. (The is_function_call() subroutine is brought in via the :classification tag.)

      return if ! is_function_call($elem);

Now that we know this element is a call to the 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 PPI; after that, we'll show how this Policy actually uses facilities provided by 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 snext_sibling() to get the next code-sibling (the 's' in snext_sibling stands for 'significant').

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

In Perl, the parenthesis around argument lists are usually optional, and PPI packs the elements into a PPI::Structure::List object when parens are used. So if the sibling is a PPI::Structure::List, we pull out the first (significant) child of that list. This child will be the first argument to grep. If parens 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 "first_arg" in Perl::Critic::Utils subroutine available via the :ppi tag. So we use that instead.

      my $arg = first_arg($elem);

Finally, we now have a reference to the first argument to grep. If that argument is a block (i.e. something in curly braces), then it will be a 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 PPI::Structure::Block, then we know that this call to grep must be using the expression form, and that violates our Policy. So we create and return a new Perl::Critic::Violation object via the "violation" in Perl::Critic::Policy 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 perlcritic with a --verbose parameter of 10 or 11 to see it along with the rest of the output for violations of your policy.

AUTHOR

Jeffrey Ryan Thalhammer <thaljef@cpan.org>

COPYRIGHT

Copyright (c) 2005-2007 Jeffrey Ryan Thalhammer. 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.

cpanm Perl::Critic

perl -MCPAN -e shell
install Perl::Critic