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Always turn off hyphenation; it makes .\" way too many mistakes in technical documents. .if n .ad l .nh .SH "NAME" Moose::Cookbook::Roles::Comparable_CodeReuse \- Using roles for code reuse .SH "VERSION" .IX Header "VERSION" version 2.1213 .SH "SYNOPSIS" .IX Header "SYNOPSIS" .Vb 2 \& package Eq; \& use Moose::Role; \& \& requires \*(Aqequal_to\*(Aq; \& \& sub not_equal_to { \& my ( $self, $other ) = @_; \& not $self\->equal_to($other); \& } \& \& package Comparable; \& use Moose::Role; \& \& with \*(AqEq\*(Aq; \& \& requires \*(Aqcompare\*(Aq; \& \& sub equal_to { \& my ( $self, $other ) = @_; \& $self\->compare($other) == 0; \& } \& \& sub greater_than { \& my ( $self, $other ) = @_; \& $self\->compare($other) == 1; \& } \& \& sub less_than { \& my ( $self, $other ) = @_; \& $self\->compare($other) == \-1; \& } \& \& sub greater_than_or_equal_to { \& my ( $self, $other ) = @_; \& $self\->greater_than($other) || $self\->equal_to($other); \& } \& \& sub less_than_or_equal_to { \& my ( $self, $other ) = @_; \& $self\->less_than($other) || $self\->equal_to($other); \& } \& \& package Printable; \& use Moose::Role; \& \& requires \*(Aqto_string\*(Aq; \& \& package US::Currency; \& use Moose; \& \& with \*(AqComparable\*(Aq, \*(AqPrintable\*(Aq; \& \& has \*(Aqamount\*(Aq => ( is => \*(Aqrw\*(Aq, isa => \*(AqNum\*(Aq, default => 0 ); \& \& sub compare { \& my ( $self, $other ) = @_; \& $self\->amount <=> $other\->amount; \& } \& \& sub to_string { \& my $self = shift; \& sprintf \*(Aq$%0.2f USD\*(Aq => $self\->amount; \& } .Ve .SH "DESCRIPTION" .IX Header "DESCRIPTION" Roles have two primary purposes: as interfaces, and as a means of code reuse. This recipe demonstrates the latter, with roles that define comparison and display code for objects. .PP Let's start with \f(CW\*(C`Eq\*(C'\fR. First, note that we've replaced \f(CW\*(C`use Moose\*(C'\fR with \f(CW\*(C`use Moose::Role\*(C'\fR. We also have a new sugar function, \f(CW\*(C`requires\*(C'\fR: .PP .Vb 1 \& requires \*(Aqequal_to\*(Aq; .Ve .PP This says that any class which consumes this role must provide an \&\f(CW\*(C`equal_to\*(C'\fR method. It can provide this method directly, or by consuming some other role. .PP The \f(CW\*(C`Eq\*(C'\fR role defines its \f(CW\*(C`not_equal_to\*(C'\fR method in terms of the required \f(CW\*(C`equal_to\*(C'\fR method. This lets us minimize the methods that consuming classes must provide. .PP The next role, \f(CW\*(C`Comparable\*(C'\fR, builds on the \f(CW\*(C`Eq\*(C'\fR role. We include \&\f(CW\*(C`Eq\*(C'\fR in \f(CW\*(C`Comparable\*(C'\fR using \f(CW\*(C`with\*(C'\fR, another new sugar function: .PP .Vb 1 \& with \*(AqEq\*(Aq; .Ve .PP The \f(CW\*(C`with\*(C'\fR function takes a list of roles to consume. In our example, the \f(CW\*(C`Comparable\*(C'\fR role provides the \f(CW\*(C`equal_to\*(C'\fR method required by \&\f(CW\*(C`Eq\*(C'\fR. However, it could opt not to, in which case a class that consumed \f(CW\*(C`Comparable\*(C'\fR would have to provide its own \f(CW\*(C`equal_to\*(C'\fR. In other words, a role can consume another role \fIwithout\fR providing any required methods. .PP The \f(CW\*(C`Comparable\*(C'\fR role requires a method, \f(CW\*(C`compare\*(C'\fR: .PP .Vb 1 \& requires \*(Aqcompare\*(Aq; .Ve .PP The \f(CW\*(C`Comparable\*(C'\fR role also provides a number of other methods, all of which ultimately rely on \f(CW\*(C`compare\*(C'\fR. .PP .Vb 4 \& sub equal_to { \& my ( $self, $other ) = @_; \& $self\->compare($other) == 0; \& } \& \& sub greater_than { \& my ( $self, $other ) = @_; \& $self\->compare($other) == 1; \& } \& \& sub less_than { \& my ( $self, $other ) = @_; \& $self\->compare($other) == \-1; \& } \& \& sub greater_than_or_equal_to { \& my ( $self, $other ) = @_; \& $self\->greater_than($other) || $self\->equal_to($other); \& } \& \& sub less_than_or_equal_to { \& my ( $self, $other ) = @_; \& $self\->less_than($other) || $self\->equal_to($other); \& } .Ve .PP Finally, we define the \f(CW\*(C`Printable\*(C'\fR role. This role exists solely to provide an interface. It has no methods, just a list of required methods. In this case, it just requires a \f(CW\*(C`to_string\*(C'\fR method. .PP An interface role is useful because it defines both a method and a \&\fIname\fR. We know that any class which does this role has a \&\f(CW\*(C`to_string\*(C'\fR method, but we can also assume that this method has the semantics we want. Presumably, in real code we would define those semantics in the documentation for the \f(CW\*(C`Printable\*(C'\fR role. (1) .PP Finally, we have the \f(CW\*(C`US::Currency\*(C'\fR class which consumes both the \&\f(CW\*(C`Comparable\*(C'\fR and \f(CW\*(C`Printable\*(C'\fR roles. .PP .Vb 1 \& with \*(AqComparable\*(Aq, \*(AqPrintable\*(Aq; .Ve .PP It also defines a regular Moose attribute, \f(CW\*(C`amount\*(C'\fR: .PP .Vb 1 \& has \*(Aqamount\*(Aq => ( is => \*(Aqrw\*(Aq, isa => \*(AqNum\*(Aq, default => 0 ); .Ve .PP Finally we see the implementation of the methods required by our roles. We have a \f(CW\*(C`compare\*(C'\fR method: .PP .Vb 4 \& sub compare { \& my ( $self, $other ) = @_; \& $self\->amount <=> $other\->amount; \& } .Ve .PP By consuming the \f(CW\*(C`Comparable\*(C'\fR role and defining this method, we gain the following methods for free: \f(CW\*(C`equal_to\*(C'\fR, \f(CW\*(C`greater_than\*(C'\fR, \&\f(CW\*(C`less_than\*(C'\fR, \f(CW\*(C`greater_than_or_equal_to\*(C'\fR and \&\f(CW\*(C`less_than_or_equal_to\*(C'\fR. .PP Then we have our \f(CW\*(C`to_string\*(C'\fR method: .PP .Vb 4 \& sub to_string { \& my $self = shift; \& sprintf \*(Aq$%0.2f USD\*(Aq => $self\->amount; \& } .Ve .SH "CONCLUSION" .IX Header "CONCLUSION" Roles can be very powerful. They are a great way of encapsulating reusable behavior, as well as communicating (semantic and interface) information about the methods our classes provide. .SH "FOOTNOTES" .IX Header "FOOTNOTES" .IP "(1)" 4 .IX Item "(1)" Consider two classes, \f(CW\*(C`Runner\*(C'\fR and \f(CW\*(C`Process\*(C'\fR, both of which define a \&\f(CW\*(C`run\*(C'\fR method. If we just require that an object implements a \f(CW\*(C`run\*(C'\fR method, we still aren't saying anything about what that method \&\fIactually does\fR. If we require an object that implements the \&\f(CW\*(C`Executable\*(C'\fR role, we're saying something about semantics. .SH "AUTHORS" .IX Header "AUTHORS" .IP "\(bu" 4 Stevan Little .IP "\(bu" 4 Dave Rolsky .IP "\(bu" 4 Jesse Luehrs .IP "\(bu" 4 Shawn M Moore .IP "\(bu" 4 \&\s-1XXXX XXX\s0'\s-1XX \s0(Yuval Kogman) .IP "\(bu" 4 Karen Etheridge .IP "\(bu" 4 Florian Ragwitz .IP "\(bu" 4 Hans Dieter Pearcey .IP "\(bu" 4 Chris Prather .IP "\(bu" 4 Matt S Trout .SH "COPYRIGHT AND LICENSE" .IX Header "COPYRIGHT AND LICENSE" This software is copyright (c) 2006 by Infinity Interactive, Inc.. .PP This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.