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44 <br /><span xmlns:dc="http://purl.org/dc/elements/1.1/" href="http://purl.org/dc/dcmitype/Text" property="dc:title" rel="dc:type">Introduction to Moose</span> by <span xmlns:cc="http://creativecommons.org/ns#" property="cc:attributionName">David Rolsky</span> is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-sa/3.0/us/">Creative Commons Attribution-Share Alike 3.0 United States License</a>.
47 <h2>Introduction to Moose</h2>
51 <div class="presentation">
54 <h1>Introduction to Moose</h1>
59 <h1>Moose Summed Up</h1>
62 <li><strong>Declarative</strong> OO sugar</li>
63 <li>Introspectable</li>
64 <li>Extensible (MooseX::* on CPAN)</li>
69 <h1>Moose Background</h1>
72 <li>Created by Stevan Little, first released in 2006</li>
73 <li>Moose builds on Perl 5's native OO</li>
74 <li>Borrows ideas from other languages, notably Perl 6</li>
75 <li>Provides semantics for common operations</li>
79 <div class="slide fake-slide0">
80 <h1>Part 0: Moose Concepts</h1>
93 <li>Method modifiers</li>
94 <li>Constructor and destructor</li>
95 <li>One metaclass object</li>
98 <li>Classes do roles</li>
103 <h1>Class Example</h1>
105 <pre><code>package Person;
106 <span class="highlight">use Moose;</span></code></pre>
109 <li>Poof, a Moose-based class!</li>
117 <li>Aka property, slot, field, member variable</li>
118 <li>A piece of data owned by an object</li>
129 <li>Access-control (read-only vs read-write)</li>
130 <li>An optional type</li>
131 <li>Accessor methods</li>
132 <li>Delegation methods</li>
133 <li>Optional default value</li>
134 <li>Many more features</li>
137 <li>Stored in the object, but don't worry about that</li>
142 <h1>Attribute Example</h1>
144 <pre><code>package Person;
147 <span class="highlight">has first_name => ( is => 'rw' );</span></code></pre>
155 <li>Nothing fancy here, just Perl subroutines</li>
158 <pre><code>package Person;
161 <span class="highlight">sub greet { ... }</span></code></pre>
168 <li>Classes <strong>do</strong> (or consume) roles</li>
169 <li>Similar to mixins and Java interfaces</li>
177 <li>Like classes, can have attributes, methods, do roles</li>
178 <li>Roles can require methods</li>
179 <li>Roles are composed (flattened) into classes</li>
184 <h1>Role Example</h1>
186 <pre><code>package HasPermissions;
187 <span class="highlight">use Moose::Role;</span>
189 has is_admin => ( is => 'rw' );</code></pre>
193 <h1>Role Example</h1>
199 <pre><code>package Person;
202 <span class="highlight">with 'HasPermissions';</span></code></pre>
206 <h1>Method Modifiers</h1>
210 <li>"<strong>Before</strong> foo(), do this first"</li>
211 <li>"Do this <strong>after</strong> foo()</li>
212 <li>"Put this code <strong>around</strong> foo()"</li>
217 <h1>Before & After</h1>
219 <pre><code>before 'foo'
220 => sub { warn 'About to call foo()' };
223 => sub { warn 'Leaving foo()' };</code></pre>
230 <pre><code>around 'foo' => sub {
231 my $real_foo = shift;
234 warn 'Just before foo()';
236 $self->$real_foo( @_, bar => 42 );
238 return ( @return, 'modify return values' );
243 <h1>Type Constraints</h1>
246 <li>NOT A FULL-BLOWN TYPE SYSTEM!</li>
247 <li>But still darn useful</li>
248 <li>Constrain attribute values</li>
249 <li>Coerce from other types</li>
254 <h1>Type Constraint Example</h1>
256 <pre><code>package Person;
261 <span class="highlight">isa => 'Int'</span>,
265 Person->new( weight => 'fat' );</code></pre>
272 <li>Attributes can define delegations</li>
273 <li>Lets you hide some implementation details</li>
274 <li>Fewer objects to chase around</li>
281 <pre><code>package Person;
287 <span class="highlight">handles => { 'blog_hostname' => 'host' },</span>
290 <span class="highlight">$person->blog_hostname;</span>
291 # really calls $person->blog_uri->host</code></pre>
295 <h1>Constructors</h1>
298 <li>Moose creates <code>new()</code> for you</li>
299 <li>Provide an optional <code>BUILDARGS()</code> and <code>BUILD()</code></li>
307 <li>Provide an optional <code>DEMOLISH()</code></li>
312 <h1>Moose Meta-API</h1>
315 <li>Answers questions like ...
317 <li>What methods does this class have?</li>
318 <li>What are its parents?</li>
319 <li>What attributes does it have (including inherited attributes)?</li>
320 <li>What roles does it do?</li>
321 <li>Much, much, more</li>
328 <h1>Moose Meta-API</h1>
331 <li>Not just for introspection ...
333 <li>Add methods, attributes, roles, etc</li>
334 <li>Extend and alter core features</li>
344 <li>A quick bit of propoganda ...</li>
351 <pre><code>package Person;
354 has last_name => (
361 <h1>Without Moose</h1>
363 <pre class="small"><code>package Person;
373 if (exists $args{last_name}) {
374 confess "Attribute (last_name) does not pass the type constraint because: "
375 . "Validation failed for 'Str' with value $args{last_name}"
376 if ref($args{last_name});
377 $self->{last_nane} = $args{last_name};
380 return bless $self, $class;
388 confess "Attribute (last_name) does not pass the type constraint because: "
389 . "Validation failed for 'Str' with value $value"
391 $self->{last_name} = $value;
394 return $self->{last_name};
400 <h1>Side by side</h1>
402 <table class="side-by-side">
405 <pre><code>package Person;
408 has last_name => (
414 <pre class="small"><code>package Person;
424 if (exists $args{last_name}) {
425 confess "Attribute (last_name) does not pass the type constraint because: "
426 . "Validation failed for 'Str' with value $args{last_name}"
427 if ref($args{last_name});
428 $self->{last_nane} = $args{last_name};
431 return bless $self, $class;
439 confess "Attribute (last_name) does not pass the type constraint because: "
440 . "Validation failed for 'Str' with value $value"
442 $self->{last_name} = $value;
445 return $self->{last_name};
454 <h1>Side by side</h1>
456 <table class="side-by-side">
459 <pre><code><span class="match-moose">package Person;</span>
462 has last_name => (
468 <pre class="small"><code><span class="match-unsweet">package Person;</span>
478 if (exists $args{last_name}) {
479 confess "Attribute (last_name) does not pass the type constraint because: "
480 . "Validation failed for 'Str' with value $args{last_name}"
481 if ref($args{last_name});
482 $self->{last_nane} = $args{last_name};
485 return bless $self, $class;
493 confess "Attribute (last_name) does not pass the type constraint because: "
494 . "Validation failed for 'Str' with value $value"
496 $self->{last_name} = $value;
499 return $self->{last_name};
507 <h1>Side by side</h1>
509 <table class="side-by-side">
512 <pre><code>package Person;
513 <span class="match-moose">use Moose;</span>
515 has last_name => (
521 <pre class="small"><code>package Person;
522 <span class="match-unsweet">use strict;
529 my $self = {};</span>
531 if (exists $args{last_name}) {
532 confess "Attribute (last_name) does not pass the type constraint because: "
533 . "Validation failed for 'Str' with value $args{last_name}"
534 if ref($args{last_name});
535 $self->{last_nane} = $args{last_name};
538 <span class="match-unsweet">return bless $self, $class;
546 confess "Attribute (last_name) does not pass the type constraint because: "
547 . "Validation failed for 'Str' with value $value"
549 $self->{last_name} = $value;
552 return $self->{last_name};
560 <h1>Side by side</h1>
562 <table class="side-by-side">
565 <pre><code>package Person;
568 <span class="match-moose">has last_name => (</span>
571 <span class="match-moose">);</span></code></pre>
574 <pre class="small"><code>package Person;
584 <span class="match-unsweet">if (exists $args{last_name}) {</span>
585 confess "Attribute (last_name) does not pass the type constraint because: "
586 . "Validation failed for 'Str' with value $args{last_name}"
587 if ref($args{last_name});
588 <span class="match-unsweet">$self->{last_nane} = $args{last_name};
591 return bless $self, $class;
599 confess "Attribute (last_name) does not pass the type constraint because: "
600 . "Validation failed for 'Str' with value $value"
602 $self->{last_name} = $value;
605 return $self->{last_name};
613 <h1>Side by side</h1>
615 <table class="side-by-side">
618 <pre><code>package Person;
621 has last_name => (
622 <span class="match-moose">is => 'rw',</span>
627 <pre class="small"><code>package Person;
637 if (exists $args{last_name}) {
638 confess "Attribute (last_name) does not pass the type constraint because: "
639 . "Validation failed for 'Str' with value $args{last_name}"
640 if ref($args{last_name});
641 $self->{last_nane} = $args{last_name};
644 return bless $self, $class;
647 <span class="match-unsweet">sub last_name {
651 my $value = shift;</span>
652 confess "Attribute (last_name) does not pass the type constraint because: "
653 . "Validation failed for 'Str' with value $value"
655 <span class="match-unsweet">$self->{last_name} = $value;
658 return $self->{last_name};
659 }</span></code></pre>
666 <h1>Side by side</h1>
668 <table class="side-by-side">
671 <pre><code>package Person;
674 has last_name => (
676 <span class="match-moose">isa => 'Str',</span>
680 <pre class="small"><code>package Person;
690 if (exists $args{last_name}) {
691 <span class="match-unsweet">confess "Attribute (last_name) does not pass the type constraint because: "
692 . "Validation failed for 'Str' with value $args{last_name}"
693 if ref($args{last_name});</span>
694 $self->{last_nane} = $args{last_name};
697 return bless $self, $class;
705 <span class="match-unsweet">confess "Attribute (last_name) does not pass the type constraint because: "
706 . "Validation failed for 'Str' with value $value"
707 if ref($value);</span>
708 $self->{last_name} = $value;
711 return $self->{last_name};
719 <h1>Side by side</h1>
721 <table class="side-by-side">
722 <tr class="incremental">
726 <tr class="incremental">
727 <td>92 characters</td>
728 <td>741 characters</td>
732 <pre><code>package Person;
735 has last_name => (
741 <pre class="small"><code>package Person;
751 if (exists $args{last_name}) {
752 confess "Attribute (last_name) does not pass the type constraint because: "
753 . "Validation failed for 'Str' with value $args{last_name}"
754 if ref($args{last_name});
755 $self->{last_nane} = $args{last_name};
758 return bless $self, $class;
766 confess "Attribute (last_name) does not pass the type constraint because: "
767 . "Validation failed for 'Str' with value $value"
769 $self->{last_name} = $value;
772 return $self->{last_name};
782 <pre class="small"><code>sub new {
787 if (exists $args{last_name}) {
788 confess "Attribute (last_name) does not pass the type constraint because: "
789 . "Validation failed for 'Str' with value $args{last_name}"
790 if ref($args{last_name});
791 $self->{last_nane} = $args{last_name};
794 return bless $self, $class;
801 <pre class="small"><code>if (exists $args{last_name}) {
802 confess "Attribute (last_name) does not pass the type constraint because: "
803 . "Validation failed for 'Str' with value $args{last_name}"
804 if ref($args{last_name});
805 $self->{last_nane} = $args{last_name};
812 <code>$self->{last_nane} = $args{last_name};</code>
817 <code>$self->{last_na<span class="wrong">n</span>e}</code>
823 <pre><code>package Person;
826 has last_name => (
832 <div class="slide fake-slide0">
833 <h1>Part 1: Moose Classes</h1>
837 <h1>Moose Classes</h1>
840 <li>Moose classes are Perl packages which <code>use Moose</code></li>
845 <h1>Moose.pm and Your Class</h1>
847 <pre><code>package Person;
848 use Moose;</code></pre>
851 <li><code>Moose.pm</code> provides declarative sugar</li>
852 <li>Turns on <code>strict</code> and <code>warnings</code></li>
853 <li>Creates metaclasses for your class: <code>Person->meta</code></li>
854 <li>Moose classes automatically inherit from <code>Moose::Object</code></li>
859 <h1>What <code>Moose::Object</code> Provides</h1>
862 <li>Constructor - <code>new()</code></li>
863 <li>Calls your <code>BUILDARGS()</code> and/or <code>BUILD()</code></li>
864 <li>Calls your <code>DEMOLISH</code> during object destruction</li>
872 <li><code>extends</code> is sugar for declaring parent classes</li>
875 <pre><code>package Employee;
877 <span class="highlight">extends 'Person';</span></code></pre>
884 <li>Each call to <code>extends</code> <strong>resets</strong> your parents</li>
887 <h2 class="wrong">WRONG</h2>
889 <pre><code>package EvilEmployee;
892 extends 'Thief';</code></pre>
894 <h2 class="right">RIGHT</h2>
896 <pre><code>package EvilEmployee;
898 extends 'Person', 'Thief';</code></pre>
902 <h1>Extending un-Moose-y Parents</h1>
904 <pre><code>package My::LWP;
906 extends 'LWP';</code></pre>
909 <li>No <code>Moose::Object</code>, so ...
911 <li>No attribute-handling <code>new()</code></li>
912 <li>No <code>BUILDARGS()</code> or <code>BUILD()</code></li>
913 <li>No <code>DEMOLISH()</code></li>
916 <li>But see <code>MooseX::NonMoose</code> for a workaround</li>
921 <h1><code>overrides</code> and <code>super</code></h1>
924 <li><code>overrides</code> is another method modifier</li>
925 <li>An alternative to Perl's <code>SUPER::</code></li>
930 <h1><code>overrides</code> and <code>super</code></h1>
932 <pre><code>package Employee;
935 <span class="current">extends 'Person';</span>
937 <span class="incremental">overrides</span> work => sub {
940 die "Pay me first" unless $self->got_paid;
941 <span class="incremental">super();</span>
942 }<span class="incremental">;</span></code></pre>
946 <h1>Caveat <code>super</code></h1>
949 <li>Mostly like <code>$self->SUPER::work(@_)</code></li>
950 <li><strong>But</strong> cannot change <code>@_</code>!</li>
951 <li>Binds the parent's method at compile time</li>
956 <h1>Attributes (Part 1)</h1>
959 <li><code>has 'foo'</code></li>
960 <li>Use <code>is => 'ro'</code> or <code>is => 'rw'</code></li>
961 <li>Attributes without "is" have no accessors</li>
966 <h1>Read-write attributes</h1>
968 <pre><code>package Person;
971 has first_name => ( <span class="highlight">is => 'rw'</span> );
974 Person->new( first_name => 'Dave' );
976 $person->first_name('Stevan');
977 print $person->first_name; # Stevan</code></pre>
982 <h1>Read-only attributes</h1>
984 <pre><code>package Person;
987 has first_name => ( <span class="highlight">is => 'ro'</span> );
990 Person->new( first_name => 'Dave' );
992 $person->first_name('Stevan');
993 print $person->first_name; # Dave</code></pre>
998 <h1>There is More to Come</h1>
1001 <li>Attributes have a <em>lot</em> of features</li>
1006 <h1>Cleaning Up Moose Droppings</h1>
1008 <pre><code>package Person;
1012 Person->can('extends');</code></pre>
1015 <li>Not very hygienic</li>
1020 <h1>Cleaning Up Moose Droppings</h1>
1022 <pre><code>package Person;
1030 Person->can('extends');</code></pre>
1037 <li><code>no Moose</code> at the end of a package is a best practice</li>
1043 <h1>Immutability</h1>
1046 <li><span style="font-family: URW Chancery L; font-size: 140%">Stevan's Incantation of Fleet-Footedness</span></li>
1049 <pre><code>package Person;
1052 <span class="highlight">__PACKAGE__->meta->make_immutable;</span></code></pre>
1056 <h1>What <code>make_immutable</code> does</h1>
1060 <li>Uses <code>eval</code> to "inline" a constructor</li>
1061 <li>Memoizes a lot of meta-information</li>
1062 <li>Makes loading your class slower</li>
1063 <li>Makes object creation <em>much</em> faster</li>
1068 <h1>When to Immutabilize?</h1>
1071 <li><em>Almost</em> always</li>
1072 <li>Startup time vs execution time</li>
1077 <h1>Classes Summary</h1>
1080 <li><code>use Moose</code></li>
1081 <li><code>Class->meta</code></li>
1082 <li><code>Moose::Object</code> base class</li>
1083 <li><code>extends</code>, <code>overrides</code>, and <code>super</code></li>
1084 <li>Simple attributes: <code>has</code>, <code>is => 'ro'</code>, & <code>is => 'rw'</code></li>
1085 <li><code>no Moose</code></li>
1086 <li><code>__PACKAGE__->meta->make_immutable</code></li>
1094 $ perl bin/prove -lv t/00-prereq.t
1096 Missing anything? Install it. (see tarballs/)
1098 # perl bin/prove -lv t/01-classes.t
1100 Iterate til this passes all its tests</pre>
1103 <div class="slide fake-slide0">
1104 <h1>Part 2: Roles</h1>
1108 <h1>Just What Is a Role?</h1>
1111 <li>Mixin? Interface? Trait?</li>
1112 <li>Yes ... and more!</li>
1117 <h1>Roles Can Have State <strong>and</strong> Behavior</h1>
1119 <pre><code>package HasPermissions;
1122 <span class="current"># state
1123 has access_level => ( is => 'rw' );</span>
1125 <span class="incremental"># behavior
1128 my $required = shift;
1130 return $self->access_level >= $required;
1131 }</span></code></pre>
1136 <h1>Roles Can Define Interfaces</h1>
1138 <pre><code>package Printable;
1141 requires 'as_string';</code></pre>
1145 <h1>Roles Can Do All Three</h1>
1147 <pre><code>package Printable;
1150 requires 'as_string';
1152 has has_been_printed => ( is => 'rw' );
1156 print $self->as_string;
1157 $self->has_been_printed(1);
1162 <h1>Classes Consume Roles</h1>
1164 <pre><code>package Person;
1167 with 'HasPermissions';</code></pre>
1171 <h1>Classes Consume Roles</h1>
1173 <pre><code>my $person = Person->new(
1174 first_name => 'Kenichi',
1175 last_name => 'Asai',
1176 access_level => 42,
1179 print $person->full_name
1181 . $person->can_access(42)
1183 : 'little power';</code></pre>
1187 <h1>Roles in Practice</h1>
1190 <li>Consuming a role =~ inlining the role</li>
1195 <h1>In Other Words ...</h1>
1197 <pre><code>package Person;
1200 <span class="highlight">with 'Printable';</span></code></pre>
1204 <h1>In Other Words ...</h1>
1206 <pre><code>package Person;
1209 <span class="delete">with 'Printable';</span>
1211 <span class="highlight">has has_been_printed => ( is => 'rw' );
1215 print $self->as_string;
1216 $self->has_been_printed(1);
1217 }</span></code></pre>
1224 <li>Role consumption is introspectable</li>
1227 <pre><code>if ( Person->does('Printable') ) { ... }
1231 if ( Person->meta->does('Printable') ) { ... }</code></pre>
1236 <h1>These Names Are the Same</h1>
1239 <li>What if a role and class define the same method?</li>
1240 <li>A class's <em>local</em> methods win over the role's</li>
1241 <li>The role's methods win over the class's <em>inherited</em> methods</li>
1246 <h1>Conflicts Between Roles</h1>
1249 <li>Two roles with a method of the same name</li>
1250 <li>Generates a compile-time error when consumed by a class</li>
1255 <h1>Conflict Example</h1>
1257 <pre><code>package IsFragile;
1262 package CanBreakdance;
1265 sub break { ... }</code></pre>
1269 <h1>Conflict Example</h1>
1271 <pre><code>package FragileDancer;
1274 <span class="highlight">with 'IsFragile', 'CanBreakdance';</span></code></pre>
1277 <li>Only one <code>with</code>!</li>
1282 <h1>Conflict Resolution</h1>
1285 <li>The consuming class must resolve the conflict by implementing th emethod</li>
1286 <li>Can use some combination of method exclusion and aliasing</li>
1291 <h1>Method Aliasing</h1>
1293 <pre><code>package FragileDancer;
1296 <span class="highlight">with 'IsFragile' =>
1298 { break => 'break_bone' } },
1301 { break => 'break_it_down' } };</span></code></pre>
1304 <li>Renames the roles' methods</li>
1305 <li>Still conflicts, need to <code>exclude</code> as well</li>
1310 <h1>Method Exclusion</h1>
1312 <pre><code>package FragileDancer;
1315 <span class="highlight">with 'IsFragile' =>
1317 { break => 'break_bone' },
1318 exclude => 'break' },
1321 { break => 'break_dance' },
1322 exclude => 'break' };</span></code></pre>
1326 <h1>And then ...</h1>
1328 <pre><code>package FragileDancer;
1335 if ( rand(1) < 0.5 ) {
1342 <h1>Still Full of Fail</h1>
1345 <li>Roles are also about semantics!</li>
1346 <li>We've fulfilled the letter and lost the spirit</li>
1347 <li>Roles have a <em>meaning</em></li>
1348 <li>Think twice before blindly aliasing and excluding methods!</li>
1353 <h1>Hot Role-on-Role Action</h1>
1355 <pre><code>package Comparable;
1358 requires 'compare';</code></pre>
1362 <h1>Hot Role-on-Role Action</h1>
1364 <pre><code>package TestsEquality;
1371 return $self->compare(@_) == 0;
1376 <h1>And then ...</h1>
1378 <pre><code>package Integer;
1381 with 'TestsEquality';
1383 # Satisfies the Comparable role
1386 Integer->does('TestsEquality'); # true
1387 Integer->does('Comparable'); # also true!</code></pre>
1391 <h1>Name Conflicts Between Roles</h1>
1393 <pre><code>package HasSubProcess;
1396 <span class="highlight">sub execute { ... }</span>
1401 with 'HasSubProcess';
1403 <span class="highlight">sub execute { ... }</span></code></pre>
1407 <h1>Delayed Conflict</h1>
1409 <pre><code>package StateOfTexas;
1410 with 'Killer';</code></pre>
1413 <li><code>StateOfTexas</code> must implement its own <code>execute</code></li>
1414 <li>But loading the <code>Killer</code> role by itself does not cause an error</li>
1419 <h1>Roles as Interfaces</h1>
1422 <li>Roles can <code>require</code> methods of their consumers</li>
1423 <li>Compile-time checks</li>
1424 <li>Method must exist when the role is consumed</li>
1429 <h1>The Attribute Gotcha</h1>
1431 <pre><code>package HasSize;
1434 <span class="current">requires 'size';</span>
1439 <span class="incremental">with 'HasSize';
1441 has size => ( is => 'ro' );</span></code></pre>
1445 <h1>The Attribute Gotcha Workaround</h1>
1447 <pre><code>package HasSize;
1455 has size => ( is => 'ro' );
1457 with 'HasSize';</code></pre>
1461 <h1>Compile-time Is a Lie</h1>
1464 <li>Really, it's <em>package load</em> time</li>
1465 <li>That's run-time, but before the "real" run-time</li>
1466 <li>Moose does not rewire Perl, it's just sugar!</li>
1467 <li>(but <code>MooseX::Declare</code> <em>does</em> rewire Perl)</li>
1472 <h1>Enforcing Roles</h1>
1474 <pre><code>package Comparison;
1477 has [ 'left', 'right' ] => (
1479 <span class="highlight">does => 'Comparable',</span>
1484 <li>A sneak peek at type constraints</li>
1490 <h1>Roles Can Be Applied to Objects</h1>
1492 <pre><code>use Moose::Util qw( apply_all_roles );
1494 my $fragile_person = Person->new( ... );
1495 apply_all_roles( $fragile_person, 'IsFragile' );</code></pre>
1498 <li>Does not change the <code>Person</code> class</li>
1499 <li>Works with non-Moose classes, great for monkey-patching!</li>
1504 <h1>Roles Are Dirty Too</h1>
1507 <li>Once again, clean up those Moose droppings</li>
1510 <pre><code>package Comparable;
1515 <span class="highlight">no Moose::Role;</span></code></pre>
1518 <li>But roles cannot be made immutable</li>
1523 <h1>The Zen of Roles</h1>
1526 <li>Roles represent discrete units of ...
1533 <li>Roles are shareable between unrelated classes</li>
1534 <li>Roles are what a class <em>does</em>, not what it <em>is</em></li>
1535 <li>Roles <em>add</em> functionality, inheritance <em>specializes</em></li>
1540 <h1>Abstract Examples</h1>
1543 <li>Human <em>@ISA</em> Animal</li>
1544 <li>Human <em>does</em> Toolmaker (as <em>does</em> Chimpanzee)</li>
1545 <li>Car <em>@ISA</em> Vehicle</li>
1546 <li>Car <em>does</em> HasEngine</li>
1551 <h1>Real Examples</h1>
1554 <li>Objects representing SQL database components and queries
1556 <li>Schema, Table, Column, ColumnAlias</li>
1557 <li>Select, Insert, Update, Delete</li>
1564 <h1>Real Examples</h1>
1567 <li>Column and ColumnAlias both <em>do</em> ColumnLike</li>
1568 <li>ColumnLike things can be used in certain parts of queries</li>
1569 <li>All queries <em>do</em> HasWhereClause</li>
1570 <li>Select <em>does</em> Comparable and Selectable (for subselects)</li>
1571 <li>A where clause requires its components to <em>do</em> Comparable</li>
1579 # perl bin/prove -lv t/02-roles.t
1581 Iterate til this passes all its tests</pre>
1584 <div class="slide fake-slide0">
1585 <h1>Part 3: Basic Attributes</h1>
1589 <h1>Attributes Are Huge</h1>
1592 <li>Moose's biggest feature</li>
1593 <li>The target of <em>many</em> MooseX modules</li>
1598 <h1>Quick Review</h1>
1601 <li>Declared with <code>has</code></li>
1602 <li>Read-only or read-write</li>
1605 <pre><code>package Shirt;
1608 has 'color' => ( is => 'ro' );
1609 has 'is_ripped' => ( is => 'rw' );</code></pre>
1613 <h1>Required-ness</h1>
1616 <li>Required means "must be passed to the constructor"</li>
1617 <li>But can be <code>undef</code></li>
1622 <h1>Required-ness</h1>
1624 <pre><code>package Person;
1627 has first_name => (
1629 <span class="current">required => 1,</span>
1632 <span class="incremental">Person->new( first_name => undef ); # ok
1633 Person->new(); # kaboom</span></code></pre>
1637 <h1>Default and Builder</h1>
1640 <li>Attributes can have defaults</li>
1641 <li>Simple non-referecne scalars (number, string)</li>
1642 <li>Subroutine reference</li>
1643 <li>A builder method</li>
1651 <li>Can be a non-reference scalar (including <code>undef</code>)</li>
1654 <pre><code>package Person;
1659 default => 'Spire FCU',
1667 <li>Can be a subroutine reference</li>
1670 <pre><code>package Person;
1677 name => 'Spire FCU' ) },
1682 <h1>Default as a Subroutine Reference</h1>
1685 <li>Called as a method on the object</li>
1686 <li>Called anew for each object</li>
1691 <h1>Why No Other Reference Types?</h1>
1693 <pre><code>package Person;
1698 <span class="wrong">default => Bank->new(
1699 name => 'Spire FCU' ),</span>
1703 <li>Now <strong>every</strong> person shares the <strong>same</strong> Bank object!</li>
1708 <h1>Defaulting to an Empty Reference</h1>
1710 <pre><code>package Person;
1713 has packages => (
1715 default => <span class="highlight">sub { [] }</span>,
1720 <h1>What if I Want to Share?</h1>
1722 <pre><code>package Person;
1725 my $highlander_bank =
1726 Bank->new( name => 'Spire FCU' );
1730 default => sub { $highlander_bank },
1738 <li>A method <em>name</em> which returns the default</li>
1745 <pre><code>package Person;
1750 builder => '_build_bank',
1755 return Bank->new( name => 'Spire FCU' );
1760 <h1>Default vs Builder</h1>
1763 <li>Use default for simple scalars</li>
1764 <li>Use default to return empty references</li>
1765 <li>Use default for <em>very</em> trivial subroutine references</li>
1766 <li>Use builder for everything else</li>
1771 <h1>Builder Bonuses</h1>
1774 <li>Can be overridden and method modified, because it's called by <em>name</em></li>
1775 <li>Roles can require a builder</li>
1780 <h1>Role Requires Builder</h1>
1782 <pre><code>package HasBank;
1785 requires '_build_bank';
1789 builder => '_build_bank',
1794 <h1>Lazy, Good for Nothing Attributes</h1>
1797 <li>Normally, defaults are generated during object construction</li>
1798 <li>This can be expensive</li>
1799 <li>We want to default to <code>$self->size * 2</code>, but attribute initialization order is unpredictable</li>
1800 <li>Use lazy attributes!</li>
1805 <h1>The Power of Dynamic Defaults</h1>
1807 <pre><code>package Person;
1810 has shoe_size => (
1816 <h1>The Power of Dynamic Defaults</h1>
1818 <pre><code>has shoes => (
1820 <span class="highlight">lazy => 1,</span>
1821 builder => '_build_shoes',
1827 return Shoes->new(
1828 size => <span class="highlight">$_[0]->shoe_size</span> );
1833 <h1>Lazy is Good</h1>
1836 <li>Lazy defaults are executed when the attribute is read</li>
1837 <li>Can see other object attributes</li>
1838 <li>Still need to watch out for circular laziness</li>
1843 <h1>Clearer and Predicate</h1>
1846 <li>Attributes can have a value, including <code>undef</code>, or not</li>
1847 <li>Can clear the value with a clearer method</li>
1848 <li>Can check for the existence of a value with a predicate method</li>
1849 <li>By default, these methods are not created</li>
1854 <h1>Clearer and Predicate</h1>
1856 <pre><code>package Person;
1862 builder => '_build_account',
1863 <span class="highlight">clearer => '_clear_account',
1864 predicate => 'has_account',</span>
1869 <h1>Clearer and Lazy Defaults</h1>
1872 <li>Lazy defaults are good for computed attributes</li>
1873 <li>Clear the attribute when the source data changes</li>
1874 <li>Recalculated at next access</li>
1879 <h1>Renaming constructor arguments</h1>
1882 <li>By default, constructor names = attribute names</li>
1883 <li>Use <code>init_arg</code> to change this</li>
1884 <li>Set <code>init_arg => undef</code> to make it unconstructable</li>
1889 <h1>Some <code>init_arg</code> examples</h1>
1891 <pre><code>package Person;
1896 <span class="highlight">init_arg => 'foot_size',</span>
1899 Person->new( <span class="wrong">shoe_size => 13</span> );
1902 Person->new( <span class="right">foot_size => 13</span> );
1903 print $person->shoe_size;</code></pre>
1907 <h1>Some <code>init_arg</code> examples</h1>
1909 <pre><code>package Person;
1914 <span class="highlight">init_arg => undef,</span>
1917 Person->new( <span class="wrong">shoes => Shoes->new</span> );</code></pre>
1921 <h1>Why Set <code>init_arg => undef</code>?</h1>
1924 <li>Use this with a lazy default for attributes-as-cache</li>
1925 <li>Compute the value as needed</li>
1926 <li>Ensure that it is always generated correctly (not set by constructor)</li>
1927 <li>Use triggers or method modifiers (coming soon) to clear the value</li>
1932 <h1>Attribute Inheritance</h1>
1935 <li>By default, subclasses inherit attribute as-is</li>
1936 <li>Can change some attribute parameters in subclasses
1942 <li>others we've not yet covered</li>
1949 <h1>Attribute Inheritance Example</h1>
1951 <pre><code>package Employee;
1956 has '<span class="highlight">+first_name</span>' => (
1957 default => 'Joe',
1962 <h1>Attribute Inheritance Warning</h1>
1965 <li>An attribute is a contract about a class's API</li>
1966 <li>Don't break that contract in a subclass</li>
1967 <li>Especially important in the context of types</li>
1972 <h1>Changing Accessor Names</h1>
1974 <pre><code>package Person;
1977 has first_name => (
1978 <span class="highlight">reader</span> => 'first_name',
1979 <span class="highlight">writer</span> => 'first_name',
1983 <li>The long-hand version of <code>is => 'rw'</code></li>
1988 <h1>Changing Accessor Names</h1>
1990 <pre><code>package Person;
1993 has first_name => (
1994 <span class="highlight">reader</span> => 'first_name',
1995 <span class="highlight">writer</span> => undef,
1999 <li>The long-hand version of <code>is => 'ro'</code></li>
2005 <h1>Changing Accessor Names</h1>
2007 <pre><code>package Person;
2010 has first_name => (
2011 <span class="highlight">reader</span> => 'get_first_name',
2012 <span class="highlight">writer</span> => 'set_first_name,
2017 <h1>Changing Accessor Names</h1>
2019 <pre><code>package Person;
2022 has first_name => (
2023 <span class="highlight">is</span> => 'rw',
2024 <span class="highlight">writer</span> => '_first_name',
2028 <li>Can also mix-and-match</li>
2033 <h1>ETOOMUCHTYPING</h1>
2036 <li><code>MooseX::FollowPBP</code><br /><code>get_foo</code> and <code>set_foo</code></li>
2037 <li><code>MooseX::SemiAffordanceAccessor</code><br /><code>foo</code> and <code>set_foo</code></li>
2042 <h1>ETOOMUCHTYPING</h1>
2044 <pre><code>package Person;
2046 <span class="highlight">use MooseX::SemiAffordanceAccessor;</span>
2048 has first_name => (
2053 <li>Creates <code>first_name</code> and <code>set_first_name</code></li>
2061 # perl bin/prove -lv t/03-basic-attributes.t
2063 Iterate til this passes all its tests</pre>
2072 Copyright 2009 David Rolsky. All Rights Reserved.
2074 This work is licensed under a Creative Commons Attribution-Share Alike
2075 3.0 United States License See
2076 http://creativecommons.org/licenses/by-sa/3.0/us/ for details.