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POHow do I best make triggered accessors with defaults in Moose?
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  1. POHow do I best make triggered accessors with defaults in Moose?
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    copied!<p>I have a situation where I'd like to cache some calculations for use later. Let's say I have a list of allowed values. Since I'm going to be checking to see if anything is in that list I'm going to want it as a hash for efficiency and convenience. Otherwise I'd have to grep.</p> <p>If I'm using Moose it would be nice if the cache was recalculated each time the list of allowed values is changed. I can do that with a trigger easy enough...</p> <pre><code>has allowed_values =&gt; ( is =&gt; 'rw', isa =&gt; 'ArrayRef', trigger =&gt; sub { my %hash = map { $_ =&gt; 1 } @{$_[1]}; $_[0]-&gt;allowed_values_cache(\%hash); } ); has allowed_values_cache =&gt; ( is =&gt; 'rw', isa =&gt; 'HashRef', ); </code></pre> <p>And the two will stay in sync...</p> <pre><code>$obj-&gt;allowed_values([qw(up down left right)]); print keys %{ $obj-&gt;allowed_values_cache }; # up down left right </code></pre> <p>Now let's say I want a default for <code>allowed_values</code>, simple enough change...</p> <pre><code>has allowed_values =&gt; ( is =&gt; 'rw', isa =&gt; 'ArrayRef', trigger =&gt; sub { my %hash = map { $_ =&gt; 1 } @{$_[1]}; $_[0]-&gt;allowed_values_cache(\%hash); }, default =&gt; sub { return [qw(this that whatever)] }, ); </code></pre> <p>...except setting the default doesn't call the trigger. To get it to DWIM I need to duplicate the caching.</p> <pre><code>has allowed_values =&gt; ( is =&gt; 'rw', isa =&gt; 'ArrayRef', trigger =&gt; sub { $_[0]-&gt;cache_allowed_values($_[1]); }, default =&gt; sub { my $default = [qw(this that whatever)]; $_[0]-&gt;cache_allowed_values($default); return $default; }, ); sub cache_allowed_values { my $self = shift; my $values = shift; my %hash = map { $_ =&gt; 1 } @$values; $self-&gt;allowed_values_cache(\%hash); return; } </code></pre> <p>The Moose docs are explicit about <code>trigger</code> not getting called when the default is set, but it gets in the way. I don't like the duplication there.</p> <p>Is there a better way to do it?</p>
 

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