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# Register all of our own type checks at build time
sub BUILD {
    my $self = shift;

    $self->register_type_check 'Str'         => sub {...};
    $self->register_type_check 'ShortStr'    => sub {...};
    $self->register_type_check 'PositiveInt' => sub {...};
    $self->register_type_check 'SHA1'        => sub {...};

    # Maybe more?
    ...
}</pre>

<p>Now we have our own plugin that also provides <code>with_types</code> which uses
the original plugin with a set of registered type checks.</p>
<pre class="prettyprint">package MyApp;
use Dancer2;
use Dancer2::Plugin::MyCommonTypes;

post '/:entity/update/:id' =&gt; with_types [
    [ 'route', 'entity',  'Str'         ],
    [ 'route', 'id',      'PositiveInt' ],
    [ 'body',  'message', 'Str'         ],

    'optional' =&gt; [ 'body', 'sid', 'SHA1' ],
] =&gt; sub {
    my ( $entity, $id ) = @{ route_parameters() }{qw&lt; id entity &gt;};
    my $message = body_parameters-&gt;{'message'};
    my $sid     = body_parameters-&gt;{'sid'} || '';

    # everything is validated and required parameters are checked
    ...
};</pre>

<h2><a name="could_i_do_more_with_it"></a>Could I do more with it?</h2>

<p>Absolutely!</p>
<h3><a name="handle_multiple_sources"></a>Handle multiple sources</h3>

<p>Normally, you would dictate to a user how they should send their
paramters (in the query, in the body, or as part of the path - in the
route), but sometimes you cannot control this. Maybe you're maintaining
an old interface or supporting outdated APIs.</p>
<p><a href="https://metacpan.org/module/Dancer2::Plugin::ParamTypes">Dancer2::Plugin::ParamTypes</a> is flexible enough to support multiple
sources for an argument:</p>
<pre class="prettyprint">any [ 'get', 'post' ] =&gt; '/:entity/:id' =&gt; with_types [
    [ 'route',             'entity', 'Str' ],
    [ 'route',             'id',     'PositiveInt' ],
    [ [ 'query', 'body' ], 'format', 'Str' ],

    'optional' =&gt; [ 'body', 'sid', 'SHA1' ],
] =&gt; sub {...};</pre>

<p>In this form, the parameter <code>format</code> can be provided either in the
query string or in the body, because your route might be either a
<b>GET</b> or a <b>POST</b>.</p>
<h3><a name="register_type_actions"></a>Register type actions</h3>

<p>Type checking itself is the main role of this plugin, but you can also
control how it behaves.</p>
<p>The default action to perform when a type check fails is to error out,
but you can decide to act differently by registering a different
action.</p>
<pre class="prettyprint">register_type_action 'SoftError' =&gt; sub {
    my ( $self, $details ) = @_;

    warning "Parameter $details-&gt;{'name'} from $details-&gt;{'source'} "
          . "failed checking for type $details-&gt;{'type'}, called "
          . "action $details-&gt;{'action'}";

    return;
};

get '/:id' =&gt; with_types [
    [ 'query', 'age', 'Int', 'SoftError',
] =&gt; sub {...};</pre>

<p>On a bad <code>age</code> parameter, it will print out the following warning:</p>
<pre class="prettyprint">Parameter age from query failed checking for type Int, called action SoftError</pre>

<p>This means you can also register a set of actions that you want to call
in different cases.</p>
<h2><a name="conclusion"></a>Conclusion</h2>

<p><a href="https://metacpan.org/module/Dancer2::Plugin::ParamTypes">Dancer2::Plugin::ParamTypes</a> allows you to define your own types and
your own actions, to create your own plugin that helps you maintain
reusability and consistency across your application with fewer code
duplication and less effort.</p>
<h2><a name="author"></a>Author</h2>

<p>This article has been written by Sawyer X for the Perl
Dancer Advent Calendar 2018.</p>
<h2><a name="copyright"></a>Copyright</h2>

<p>No copyright retained. Enjoy.</p>
<p>2018 // Sawyer X <code>&lt;xsawyerx@cpan.org&gt;</code></p>
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