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<title>Nine Nines: Constraints</title>
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<h1 class="lined-header"><span>Constraints</span></h1>
<div class="paragraph"><p>Cowboy provides an optional constraints based validation feature
when interacting with user input.</p></div>
<div class="paragraph"><p>Constraints are first used during routing. The router uses
constraints to more accurately match bound values, allowing
to create routes where a segment is an integer for example,
and rejecting the others.</p></div>
<div class="paragraph"><p>Constraints are also used when performing a match operation
on input data, like the query string or cookies. There, a
default value can also be provided for optional values.</p></div>
<div class="paragraph"><p>Finally, constraints can be used to not only validate input,
but also convert said input into proper Erlang terms, all in
one step.</p></div>
<div class="sect1">
<h2 id="_structure">Structure</h2>
<div class="sectionbody">
<div class="paragraph"><p>Constraints are provided as a list of fields and for each
field a list of constraints for that field can be provided.</p></div>
<div class="paragraph"><p>Fields are either the name of the field; the name and
one or more constraints; or the name, one or more constraints
and a default value.</p></div>
<div class="paragraph"><p>When no default value is provided then the field is required.
Otherwise the default value is used.</p></div>
<div class="paragraph"><p>All constraints for a field will be used to match its value
in the order they are given. If the value is modified by a
constraint, the next constraint receives the updated value.</p></div>
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<div class="sect1">
<h2 id="_built_in_constraints">Built-in constraints</h2>
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<div class="tableblock">
<table rules="all"
width="100%"
frame="border"
cellspacing="0" cellpadding="4">
<col width="50%" />
<col width="50%" />
<thead>
<tr>
<th align="left" valign="top"> Constraint </th>
<th align="left" valign="top"> Description</th>
</tr>
</thead>
<tbody>
<tr>
<td align="left" valign="top"><p class="table">int</p></td>
<td align="left" valign="top"><p class="table">Convert binary value to integer.</p></td>
</tr>
<tr>
<td align="left" valign="top"><p class="table">nonempty</p></td>
<td align="left" valign="top"><p class="table">Ensures the binary value is non-empty.</p></td>
</tr>
</tbody>
</table>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_custom_constraint">Custom constraint</h2>
<div class="sectionbody">
<div class="paragraph"><p>In addition to the predefined constraints, Cowboy will accept
a fun. This fun must accept one argument and return one of
<code>true</code>, <code>{true, NewValue}</code> or <code>false</code>. The result indicates
whether the value matches the constraint, and if it does it
can optionally be modified. This allows converting the value
to a more appropriate Erlang term.</p></div>
<div class="paragraph"><p>Note that constraint functions SHOULD be pure and MUST NOT crash.</p></div>
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