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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>Constraints are validation and conversion functions applied
to user input.</p></div>
<div class="paragraph"><p>They are used in various places in Cowboy, including the
router and the request match functions.</p></div>
<div class="sect1">
<h2 id="_syntax">Syntax</h2>
<div class="sectionbody">
<div class="paragraph"><p>Constraints are provided as a list of fields. For each field
in the list, specific constraints can be applied, as well as
a default value if the field is missing.</p></div>
<div class="paragraph"><p>A field can take the form of an atom <code>field</code>, a tuple with
constraints <code>{field, Constraints}</code> or a tuple with constraints
and a default value <code>{field, Constraints, Default}</code>.
The <code>field</code> form indicates the field is mandatory.</p></div>
<div class="paragraph"><p>Note that when used with the router, only the second form
makes sense, as it does not use the default and the field
is always defined.</p></div>
<div class="paragraph"><p>Constraints for each field are provided as an ordered list
of atoms or funs to apply. Built-in constraints are provided
as atoms, while custom constraints are provided as funs.</p></div>
<div class="paragraph"><p>When multiple constraints are provided, they are applied in
the order given. If the value has been modified by a constraint
then the next one receives the new value.</p></div>
<div class="paragraph"><p>For example, the following constraints will first validate
and convert the field <code>my_value</code> to an integer, and then
check that the integer is positive:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="color: #009900">PositiveFun</span> <span style="color: #990000">=</span> <span style="font-weight: bold"><span style="color: #0000FF">fun</span></span>(<span style="color: #009900">V</span>) <span style="font-weight: bold"><span style="color: #0000FF">when</span></span> <span style="color: #009900">V</span> <span style="color: #990000">></span> <span style="color: #993399">0</span> <span style="color: #990000">-></span> <span style="color: #000080">true</span>; (<span style="color: #990000">_</span>) <span style="color: #990000">-></span> <span style="color: #000080">false</span> <span style="font-weight: bold"><span style="color: #0000FF">end</span></span>,
{<span style="color: #FF6600">my_value</span>, [<span style="color: #FF6600">int</span>, <span style="color: #009900">PositiveFun</span>]}<span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>When there’s only one constraint, it can be provided directly
without wrapping it into a list:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>{<span style="color: #FF6600">my_value</span>, <span style="color: #FF6600">int</span>}</tt></pre></div></div>
</div>
</div>
<div class="sect1">
<h2 id="_built_in_constraints">Built-in constraints</h2>
<div class="sectionbody">
<div class="paragraph"><p>Built-in constraints are specified as an atom:</p></div>
<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">Converts 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_constraints">Custom constraints</h2>
<div class="sectionbody">
<div class="paragraph"><p>Custom constraints are specified as a fun. This fun takes
a single argument and must return one of <code>true</code>, <code>{true, NewValue}</code>
or <code>false</code>.</p></div>
<div class="paragraph"><p><code>true</code> indicates the input is valid, <code>false</code> otherwise.
The <code>{true, NewValue}</code> tuple is returned when the input
is valid and the value has been converted. For example,
the following constraint will convert the binary input
to an integer:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="font-weight: bold"><span style="color: #0000FF">fun</span></span> (<span style="color: #009900">Value0</span>) <span style="font-weight: bold"><span style="color: #0000FF">when</span></span> <span style="font-weight: bold"><span style="color: #000080">is_binary</span></span>(<span style="color: #009900">Value0</span>) <span style="color: #990000">-></span>
<span style="font-weight: bold"><span style="color: #0000FF">try</span></span> <span style="font-weight: bold"><span style="color: #000000">binary_to_integer</span></span>(<span style="color: #009900">Value0</span>) <span style="font-weight: bold"><span style="color: #0000FF">of</span></span>
<span style="color: #009900">Value</span> <span style="color: #990000">-></span> {<span style="color: #000080">true</span>, <span style="color: #009900">Value</span>}
<span style="font-weight: bold"><span style="color: #0000FF">catch</span></span> <span style="color: #990000">_:_</span> <span style="color: #990000">-></span>
<span style="color: #000080">false</span>
<span style="font-weight: bold"><span style="color: #0000FF">end</span></span><span style="color: #990000">.</span></tt></pre></div></div>
<div class="paragraph"><p>Constraint functions should only crash because the programmer
made an error when chaining constraints incorrectly (for example
if the constraints were <code>[int, int]</code>, and not because of input.
If the input is invalid then <code>false</code> must be returned.</p></div>
<div class="paragraph"><p>In our snippet, the <code>is_binary/1</code> guard will crash only
because of a programmer error, and the try block is there
to ensure that we do not crash when the input is invalid.</p></div>
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