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authorLoïc Hoguin <[email protected]>2018-04-04 13:13:37 +0200
committerLoïc Hoguin <[email protected]>2018-04-04 13:13:37 +0200
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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 <code>cowboy_req</code> 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
+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: #990000">_</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">&gt;</span> <span style="color: #993399">0</span> <span style="color: #990000">-&gt;</span>
+ {<span style="color: #FF6600">ok</span>, <span style="color: #009900">V</span>};
+ (<span style="color: #990000">_</span>, <span style="color: #990000">_</span>) <span style="color: #990000">-&gt;</span>
+ {<span style="color: #FF6600">error</span>, <span style="color: #FF6600">not_positive</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>We ignore the first fun argument in this snippet. We shouldn&#8217;t.
+We will simply learn what it is later in this chapter.</p></div>
+<div class="paragraph"><p>When there&#8217;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
+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
+two arguments. The first argument indicates the operation
+to be performed, and the second is the value. What the
+value is and what must be returned depends on the operation.</p></div>
+<div class="paragraph"><p>Cowboy currently defines three operations. The operation
+used for validating and converting user input is the <code>forward</code>
+operation.</p></div>
+<div class="listingblock">
+<div class="content"><!-- Generator: GNU source-highlight
+by Lorenzo Bettini
+http://www.lorenzobettini.it
+http://www.gnu.org/software/src-highlite -->
+<pre><tt><span style="font-weight: bold"><span style="color: #000000">int</span></span>(<span style="color: #FF6600">forward</span>, <span style="color: #009900">Value</span>) <span style="color: #990000">-&gt;</span>
+ <span style="font-weight: bold"><span style="color: #0000FF">try</span></span>
+ {<span style="color: #FF6600">ok</span>, <span style="font-weight: bold"><span style="color: #000000">binary_to_integer</span></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">-&gt;</span>
+ {<span style="color: #FF6600">error</span>, <span style="color: #FF6600">not_an_integer</span>}
+ <span style="font-weight: bold"><span style="color: #0000FF">end</span></span>;</tt></pre></div></div>
+<div class="paragraph"><p>The value must be returned even if it is not converted
+by the constraint.</p></div>
+<div class="paragraph"><p>The <code>reverse</code> operation does the opposite: it
+takes a converted value and changes it back to what the
+user input would have been.</p></div>
+<div class="listingblock">
+<div class="content"><!-- Generator: GNU source-highlight
+by Lorenzo Bettini
+http://www.lorenzobettini.it
+http://www.gnu.org/software/src-highlite -->
+<pre><tt><span style="font-weight: bold"><span style="color: #000000">int</span></span>(<span style="color: #FF6600">reverse</span>, <span style="color: #009900">Value</span>) <span style="color: #990000">-&gt;</span>
+ <span style="font-weight: bold"><span style="color: #0000FF">try</span></span>
+ {<span style="color: #FF6600">ok</span>, <span style="font-weight: bold"><span style="color: #000000">integer_to_binary</span></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">-&gt;</span>
+ {<span style="color: #FF6600">error</span>, <span style="color: #FF6600">not_an_integer</span>}
+ <span style="font-weight: bold"><span style="color: #0000FF">end</span></span>;</tt></pre></div></div>
+<div class="paragraph"><p>Finally, the <code>format_error</code> operation takes an error
+returned by any other operation and returns a formatted
+human-readable error message.</p></div>
+<div class="listingblock">
+<div class="content"><!-- Generator: GNU source-highlight
+by Lorenzo Bettini
+http://www.lorenzobettini.it
+http://www.gnu.org/software/src-highlite -->
+<pre><tt><span style="font-weight: bold"><span style="color: #000000">int</span></span>(<span style="color: #FF6600">format_error</span>, {<span style="color: #FF6600">not_an_integer</span>, <span style="color: #009900">Value</span>}) <span style="color: #990000">-&gt;</span>
+ <span style="font-weight: bold"><span style="color: #000000">io_lib:format</span></span>(<span style="color: #FF0000">"The value ~p is not an integer."</span>, [<span style="color: #009900">Value</span>])<span style="color: #990000">.</span></tt></pre></div></div>
+<div class="paragraph"><p>Notice that for this case you get both the error and
+the value that was given to the constraint that produced
+this error.</p></div>
+<div class="paragraph"><p>Cowboy will not catch exceptions coming from constraint
+functions. They should be written to not emit any exceptions.</p></div>
+</div>
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