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POUsing Reads, Writes and Formats in Play framework
 

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    <p>I have some models in a Play! application that I would like to serialize/deserialize to and from JSON. I used to have separate methods for that, but I have seen that the preferred way is to give an implicit instance of <code>Formats[T]</code> or <code>Reads[T]</code>, like</p> <pre><code>import play.api.libs.json.{ JsValue, Reads } case class Foo(bar: Int, ...) object Foo { implicit object FooReads extends Reads[Foo] { def reads(json: JsValue): Foo = //whatever } } </code></pre> <p>Now, it may happen that the model has the correct fields in the JSON, but it does not validate. In this case, I am not able to deserialize - I should get an exception when using <code>json.as[Foo]</code> or <code>None</code> when using <code>json.asOpt[Foo]</code>.</p> <p>If I throw an exception when I find a field that does not validate, everything seems to work as expected. But I took the care of trying to find out what exception I should throw, and in the source for <code>JsValue</code> I found this</p> <pre><code>def asOpt[T](implicit fjs: Reads[T]): Option[T] = fjs.reads(this).fold( valid = v =&gt; Some(v), invalid = _ =&gt; None ).filter { case JsUndefined(_) =&gt; false case _ =&gt; true } </code></pre> <p>Now, I cannot understand how this is supposed to work. The implicit instance of <code>fjs</code> is supplied by myself in the companion object, so I know that <code>fjs.reads(this)</code> either returns a <code>Foo</code> or throws an exception.</p> <p>Where is this <code>fold</code> coming from? It certainly is not a method on <code>Foo</code>. I guess one could have an implicit conversion, but it should be from <code>Any</code> to something with a <code>fold</code> method, so it could not be of much interest. Even worse, if <code>fjs.reads(this)</code> throws an exception, there is nothing to catch it!</p> <blockquote> <p>So, how should one handle invalid input in the JSON in an instance of <code>Reads[T]</code>? And how does the mechanism above actually work?</p> </blockquote>
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