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The reason the long handed version works is because scala's automatic type inference triggers an implicit conversion <code>toJsFieldJsValueWrapper</code>. For example <pre><code>scala> import play.api.libs.json._ import play.api.libs.functional.syntax._ import play.api.libs.json._ scala> import play.api.libs.functional.syntax._ import play.api.libs.functional.syntax._ scala> import Json._ import Json._ scala> arr("invalid", "tooshort") res0: play.api.libs.json.JsArray = ["invalid","tooshort"] scala> obj("name" -> arr("invalid", "tooshort")) res1: play.api.libs.json.JsObject = {"name":["invalid","tooshort"]} scala> obj _ res3: Seq[(String, play.api.libs.json.Json.JsValueWrapper)] => play.api.libs.json.JsObject = <function1> </code></pre> Notice that <code>arr</code> returns a <code>JsArray</code>, however <code>obj</code> requires a <code>JsValueWrapper</code>. Scala is able to convert the <code>JsArray</code> to <code>JsValueWrapper</code> when it constructs the arguments for <code>obj</code>. However it is not able to convert a <code>Seq[(String, JsArray)]</code> to a `Seq[(String, JsValueWrapper)]. If you provide the expected type when of the sequence in advance, the Scala compiler's type inferencer will perform the conversion as it creates the sequence. <pre><code>scala> Seq[(String, JsValueWrapper)]("name" -> arr("invalid", "tooshort"), "email" -> arr("invalid")) res4: Seq[(String, play.api.libs.json.Json.JsValueWrapper)] = List((name,JsValueWrapperImpl(["invalid","tooshort"])), (email,JsValueWrapperImpl(["invalid"]))) </code></pre> However once the sequence is created it cannot be converted unless there is an implicit conversion in scope that can convert sequences. The final code snippet looks like: <pre><code>def error = Action { val e = Seq[(String, JsValueWrapper)]("name" -> arr("invalid", "tooshort"), "email" -> arr("invalid")) // in real app it will be Seq[(JsPath, Seq[ValidationError])] BadRequest(obj( "errors" -> obj(e: _*) )) } </code></pre>
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1. POScala play json api: can't implicitly convert JsArray to JsValueWrapper
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1. COThanks for an explanation! Implicit conversion to JsValueWrapper played a trick with my mind at first.
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