播放Json Reads和String

Question

I have the following JSON reader in Play 2.3:

import play.api.libs.json._
import play.api.libs.json.Reads._
val airportSearchReads: Reads[String] = (JsPath \ "search").read[String](minLength(3))

and the compiler gives me the error

diverging implicit expansion for type play.api.libs.json.Reads[M]
starting with method ArrayReads in trait DefaultReads

if I use an implicit val I get

 ambiguous implicit values:
 both value uuidReads in trait DefaultReads of type => play.api.libs.json.Reads[java.util.UUID]
 and value airportSearchReads in object AirportSearch of type => play.api.libs.json.Reads[String]
 match expected type play.api.libs.json.Reads[M]

How do I get it to work?

Answer 1

I get a different error, but it works fine for me if I add an explicit type parameter to minLength :

scala> val airportSearchReads: Reads[String] = (JsPath \ "search").read[String](minLength[String](3))
airportSearchReads: play.api.libs.json.Reads[String] = play.api.libs.json.Reads$$anon$8@3fee86da

I think the problem with leaving that up to the compiler is that there are different combinations of implicits in scope that would satisfy the implicit parameter list of minLength .

Answer 2

DefaultReads provides readers you need for transforming json values to common types ( String , Option , Array , etc.). So providing new readers for String is not necessary.

Hence, for accessing a field in your json object you don't need to define a reader, unless you want to read that field into an arbitrary type of yours.

All you need in this case is the constraint which is defined both in Reads and Constraints . So assuming that your json object is jsValue the following code gives you what you want:

// val jsValue = ...

(jsValue \ "search").as[String](Reads.minLength(3))