scala – 使用json4s提取字符串值

scala> import org.json4s._
import org.json4s._

scala> import org.json4s.native.JsonMethods._
import org.json4s.native.JsonMethods._

scala> val s = """ {"a": "hello"} """
s: String = " {"a": "hello"} "

scala> val json = parse(s)
json: org.json4s.JValue = JObject(List((a,JString(hello))))

scala> json  "a"
res0: org.json4s.JValue = JString(hello)

scala> res0.extract[String]
<console>:17: error: No org.json4s.Formats found. Try to bring an instance of org.json4s.Formats in scope or use the org.json4s.DefaultFormats.
              res0.extract[String]
                          ^

scala> import org.json4s.Formats._
import org.json4s.Formats._

scala> res0.extract[String]
<console>:20: error: No org.json4s.Formats found. Try to bring an instance of org.json4s.Formats in scope or use the org.json4s.DefaultFormats.
              res0.extract[String]
                          ^

scala> import org.json4s.DefaultFormats._
import org.json4s.DefaultFormats._

scala> res0.extract[String]
<console>:23: error: No org.json4s.Formats found. Try to bring an instance of org.json4s.Formats in scope or use the org.json4s.DefaultFormats.
              res0.extract[String]
                          ^

org.json4s.DefaultFormats,org.json4s.Formats和成员已经在范围内.如何解决这个问题？

EDIT1

从@mfirry的答案,这样做：

scala> implicit val formats = DefaultFormats
formats: org.json4s.DefaultFormats.type = org.json4s.DefaultFormats$@12b195f5

scala> val json = parse(""" {"a": "hello","b": 1.2} """)
json: org.json4s.JValue = JObject(List((a,JString(hello)),(b,JDouble(1.2))))

scala> (json  "b").extract[String]
res6: String = 1.2

scala> (json  "b").extract[Double]
res7: Double = 1.2