Scala type parameter seems to be getting "stuck"

Question

I'm using lift-json to de-serialize JSON strings into Scala case classes. The JSON I'm parsing has a common structure: a data, success and error field, where data contains the interesting bits. I've created an APIResponse class to account for this structure, and a simple parse method which takes a type parameter for the type of the object contained in 'data':

import net.liftweb.json.Serialization.read
import net.liftweb.json.DefaultFormats

object JSONParseTest extends App {
  implicit val formats = DefaultFormats

  def parse[T: Manifest](json: String) = {
    read[APIResponse[T]](json)
  }

  val resultA = parse[TypeA](""" { "data": { "foo": "string" }, "success": true } """)
  println(resultA)
  val resultB = parse[TypeB](""" { "data": { "bar": "string" }, "success": true } """)
  println(resultB)
}

case class TypeA(foo: String)
case class TypeB(bar: String)

case class APIResponse[D](data: D, success: Boolean, error: Option[String]) {
  override def toString: String =
    if(success) {
      "SUCCESSFUL: " + data.toString
    } else {
      "ERROR: " + error.get
    }
}

Everything works great...for the first object parsed. For some reason, though, the second object parsed seems to get "stuck", performing as if it was passed the first type parameter instead of the second. As you can see in the output below, lift-json is looking for a 'foo' field in the JSON string and can't find it; 'foo' exists on TypeA, but not on TypeB. I've checked the manifest in the parse method, and it's got the proper type there. If I comment out the first parse/print, the second starts working. Any pointers on why this isn't behaving as expected? This is sorta boggling my mind.

SUCCESSFUL: TypeA(string)

Exception in thread "main" net.liftweb.json.MappingException: No usable value for data
No usable value for foo
Did not find value which can be converted into java.lang.String
    at net.liftweb.json.Meta$.fail(Meta.scala:191)
    at net.liftweb.json.Extraction$.mkValue$1(Extraction.scala:357)
    at net.liftweb.json.Extraction$.build$1(Extraction.scala:317)
    at net.liftweb.json.Extraction$$anonfun$12.apply(Extraction.scala:253)
    at net.liftweb.json.Extraction$$anonfun$12.apply(Extraction.scala:253)
    at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:233)
    at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:233)
    at scala.collection.LinearSeqOptimized$class.foreach(LinearSeqOptimized.scala:59)
    at scala.collection.immutable.List.foreach(List.scala:76)
    at scala.collection.TraversableLike$class.map(TraversableLike.scala:233)
    at scala.collection.immutable.List.map(List.scala:76)
    at net.liftweb.json.Extraction$.instantiate$1(Extraction.scala:253)
    at net.liftweb.json.Extraction$.newInstance$1(Extraction.scala:286)
    at net.liftweb.json.Extraction$.build$1(Extraction.scala:315)
    at net.liftweb.json.Extraction$.net$liftweb$json$Extraction$$extract0(Extraction.scala:366)
    at net.liftweb.json.Extraction$.net$liftweb$json$Extraction$$extract0(Extraction.scala:199)
    at net.liftweb.json.Extraction$.extract(Extraction.scala:43)
    at net.liftweb.json.JsonAST$JValue.extract(JsonAST.scala:300)
    at net.liftweb.json.Serialization$.read(Serialization.scala:58)
    at jmullin.api.Test$.parse(API.scala:11)
    at jmullin.api.Test$delayedInit$body.apply(API.scala:16)
    at scala.Function0$class.apply$mcV$sp(Function0.scala:34)
    at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:12)
    at scala.App$$anonfun$main$1.apply(App.scala:60)
    at scala.App$$anonfun$main$1.apply(App.scala:60)
    at scala.collection.LinearSeqOptimized$class.foreach(LinearSeqOptimized.scala:59)
    at scala.collection.immutable.List.foreach(List.scala:76)
    at scala.collection.generic.TraversableForwarder$class.foreach(TraversableForwarder.scala:30)
    at scala.App$class.main(App.scala:60)
    at jmullin.api.Test$.main(API.scala:7)
    at jmullin.api.Test.main(API.scala)
Caused by: net.liftweb.json.MappingException: No usable value for foo
Did not find value which can be converted into java.lang.String
    at net.liftweb.json.Meta$.fail(Meta.scala:191)
    at net.liftweb.json.Extraction$.mkValue$1(Extraction.scala:357)
    at net.liftweb.json.Extraction$.build$1(Extraction.scala:317)
    at net.liftweb.json.Extraction$$anonfun$12.apply(Extraction.scala:253)
    at net.liftweb.json.Extraction$$anonfun$12.apply(Extraction.scala:253)
    at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:233)
    at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:233)
    at scala.collection.LinearSeqOptimized$class.foreach(LinearSeqOptimized.scala:59)
    at scala.collection.immutable.List.foreach(List.scala:76)
    at scala.collection.TraversableLike$class.map(TraversableLike.scala:233)
    at scala.collection.immutable.List.map(List.scala:76)
    at net.liftweb.json.Extraction$.instantiate$1(Extraction.scala:253)
    at net.liftweb.json.Extraction$.newInstance$1(Extraction.scala:286)
    at net.liftweb.json.Extraction$.build$1(Extraction.scala:315)
    at net.liftweb.json.Extraction$.mkValue$1(Extraction.scala:351)
    ... 29 more
Caused by: net.liftweb.json.MappingException: Did not find value which can be converted into java.lang.String
    at net.liftweb.json.Meta$.fail(Meta.scala:191)
    at net.liftweb.json.Extraction$.convert(Extraction.scala:403)
    at net.liftweb.json.Extraction$.build$1(Extraction.scala:314)
    at net.liftweb.json.Extraction$.mkValue$1(Extraction.scala:351)
    ... 42 more

Answer 1

It turns out lift-json is doing some memoization of class to constructor mappings behind the scenes, ignoring type args. When the lookup is performed for the same class with different type parameters, the old constructor is returned, causing the confusion seen here. Mystery solved (though the problem isn't).