Can't even try to load a class in Scala - scala

Question

This is one of those times where you go, What do you mean, it doesn't compile?
That's not a rhetorical question: What's the shortest or idiomatickest fix? For bonus points, why is it necessary?
scala> import scala.util.Try
import scala.util.Try
scala> Try { getClass.getClassLoader loadClass "scala.util.Try" }
I hope this doesn't give the game away, but here's the message:
<console>:9: error: type mismatch;
found : Class[_]
required: Class[?0(in value res0)] where type ?0(in value res0)
Note: Any >: ?0, but Java-defined class Class is invariant in type T.
You may wish to investigate a wildcard type such as `_ >: ?0`. (SLS 3.2.10)
Try { getClass.getClassLoader loadClass "scala.util.Try" }
By "investigate", do they mean like basic research, or just apply techniques already available in the literature?
I'm still waiting for that error message that concludes, "Left as an exercise for the reader."
Update:
This is an exercise for Scala 2.10.
As usual, all good things come to those that wait:
apm#mara:~/tmp$ skala
Welcome to Scala version 2.11.0-20130622-103744-990c2b024a (OpenJDK 64-Bit Server VM, Java 1.7.0_21).
Type in expressions to have them evaluated.
Type :help for more information.
scala> import scala.language.existentials
import scala.language.existentials
scala> import scala.util.Try
import scala.util.Try
scala> Try { getClass.getClassLoader loadClass "scala.util.Try" }
res0: scala.util.Try[Class[?0]] forSome { type ?0 } = Success(class scala.util.Try)

Answer 1

This must surely be a duplicate question. Maybe someone can point it out, or exactly how type inference is not doing what comes naturally here.
Someone left an answer (which seems to have disappeared?) with a helpful link to MacIver on existential types. Possibly, I also need help from MacGyver.
Here are some variants I tried on the way to the forum.
package classy
import scala.util._
class Foo
object Test {
/* DNC
def loadTry(n: String, loader: ClassLoader) = Try { loader loadClass n }
def loadTry(n: String, loader: ClassLoader): Try[Class[_]] = Try { loader loadClass n }
*/
def main(args: Array[String]) {
val cl = getClass.getClassLoader
println(loadTry("classy.Foo", cl))
println(loadTry("classy.Bar", cl))
println(cl loadClass "classy.Foo")
println(loadOpt("classy.Foo", cl))
println(loadTryAgain("classy.Foo", cl))
println(loadTryYetAgain("classy.Foo", cl))
}
def loadOpt(n: String, loader: ClassLoader): Option[Class[_]] =
try Some(loader loadClass n) catch {
case _: Exception => None
}
def loadTryAgain(n: String, loader: ClassLoader): Try[Class[_]] = {
val res: Option[Class[_]] = try Some(loader loadClass n) catch {
case _: Exception => None
}
res match {
case None =>
Failure(new RuntimeException(s"Warning: class not found: ${n})"))
case Some(x) =>
Success(x)
}
}
def loadTryYetAgain(n: String, loader: ClassLoader): Try[Class[_]] = {
val res = try loader loadClass n catch {
case _: Exception => null
}
res match {
case null =>
Failure(new RuntimeException(s"Warning: class not found: ${n})"))
case x =>
Success(x)
}
}
def loadTry(n: String, loader: ClassLoader) =
Try[Class[_]] {
loader loadClass n
} recoverWith {
case e: Exception =>
Failure(new RuntimeException(s"Warning: class not found: ${n} (${e.getMessage})"))
}
}

Answer 2

the Try is causing it. for me in scala 2.10.0:
scala> import scala.util.Try
scala> val typeName = "scala.util.Try"
error:
scala> Try(Class.forName(typeName))
<console>:10: error: type mismatch;
found : Class[_]
required: Class[?0(in value res1)] where type ?0(in value res1)
Note: Any >: ?0, but Java-defined class Class is invariant in type T.
You may wish to investigate a wildcard type such as `_ >: ?0`. (SLS 3.2.10)
Try(Class.forName(typeName))
^
no error:
scala> Try[Class[_]](Class.forName(typeName))
res2: scala.util.Try[Class[_]] = Success(class scala.util.Try)
catching also has the same problem:
scala> import scala.util.control.Exception._
scala> catching(classOf[Throwable]) opt Class.forName(typeName)
<console>:13: error: type mismatch;
found : Class[_]
required: Class[?0(in value res4)] where type ?0(in value res4)
Note: Any >: ?0, but Java-defined class Class is invariant in type T.
You may wish to investigate a wildcard type such as `_ >: ?0`. (SLS 3.2.10)
catching(classOf[Throwable]) opt Class.forName(typeName)