Finding type parameters via reflection in Scala 2.

Question

Using type tags, I'm able to see the parameters of some type:

scala> import scala.reflect.runtime.universe._
import scala.reflect.runtime.universe._

scala> typeOf[List[Int]]
res0: reflect.runtime.universe.Type = List[Int]

But I just can't quite figure out how to programmatically get that "Int" out of there, in a general way.

(I've been wandering around in REPL for an hour now, trying permutations on Type, to see what I can obtain from it... I get a lot of things which indicate this is a "List", but good luck on finding that "Int"! And I don't really want to resort to parsing the toString() output...)

Daniel Sobral has an excellent (as usual) quick overview here, in which he gets tantalizingly close to what I'm looking for, but (apparently) only if you happen to know, for that particular class, some specific method whose type can be interrogated:

scala> res0.member(newTermName("head"))
res1: reflect.runtime.universe.Symbol = method head

scala> res1.typeSignatureIn(res0)
res2: reflect.runtime.universe.Type = => Int

But I'm hoping for something more general, which doesn't involve rooting around in the list of declared methods and hoping that one of them will capture (and thus divulge) the tag's current type information somewhere.

If Scala can so easily print "List[Int]", why on earth is it so hard to discover that "Int" part of that -- without resorting to string pattern matching? Or am I just missing something really, really obvious?

scala> res0.typeSymbol.asInstanceOf[ClassSymbol].typeParams
res12: List[reflect.runtime.universe.Symbol] = List(type A)

scala> res12.head.typeSignatureIn(res0)
res13: reflect.runtime.universe.Type = 

Grr...

Answer 1

Starting with Scala 2.11, you can simply use:

yourGenericType.typeArgs.head

See the macros changelog point number 14.

Answer 2

Sadly, I don't think that there's a method that will give you the parameters, but you can get hold of them this way:

Welcome to Scala version 2.10.0-20121007-145615-65a321c63e (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_35).
Type in expressions to have them evaluated.
Type :help for more information.

scala> import scala.reflect.runtime.universe._
import scala.reflect.runtime.universe._

scala> typeOf[List[Int]]
res0: reflect.runtime.universe.Type = scala.List[Int]

scala> res0 match { case TypeRef(_, _, args) => args }
res1: List[reflect.runtime.universe.Type] = List(Int)

scala> res1.head
res2: reflect.runtime.universe.Type = Int

Edit Here's a slightly nicer way to achieve the same thing (following a discussion on scala-internals):

scala> res0.asInstanceOf[TypeRefApi].args
res1: List[reflect.runtime.universe.Type] = List(Int)