I was under the impression that Scala just used Manifests and type tokens (a la my colleague Neal Gafter's Super Type Tokens) to get around the problem of erasure. I don't see any evidence that this actually works better, though.

    scala> val list = List(null, 1, 2, "any")
    list: List[Any] = List(null, 1, 2, any)

    scala> list.collect { case x: Int => x }
    res0: List[Int] = List(1, 2)

    scala> Vector(null, "hello", 3).collectFirst { case i: Int => i * 2 }
    res1: Option[Int] = Some(6)

    scala> import collection.immutable.BitSet
    
    scala> BitSet(1,2,3).map(_ + 1)
    res2: immutable.BitSet = BitSet(2, 3, 4)

    scala> BitSet(1,2,3).map(_.toString)
    res3: immutable.SortedSet[String] = TreeSet(1, 2, 3)

    scala> import scala.reflect.ClassTag
    scala> def myAwesomeArray[T : ClassTag] = new Array[T](10)

    scala> myAwesomeArray[Int]
    res4: Array[Int] = Array(0, 0, 0, 0, 0, 0, 0, 0, 0, 0)

    def sum(list: List[String]) = ???
    def sum(list: List[Int]) = ???

    trait Monoid[T] { 
      def plus(x: T, y: T): T
      def zero: T 
    }

    implicit object IntMonoid extends Monoid[Int] { 
      def plus(x: Int, y: Int) = x + y
      val zero = 0 
    }

    implicit object StringMonoid extends Monoid[String] { 
      def plus(x: String, y: String) = x + y
      val zero = "" 
    }

    def sum[T : Monoid](list: List[T]) = {
      val monoid = implicitly[Monoid[T]]
      list.foldLeft(monoid.zero)(monoid.plus)
    }

    scala> sum(List("a", "b", "c", "d"))
    res5: String = abcd

    scala> sum(List(1,2,3,4))
    res6: Int = 10

    trait SetBuilder[T, R] {
      def empty: R
      def buildInstance(param: T): R
      def concat(s1: R, s2: R): R
    }

    implicit object IntSetBuilder extends SetBuilder[Int, BitSet] {
      def empty = BitSet.empty
      def buildInstance(param: Int) = BitSet(param)
      def concat(s1: BitSet, s2: BitSet) = 
        s1 ++ s2
    }

    implicit object StringSetBuilder extends SetBuilder[String, Set[String]] {
      def empty = Set.empty[String]
      def buildInstance(param: String) = Set(param)
      def concat(s1: Set[String], s2: Set[String]) =
        s1 ++ s2
    }

    def buildSetFrom[T,R](params: T*)(implicit builder: SetBuilder[T,R]): R = {
      if (params.nonEmpty)
        builder.concat(
          builder.buildInstance(params.head),
          buildSetFrom(params.tail : _*)
        )
      else
        builder.empty
    }  

    scala> buildSetFrom(1,2,3)
    res7: scala.collection.immutable.BitSet = BitSet(1, 2, 3)

    scala> buildSetFrom("a", "b", "c")
    res8: Set[String] = Set(a, b, c)

At worst I could see how it could be a trivial implementation detail, but the fact that Scala went out of their way to use tokens and manifests suggests that the functionality is desired. Whether that's provided by the runtime or by the compiler writer is only a difference in effort (or rather, whose effort).

> The big ML-.NET type system mismatches have to do with things like pervasive overloading in .NET making type inference difficult, nothing to do with reified generics

You've hinted at a side-effect, but no, it has to do with subtyping [1], which naturally leads to co/contra-variance [2]. Or to put it bluntly, in a nominal type system that has sub-typing, Hindley-Milner type-inference is not only "difficult" but actually impossible. And because F# had to be interoperable with .NET, then it needed C# generics. Pity, because there's many things that F# misses, like Ocaml functors, structural typing, type-classes, higher-kinded types and the list can continue. And I just love how List.sum<^T> is defined [3] (static member, FTW).

[1] https://en.wikipedia.org/wiki/Subtyping

[2] https://en.wikipedia.org/wiki/Covariance_and_contravariance_...

[3] http://msdn.microsoft.com/en-us/library/ee353634.aspx

In any case, it is true that F#'s type systems has concepts that the CLR doesn't natively support, but I don't see how this demonstrates any weaknesses in the CLR or F#. The exact same thing is true of Scala on the JVM, as far as I can tell - how are erased generics an improvement?