It died, because .net supports reified types, which make interoperability pretty much impossible.
Contrary to popular opinion, erasure is a superior solution to reification for many reasons, and this is one of them.
Contrary to popular opinion, erasure is a superior solution to reification for many reasons, and this is one of them.
Java's generics can't be built on top of the CLR's reified generics, but this goes for other languages as well - Haskell, ML, Scala, you name it. Basically, reified generics work well in case you design the language in combination with the VM, otherwise it becomes a huge PITA.
FYI, the CLR's reified generics is one reason for why the JVM is a better target for other languages. For dynamic languages, they are a problem because the bytecode generator has to work around them. For static languages, such as Scala or anything in the ML family, it's a problem because either (a) they are designed for co/contra-variance or (b) they don't support higher-kinded types or (c) they don't support rank-2 types or (d) in case you do need co/contra-variance rules, for some reason Microsoft chose to restrict the declarations only to generic interfaces and delegates, so you can't design a List<+T> class for example (to be honest, I'm not sure if this last one is a limitation of the language or the CLR).
Also, having reified generics is less important than you think. Scala for example can do specialization for primitive types and because the type-system is much stronger, coupled with an awesome collections library and pattern matching, the flow of the types is much better. For example, I can't remember any instance in which I felt the need to do a (obj isInstanceOf List<int>) check, immutable collections can be covariant without issues and for everything else there are type-classes, something which C# sorely lacks.
IMHO, in spite of people's opinion on the matter, Java's type-erased generics is one of its best features.
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_...
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?
What do you think about being able to use Void in Generics? Java's issue of not supporting primitive types pales in comparison to that.
Also, while reified generics and structs are fine, most of the optimizations one would reasonably suspect to be supported are just not there. Some time ago, it was still faster to pass value types by reference ... that's just embarrassing. Maybe RyuJIT does better here?