Scala: Higher kinded, open-type and wild card generics in Java, C#, Scala and C++
c#, c++, generics, java, scala
Solution
This is higher kinded type is it not:
No. A higher-kinded type is something like
class Class1<T>
{
T<String> foo; // won't compile in actual C#
}
I.e. a generic type whose parameters are required to be generic themselves. Note that in this example `Class1<IList>` should compile, but `Class1<String>` or `Class1<IDictionary>` should not.
Problem
I'd been programming in C#, but was frustrated by the limitations of its type system. One of the first things, I learned about Scala was that Scala has higher kinded generics. But even after I'd looked at a number of articles, blog entries and questions I still wasn't sure what higher kinded generics were. Anyway I'd written some Scala code which compiled fine, Does this snippet use higher kinds? ``` abstract class Descrip [T <: DTypes, GeomT[_ <: DTypes] <: GeomBase[_]](newGeom: NewGeom[GeomT]) { type GeomType = GeomT[T] val geomM: GeomT[T] = newGeom.apply[T]() } ``` And then I thought maybe I'm already using higher kinded generics. As I understand it I was, but then as I now I understand it I had already been happily using higher kinded types in C# before I'd even heard of Scala. Does this snipet use higher kinded type? ``` namespace ConsoleApplication3 { class Class1<T> { List<List<T>> listlist; } } ``` So to avert further confusion I thought it would be useful to clarify for each of Java, C# and Scala what they allow in terms of Higher kinded types, wild cards and the use of open / partially open types. As the key difference between C# and Scala seems to be that Scala allows wild cards and open types, where as C# has no wild card and requires all generic types to be closed before use. I know they are some what different but I think it would be useful to relate the existence of these features to their equivalent in C++ Templates. So is the following correct? This table has been corrected for Alexey's answer ``` Lang: Higher-kind Wild-card Open-types Scala yes yes yes C# no no no Java no yes no C++ yes yes yes ```