Are typeclasses possible on the JVM? I'm a .NET person, and I've always heard that typclasses aren't possible on the .NET CLR, so I wonder what's different about the JVM that makes this possible?
Whoever told you it's impossible in .NET is probably wrong. Don't you have interfaces in C# ?
But you don't even need interfaces. Strictly speaking, unless you use typeclasses with polymorphic recursion, you don't need a runtime representation for typeclasses at all.
Scala has typeclasses. It is not strictly equal to Haskell since it relies on the usage of `implicit`. Plus it doesn't enforce that there is only one typeclass per type in the classpath (I believe I remember that Haskell enforces that I compile time).
Implicit is basically just an implementation detail. The idea of only allowing one typeclass instance per data is generally referred as confluence and you're correct in that scala doesn't attempt to enforce it.
I understand the Haskell communities desire for coherent typeclasses, but I still find the newtype work around cludge to allow multiple implementation of, say, Monoid to be quasi hacky. What's worse, you can still fairly easily define multiple instances of the same typeclass accidentally (orphan instances) and the compiler won't catch it.
Maybe it was valueclasses you heard were not possible on the jvm? I think those are indeed possible on the CLR but not on the jvm. PS: yes I know scala has value classes, but sometimes those have to be instantiated.
