I guess Go is the only language which comes close, which has map and array/slice which cannot be implemented in the language itself. Though it's the lack of gen

I guess Go is the only language which comes close, which has map and array/slice which cannot be implemented in the language itself. Though it's the lack of generics, not ad-hoc polymorphism which is Go's lack. Then again, it wouldn't surprise me if of all the languages you've thought about, Elm is the only one which hasn't reached 1.0 yet. I think Elm will get some mechanism of ad-hoc polymorphism before it reaches 1.0, but I don't think it will be type classes. Though, I have been wrong before.