I was reading through the Hacklang docs on Collections, and came across this curious definition [paraphrased slightly]:

interface KeyedIterable<Tk, Tv> ...
interface ConstVector<+Tv> implements KeyedIterable<int, Tv>  ...

Of course, KeyedIterable must be read-only with respect to Tv, and indeed it is since its methods cannot modify its type. It could be specified to be covariant on at least Tv, but isn't.

I was wondering what impetus there could be to do this?

I was reading through the Hacklang docs on Collections, and came across this curious definition [paraphrased slightly]:

interface KeyedIterable<Tk, Tv> ...
interface ConstVector<+Tv> implements KeyedIterable<int, Tv>  ...

Of course, KeyedIterable must be read-only with respect to Tv, and indeed it is since its methods cannot modify its type. It could be specified to be covariant on at least Tv, but isn't.

I was wondering what impetus there could be to do this?

Edit: I read the definition wrong, KeyedIterable is actually defined as KeyedIterable<Tk, +Tv>, so this definition makes perfect sense. I'm just realizing now that it's not so much "should or should not", but rather "this is never possible".