Context
I'm building an application that benefits from versatile collection wrappers, so I've set my sights on the Hack collection interfaces. Two of them are particularly promising: KeyedContainer<+Tk, +Tv> and ConstIndexAccess<Tk, +Tv>. The square-bracket access syntax allows Tk to be covariant in the former which has proven very useful — getter methods otherwise would require Tk in the argument (contravariant) position. That means I can access elements in sublinear time (argument-position Tk), or loop over the collection and access keys (return-position Tk) as I wish. The downside is that KeyedContainer is impossible to extend usefully: it doesn't define any methods.
At the moment, I need a mutable keyed collection type with an immutable ancestor. With this restriction on KeyedContainer, I'm considering biting the bullet by giving up keyed foreach and using ConstIndexAccess, which defines an explicit getter and has a mutable IndexAccess child from which MutableMap and MutableVector inherit.
Issue
The invariance of Tk makes ConstIndexAccess less powerful of a type, but also suggests that maybe there is some way to read keys from them. Is this the reason that ConstIndexAccess isn't contravariant on Tk? None of its descendants are covariant on Tk, and it is not possible to iterate using foreach. All other standard Hack contracts as far as I can tell have as flexible variances as possible except this one.