You could do the polymorphic return with an enumeration. But those are distinct types with very different semantics: you can’t just say “it returns a function that you can call once or maybe a function you can call more than once. Shrug”.
I believe they're asking for "this function returns a function of the same type (Fn/FnMut/FnOnce) as its first argument, but with different arguments. A generic way to write something like `fn bind<T, F: Fn(T,...) >(f: F, arg: T) -> Fn(...)`
> But those are distinct types with very different semantics: you can’t just say “it returns a function that you can call once or maybe a function you can call more than once. Shrug”.
Right, that's why I want parametricity i.e. HKT. String and Int are different types with very different semantics, you can't just say "it returns a String or maybe an Int, shrug", but it's very useful to be able to write generic code and datastructures (e.g. collections) that work for String and Int.
