15.Answer

// It's because `foldRight`, `foldLeft`, and `foldMap` don't give us any way of constructing a value of the foldable type. In order to `map` over a structure, you need the ability to create a new structure (such as `Nil` and `Cons` in the case of a `List`). `Traverse` is able to extend `Functor` precisely because a traversal preserves the original structure. 
// An example of a Foldable that is not a functor:

    case class Iteration[A](a: A, f: A => A, n: Int)

// This class conceptually represents a sequence of `A` values, generated by repeated function application starting from some seed value. But can you see why it's not possible to define `map` for this type?