Scala By Example Chapter 9 Solutions - Exercise 9.2.1

Exercise 9.2.1 Design a tail-recursive version of length.

Ah, tail recursion again. Re-iterating that in a tail recursion, the last line of the function has to be a pure call to itself. To achieve this, an extra parameter needs to be passed to the function that keeps track of some relevant state.
We are getting good at this now, arent we?