10.Answer

/*
It's common practice to annotate functions you expect to be tail-recursive with the `tailrec` annotation. If the
function is not tail-recursive, it will yield a compile error, rather than silently compiling the code and resulting
in greater stack space usage at runtime.
*/
@annotation.tailrec
def foldLeft[A,B](l: List[A], z: B, f: (B, A) => B): B = l match
  case Nil => z
  case Cons(h,t) => foldLeft(t, f(z,h), f)