Length.hs

length' xs  = lengthHelper xs 0 -- We're passing in the full list and 0 as the starting value for rsf
        where lengthHelper (x:xs) rsf -- Separate xs into x (head) and xs (tail)
          | xs == []  = rsf -- If the tail is empty, we're at the end. At this point, return rsf — we're done
          | otherwise = lengthHelper xs (rsf + 1) -- If the tail isn't empty, pass in the tail (the rest of the list) and rsf + 1, since we're counting the head (x) right now