So how do our \hinline{sum}, \hinline{prod} and \hinline{length} functions look like if we use our \hinline{fold} abstraction?
\pause
\begin{haskellcode}
sum :: [Int] -> Int
sum xs = fold 0 (\x y -> x + y) xs
-- a Haskeller would write
sum = fold 0 (+)

prod :: [Int] -> Int
prod xs = fold 1 (\x y -> x * y) xs

length :: [a] -> Int
length xs = fold 0 (\x y -> 1 + y) xs
\end{haskellcode}