There is also a function that folds from the \emph{left} which is also in the \emph{Prelude} and called \textbf{foldl}.\\ To summarize: \begin{haskellcode} foldr f z [a,b,c] == a `f` (b `f` (c `f` z)) foldl f z [a,b,c] == ((z `f` a) `f` b) `f` c \end{haskellcode} For \code{foldl} the \code{z} is sort of the starting value. \vspace{\baselineskip} \\ \pause We can even express foldl in terms of foldr and vice versa. If you are interested, have a look here:\\ \url{http://lambda.jstolarek.com/2012/07/expressing-foldl-in-terms-of-foldr/} \vspace{\baselineskip} \\ You should definitely look them up in the Prelude and play with them: \url{https://hackage.haskell.org/package/base-4.8.0.0/docs/Prelude.html} \vspace{\baselineskip} \\ GHCi... \begin{haskellcode} > foldr (-) 0 [1, 2, 3] > foldl (-) 0 [1, 2, 3] \end{haskellcode}