Haskell Cookbook

How to do it...

1. Open src/Main.hs and add the following definitions to it. For each function such as sum, we will create two versions--sumr, which uses foldr, and suml, which uses foldl.
2. Use both, right and left folds to sum up the numerical contents of a list to write functions sumr and suml  as follows:

        sumr :: Num a => [a] -> a        sumr xs = foldr (+) 0 xs        suml :: Num a => [a] -> a        suml xs = foldl (+) 0 xs

3. Similarly, use right and left folds to calculate product of all elements in the list. This should results in functions productr and productl respectively:

        productr :: Num a => [a] -> a        productr xs = foldr (*) 1 xs        productl :: Num a => [a] -> a        productl xs = foldl (*) 1 xs

4. Define map using the folds mapr and mapl:

 mapr :: (a -> b) ...