Fold.hs

-- file: ch04/Fold.hs

myFilter :: (a -> Bool) -> [a] -> [a]

myFilter p xs = foldr step [] xs
    where step x ys | p x       = x : ys
                    | otherwise = ys


myFoldl :: (a -> b -> a) -> a -> [b] -> a




myFoldl f z xs = foldr step id xs z
    where step x g a = g (f a x)