foldr

foldr.hs

-- foldr is meant to encapsulate the following pattern:
-- f [] = v
-- f (x:xs) = x # f xs
-- where # is an operator applied to the head of the list and recursively processing the tail

-- Using sum:
-- sum [] = 0
-- sum (x:xs) = x + sum xs

-- Therefore, sum = foldr (+) 0

-- map f in terms of foldr:
-- map f [] = []
-- map f (x:xs) = f x : map f xs

-- Therefore, map f = foldr (\h rst -> f h : rst) []