foldr

The function

foldr :: (a -> b -> b) -> b -> t a -> b

is exactly the same as foldr for lists, only the argument list of type [a] has been replaced with a container of type t storing a bunch of as. foldr traverses the container from right to left and accumulates the elements in the container using the provided accumulator function.

For lists:

GHCi

>>> foldr (+) 0 [1..10]
55

For Maybe:

GHCi

>>> foldr (+) 0 Nothing
0
>>> foldr (+) 0 (Just 5)
5

For trees:

GHCi

>>> tree = Node Empty 5 (Node (Node Empty 3 Empty) 1 (Node Empty 2 Empty))
>>> foldr (+) 0 tree
11

Since addition is associative, the results in these examples are exactly the same as when using foldl in the previous subsection. For non-associative operations, the results differ:

GHCi

>>> foldr (/) 1 [1,2,4]
2.0
>>> foldl (/) 1 [1,2,4]
0.125