Haskell Cookbook

How it works...

Foldr has a type declaration as shown here:

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

Whereas, foldl has a type declaration as shown here:

    foldl :: Foldable t => (b -> a -> b) -> b -> t a -> b

The part of the declaration Foldable t denotes that folds can be implemented by any data type that would declare itself to be Foldable (Foldable is a type class). In the context of list, the preceding function can be adapted as follows:

    foldr :: (a -> b -> b) -> b -> [a] -> b    foldl :: (b -> a -> b) -> b -> [a] -> b

Foldr takes three arguments, which are described here:

• (a -> b -> b): This is the function that takes the element of type a of the list (or Foldable), another value of type b, and produces b
• b: This is the ...