A fold or a folding function is a higher-order function that processes a data structure (e.g. a list) in some order and builds a return value along the way. Therefore, a fold needs three things to work - a function that combines the elements, a starting value, and the data structure where the elements are stored.

There are two basic folding functions defined in the Haskell Prelude, foldr and foldl. The difference between them is the order in which they apply the combining function to the elements.

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