Rekursion über Bäume (Schema)

f :: Tree a -> b
f t = case t of
  Leaf {} -> ...
  Branch {} -> 
      ... (f (left t)) (key t) (f (right t))

dieses Schema ist eine Funktion höherer Ordnung:

fold :: ( ... ) -> ( ... ) -> ( Tree a -> b )
fold leaf branch = \ t -> case t of
  Leaf {} -> leaf
  Branch {} -> 
     branch (fold leaf branch (left t)) 
       (key t) (fold leaf branch (right t))
summe = fold 0 ( \ l k r -> l + k + r )