A tree fold replaces
Empty by a seed and Node by a combining function. The combining function receives the already folded left subtree, the node's label and the already folded right subtree:foldSTree :: (b -> a -> b -> b) -> b -> STree a -> b
foldSTree combine empty Empty = empty
foldSTree combine empty (Node left x right) =
combine (foldSTree combine empty left)
x
(foldSTree combine empty right)Its polymorphic type signature isThe result type need not be the label type: it may be a list, a number or another structure. No ordering constraint is needed because this tree fold follows constructors without comparing labels.
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