问题
let us say we have a tree...
data Tree a = Node a [Tree a] deriving (Show)
and that tree has some nodes
t = Node 1 [Node 2 [Node 3 []], Node 4 [], Node 5 [Node 6 []]]
the following function will collect the paths in a tree.
paths :: Tree a -> [[a]]
paths (Node n []) = [[n]]
paths (Node n ns) = map ((:) n . concat . paths) ns
like so:
*Main> paths t
[[1,2,3],[1,4],[1,5,6]]
But now how could we fold these paths? Obviously we could do this. Which folds after finding the paths.
wastefullFold :: (a -> b -> b) -> b -> Tree a -> [b]
wastefullFold f z (Node n ns) = map (foldr f z) $ paths (Node n ns)
*main> wastefullFold (+) 0 t
[6,5,12]
The closest I can some is:
foldTreePaths :: (a -> [b] -> [b]) -> [b] -> Tree a -> [[b]]
foldTreePaths f z (Node n []) = [f n z]
foldTreePaths f z (Node n ns) = map (f n . concat . foldTreePaths f z) ns
*Main> foldTreePaths (:) [] a
[1,2,3],[1,4],[1,5,6]]
*Main> foldTreePaths ((:) . (+ 1)) [] a
[[2,3,4],[2,5],[2,6,7]]
but I feel like there should be something cleaner than this below
*Main> foldTreePaths (\node base -> [node + sum base]) [0] a
[[6],[5],[12]]
Basically I do not know how to write foldTreePaths with the following signature:
foldTreePaths :: (a -> b -> b) -> b -> Tree a -> [b]
回答1:
I think this is pretty easy with comprehensions:
foldRose f z (Node x []) = [f x z]
foldRose f z (Node x ns) = [f x y | n <- ns, y <- foldRose f z n]
> foldRose (:) [] t
[[1,2,3],[1,4],[1,5,6]]
> foldRose (+) 0 t
[6,5,12]
来源：https://stackoverflow.com/questions/24032137/haskell-fold-rose-tree-paths