SubForestWithParent.hs

-- subForestWithParent returns the subforest of the input 'Tree', where each
-- node value in each 'Tree' of the subforest is paired with the value of its
-- parent node.
--
-- ghci> let tree = Node 1 [Node 2 [Node 4 []], Node 3 [Node 5 [Node 7 []], Node 6 []]]
-- ghci> putStrLn . drawTree . fmap show $ tree
-- 1
-- |
-- +- 2
-- |  |
-- |  `- 4
-- |
-- `- 3
--    |
--    +- 5
--    |  |
--    |  `- 7
--    |
--    `- 6
--
-- ghci> putStrLn . drawForest . fmap (fmap show) . subForestWithParent $ tree
-- (1,2)
-- |
-- `- (2,4)
--
-- (1,3)
-- |
-- +- (3,5)
-- |  |
-- |  `- (5,7)
-- |
-- `- (3,6)
subForestWithParent :: Tree a -> Forest (a, a)
subForestWithParent (Node n ts) = fmap (go n) ts where
  go :: a -> Tree a -> Tree (a, a)
  go parentLabel Node {rootLabel, subForest} = Node
    { rootLabel = (parentLabel, rootLabel)
    , subForest = fmap (go rootLabel) subForest
    }