メモ

TreeSet.hs

import Data.Tree
import qualified Data.Set as S

branch :: Tree a -> [[a]]
branch Node {rootLabel=n, subForest=[]} = [[n]]
branch Node {rootLabel=n, subForest=f} = map (n:) $ branchF f
  where branchF f = concat $ map branch f

powerset :: Ord a => S.Set a -> S.Set (S.Set a)
powerset s
    | s == S.empty = S.singleton S.empty
    | otherwise = S.map (S.insert x) pxs `S.union` pxs
        where (x, xs) = S.deleteFindMin s
              pxs = powerset xs

ppowerset s = S.delete s (powerset s)