Haskell语言学习笔记（14）Foldable

Foldable

class Foldable t where    fold :: Monoid m => t m -> m    fold = foldMap id    foldMap :: Monoid m => (a -> m) -> t a -> m    foldMap f = foldr (mappend . f) mempty    foldr :: (a -> b -> b) -> b -> t a -> b    foldr' :: (a -> b -> b) -> b -> t a -> b    foldl :: (b -> a -> b) -> b -> t a -> b    foldl' :: (b -> a -> b) -> b -> t a -> b    foldr1 :: (a -> a -> a) -> t a -> a    foldl1 :: (a -> a -> a) -> t a -> a    toList :: t a -> [a]    null :: t a -> Bool    length :: t a -> Int    elem :: Eq a => a -> t a -> Bool    maximum :: forall a . Ord a => t a -> a    minimum :: forall a . Ord a => t a -> a    sum :: Num a => t a -> a    PRoduct :: Num a => t a -> a
Foldable Tree
import qualified Data.Foldable as Fimport Data.Monoiddata Tree a = Empty | Node a (Tree a) (Tree a) deriving (Show, Read, Eq)instance F.Foldable Tree where    foldMap f Empty = mempty    foldMap f (Node x l r) = F.foldMap f l `mappend`                             f x           `mappend`                             F.foldMap f r                             testTree = Node 5              (Node 3                  (Node 1 Empty Empty)                  (Node 6 Empty Empty)              )              (Node 9                  (Node 8 Empty Empty)                  (Node 10 Empty Empty)              )main = do    -- print $ F.foldl (+) 0 testTree    print $ F.sum testTree    -- print $ F.foldl (*) 1 testTree    print $ F.product testTree    print $ getAny $ F.foldMap (/x -> Any $ x == 3) testTree    print $ getAny $ F.foldMap (/x -> Any $ x > 15) testTree    -- print $ F.foldMap (/x -> [x]) testTree    print $ F.toList testTree    print $ F.length testTree    print $ F.maximum testTree    print $ F.minimum testTree    print $ 3 `F.elem` testTree    print $ F.null testTree      {-4264800TrueFalse[1,3,6,5,8,9,10]TrueFalse-}