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我是Haskell的新手并拥有这段代码:

import Control.Monad

data NestedList a = Elem a | List [NestedList a] deriving (Show)

instance Monad NestedList where
    return a = List [Elem a]
    (List (Elem a: xs)) >>= f = let a' = f a in a' `joinLists` xs

func :: a -> NestedList a
func a = List ([Elem a] ++ [Elem a])

joinLists :: NestedList a -> [NestedList a] -> NestedList a
joinLists (List a) b = List (a ++ b)

main = do let a …