無法匹配預期的IO類型

Question

-- Binary Search tree

data Tree a = Empty 
    | Node a (Tree a) (Tree a) 
     deriving (Eq, Ord, Show)

leaf :: a -> Tree a
leaf x = Node x Empty Empty
insert :: (Ord a) => Tree a -> a -> Tree a
insert Empty x = leaf x
insert (Node x l r) y = case compare y x of
  GT -> Node x l (insert r y)
      _  -> Node x (insert l y) r
t1 :: Tree Int
t1 = Node 4 (leaf 3) (Node 7 (leaf 5) (leaf 10))

main = do
 insert t1 8

錯誤信息：

{-

Couldn't match expected type `IO b0' with actual type `Tree a0'
    In the return type of a call of `insert'
    In a stmt of a 'do' block: insert t1 8

-}

Answer 1

您的 main功能是Tree Int類型， Tree Int編譯的Haskell程序必須具有Type IO a 。

您可以使用return :: a -> IO a IO (Tree Int) return :: a -> IO a （為您的情況簡化簽名IO (Tree Int)將Tree Int值轉換為IO (Tree Int) ）：

main :: IO (Tree Int)
main = do
  return (insert t1 8)

但是，這不會將任何內容輸出到控制台，因為它只是將樹作為值返回。 要打印結果，您可以使用print :: Show a => a -> IO ()

main :: IO ()
main = do
  print (insert t1 8)

Answer 2

您的插入函數Tree a類型，而main通常是一系列IO操作，類型為IO a 。 由於Tree /= IO您有一個小問題。 也許您想打印樹， main = print (insert t1 8) 。