无法匹配预期的类型IO

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

错误信息:

{-

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函数是类型,Tree Int而编译的Haskell程序必须具有Type IO a.

您可以将转换Tree Int值IO (Tree Int)与return :: a -> IO a(签名简化了您的情况):

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

但是,这不会向控制台打印任何内容,因为它只是将树作为值返回.要打印您可以使用的结果print :: Show a => a -> IO ()

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