Couldn't match expected type 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

Error message:

{-

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

Your main function is of type Tree Int whereas compiled Haskell programs must have Type IO a .

You can convert your Tree Int value to IO (Tree Int) with return :: a -> IO a (signature simplified for your case):

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

However, this won't print anything to the console because it simply returns your tree as a value. To print the result you can use print :: Show a => a -> IO ()

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

Answer 2

Your insert function results in a type Tree a while main is typically a sequence of IO actions, type IO a . Since Tree /= IO you have a slight problem. Perhaps you would like to print the tree, main = print (insert t1 8) .