CODEWITHSUNDEEP
haskelltree
user3663177
user3663177

Reputation: 11

Couldn't match expected type IO

-- 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

-}

Upvotes: 1

Views: 2446

Answers (2)

eugenk
eugenk

Reputation: 472

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)

Upvotes: 5

Thomas M. DuBuisson
Thomas M. DuBuisson

Reputation: 64750

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).

Upvotes: 1

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