On Saturday 12 February 2011 15:52:29, htc2011 wrote:
Hi all,
I just started learning Haskell but am having some trouble getting the insertion into a binary search tree to work and was hoping that somebody could advise me on a possible solution. Thanks. Given the data type definition of the tree to be:
Code: data Ord a => BST a = EmptyBST | Node ( BST a ) a ( BST a ) deriving (Show, Eq) And the insertion function to be:
Code: insert v EmptyBST = Node EmptyBST v EmptyBST insert v (Node b a c)
| v<= a = Node a ( insert v b ) c | otherwise = Node a b ( insert v c )
You got the argument order wrong here, it should be tree-value-tree, but you wrote value-tree-tree on both right hand sides (that would mean that a and BST a are the same type, which would be an infinite type).
GHCI keeps telling me that I 'cannot construct an infinite type': ' Occurs check: cannot construct the infinite type: a = BST (BST a) Expected type: BST a -> BST (BST a) -> a Inferred type: BST a -> BST (BST a) -> BST (BST a) In the second argument of `Node', namely `(insert v b)' In the expression: Node a (insert v b) c' Upon adding the following signature:
Code: insert :: Ord a => a -> BST a -> BST a
I get: Occurs check: cannot construct the infinite type.
Sorry, but I'm quite lost. Any thoughts?
Thanks. Benjamin