Hi All, I am learning Haskell and can't understand the following problem. Maybe somebody could advise me on a solution? Using GHCI, I have the following definition of a BST: data Ord a => BST a = EmptyBST | Node ( BST a ) a ( BST a ) deriving (Show) I want to determine the number of leaves that a tree using the above definition has: numLeaves :: Ord a => BST a -> Int numLeaves EmptyBST = 0 numLeaves (Node b a c) | b == EmptyBST = 1 + numLeaves c | c == EmptyBST = 1 + numLeaves b | otherwise = numLeaves b + numLeaves c However whenever I load my haskell file containing the above code into GHCI, I get the following error: Could not deduce (Eq (BST a)) from the context (Ord a) arising from a use of `==' at a8.hs:17:3-15 Possible fix: add (Eq (BST a)) to the context of the type signature for `numLeaves' or add an instance declaration for (Eq (BST a)) In the expression: b == EmptyBST In a stmt of a pattern guard for the definition of `numLeaves': b == EmptyBST In the definition of `numLeaves': numLeaves (Node b a c) | b == EmptyBST = 1 + numLeaves c | c == EmptyBST = 1 + numLeaves b | otherwise = numLeaves b + numLeaves c Could anybody explain to me what this means? / How to get around this? Thank you for your time! -- View this message in context: http://haskell.1045720.n5.nabble.com/Beginner-Binary-Search-Tree-Question-tp... Sent from the Haskell - Haskell mailing list archive at Nabble.com.