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!
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