Wow, this was a fast reply :-) Thank you Mihai! This works. On 12 February 2011 10:45, Mihai Maruseac <mihai.marus...@gmail.com> wrote:
> On Sat, Feb 12, 2011 at 12:39 PM, htc2011 <jakobusbe...@gmail.com> wrote: > > > > 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! > > Hi, > > You are comparing two BST instances in the second expression for > numLeaves without declaring the Eq instance. > > numLeaves (Node b a c) will bind b and c to two BST instances. When > you are comparing b and c with EmptyBST an error is raised. > > To solve this, you'll have to declare an Eq instance, just like you've > declared a Show one: > > data Ord a => BST a = EmptyBST | Node ( BST a ) a ( BST a ) deriving (Show, > Eq) > > This will solve it :D > > -- > Mihai >
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