Haskell, finding element in bst

Question

data Bst a = Empty | Node (Bst a) a (Bst a)

lElems :: Ord a => a -> Bst a -> Int
lElems _ Empty = -1
lElems n (Node l m r)
                | n == m = n
                | n > m = lElems n r
                | n < m = lElems n l

Here you see definition of type and program. I want to return element if it's in the tree and -1 if not.

However, I had this problem 
 Couldn't match expected type `Int' with actual type `a'
      `a' is a rigid type variable bound by
          the type signature for lElems :: Ord a => a -> Bst a -> Int
          at C:\Users\User\workspace\s\src\Main.hs:3:11

Can someone explain me what is wrong here?

Answer 1

In the n == m case, lElems returns n. From the type signature, the function is declared as always returning Int. This means that n must be an Int, and thus, can't be any Ord a.

Try converting n into Int before returning it; that will probably require you to further constrain a, though.

Instead of returning -1 in the case where you don't find what you're looking for, consider changing the return type to Maybe a, or perhaps an Either value.