二叉查找树 (haskell)

data Tree = Node Int Tree Tree | Empty
insert :: Tree -> Int -> Tree
insert Empty a = (Node a Empty Empty)
insert (Node p lson rson) a
                          | p > a = Node p lson (insert rson a)
                          | p == a = Node p lson rson 
                          | otherwise = Node p (insert lson a) rson
find :: Tree -> Int -> Bool 
find Empty a = False
find (Node p lson rson) a
                        | p > a = find rson a
                        | p == a = True
                        | otherwise = find lson a
toTree :: [Int] -> Tree
toTree = foldl insert Empty 
main = do
    let tree = toTree [10,123,-1,5435,143]
    if(find tree 250)
        then putStrLn "Yes"
        else putStrLn "No"