Haskell binary search tree

bst.hs

module BST where

-- Define the BST data type
data BST a
    = Node (BST a) (BST a) a
    | Tip
    deriving (Show, Eq)

-- Make a leaf node
leaf y = Node Tip Tip y

-- Adding a node to a BST
add Tip y = leaf y

add (Node left right x) y =
    if y < x
    then Node (add left y) right         x
    else Node left         (add right y) x

-- Build a tree from a list
build xs = build' Tip xs
    where
    build' tree []     = tree
    build' tree (x:xs) = build' (add tree x) xs

preOrd Tip = ""
preOrd (Node left right x) = 
    show x ++ " => " ++ preOrd left ++ preOrd right

inOrd Tip = ""
inOrd (Node left right x) = 
    inOrd left ++ show x ++ " => " ++ inOrd right

postOrd Tip = ""
postOrd (Node left right x) = 
    postOrd left ++ postOrd right ++ show x ++ " => "