Haskell binary tree height example

data Tree a = Tree a (Maybe (Tree a)) (Maybe (Tree a))
              deriving (Show)

treeHeight :: Maybe (Tree a) -> Int
treeHeight Nothing                                      = 0
treeHeight (Just (Tree _   Nothing       Nothing    ))  = 1
treeHeight (Just (Tree _  (Just left)    Nothing    ))  = 1 + treeHeight (Just left)
treeHeight (Just (Tree _   Nothing      (Just right)))  = 1 + treeHeight (Just right)
treeHeight (Just (Tree _  (Just left)   (Just right)))
         | leftHeight > rightHeight   = 1 + leftHeight
         | leftHeight < rightHeight   = 1 + rightHeight
         | otherwise                  = 1
         where leftHeight     = treeHeight (Just left)
               rightHeight    = treeHeight (Just right)