Well, you could do something like:

```Haskell
empty :: Maybe (BT a)
empty = Nothing

bt :: Int -> Maybe (BT Int) -> Maybe (BT Int) -> Maybe (BT Int)
bt v l r = Just (BT v l r)

leaf :: Int -> Maybe (BT Int)
leaf v = bt v Nothing Nothing

t1, t2, t3, t4 :: BinTree Int
t1 = BT 1 (bt 2 empty    (leaf 3)) (leaf 4)
t2 = BT 1 (bt 5 empty    (leaf 3)) (leaf 4)
t3 = BT 1 (bt 2 (leaf 5) (leaf 3)) (leaf 4)
t4 = BT 1 (leaf 2)                 (leaf 4)
```