Binary search tree cannot be a monoid

[sjakobi]

I was surprised that the binary-search-tree exercise doesn't allow solutions where the BST can be empty.

The problem is that the bstValue function is of type BST a -> a instead of of BST a -> Maybe a.
This also means that the fromList function must have a special case where it returns an error for the empty list.

In the example solution the BST is defined as:

data BST a = Node { bstValue :: a
                  , bstLeft :: Maybe (BST a)
                  , bstRight :: Maybe (BST a) }

Wouldn't a definition like the following be nicer?

data BST a = Tip
           | Node (BST a) a (BST a)

[etrepum]

That would be reasonable if bstValue was BST a -> Maybe a