Sudoku in Haskell

I'm currently writing a Sudoku solver in Haskell for fun. The board is a list (rows) of lists (squares) of lists (possible numbers for that square) and solving proceeds by eliminating possibilities as much as possible, finding the square with the fewest possibilities, and trying each one as if it was the only possibility for that square. I implemented this in Python a while ago, so I wanted to try it out in my new favorite language.
I've noticed that I get a lot of benefit out of Haskell for this type of program. The List monad is exactly what I need for nondeterministic selection of possibilities for a square, I used the derivative of the list type (aka a zipper) to move around in the board, I don't have to make a copy of the board before changing it (as there are no side effects), and laziness allows the first solution to be the one that is used without calculating the others (which will fail as there should only be one solution) without adding some way to exit the recursive solver when a solution is found. I might look into ways to do this kind of exit, such as with a continuation, but for now I don't even have to try at all- Haskell is nice enough to do this for me in a way.
Its not done yet, so back to work!