I realized last night that I hadn't explained my own code correctly. The value field is really the minimax value of that Game board. A minimax value is never unknown -- as long as it's computed. It always tells you the result of the game: 

•  1 - X wins

•  0 - tie game

• -1 - O wins

So I changed the name of the field to minimaxValue and the type to Int, not Maybe Int.

The minimaxValue field is assigned in evaluate, which always gives it an Int value.