Player is now defined as follows.

type Player = Maybe Char

I'm about to change Player to Mark and define it as follows.

data Mark = X | O | E deriving (Eq) -- E is for the empty cell.

Like the other changes, this makes very little difference in the logic of the code. But it makes the overall code cleaner.

So that those of you planning to come in to office hours tomorrow are not thrown off, I won't make the change until after tomorrow's office hours. After the change I'll expect you all to use the new version.