factor check for move validity into Board.Apply

It feels more natural and less error-prone to have the Board.Apply
method check whether the given move is valid, instead of making it
a precondition and counting on the caller to do that.

This makes it slightly harder to distinguish whether Board.Apply
failed because the move was invalid or illegal, but that can be
resolved with better errors if it becomes important.

GitHub-Pull-Request: #2

cmd/tictactoe/main.go

@@ -87,13 +87,10 @@ func playerTurn(b *ttt.Board, player player, cellClick <-chan int) error {
 	if err != nil {
 		return fmt.Errorf("player %v (%s) failed to make a move: %v", player.Mark, player.Name(), err)
 	}
-	if err := move.Valid(); err != nil {
-		return fmt.Errorf("player %v (%s) made a move that isn't valid: %v", player.Mark, player.Name(), err)
-	}
 
 	err = b.Apply(move, player.Mark)
 	if err != nil {
-		return fmt.Errorf("player %v (%s) made a move that isn't legal: %v", player.Mark, player.Name(), err)
+		return fmt.Errorf("player %v (%s) made a move that isn't legal or isn't valid: %v", player.Mark, player.Name(), err)
 	}
 
 	return nil

tictactoe.go

@@ -105,9 +105,12 @@ type Board struct {
 	Cells [9]State
 }
 
-// Apply a valid move to this board. Mark is either X or O.
-// If it's not a legal move, the board is not modified and the error is returned.
+// Apply a move to this board. Mark is either X or O.
+// If it's not a legal, valid move, the board is not modified and an error is returned.
 func (b *Board) Apply(move Move, mark State) error {
+	if err := move.Valid(); err != nil {
+		return err
+	}
 	// Check if the move is legal for this board configuration.
 	if b.Cells[move] != F {
 		return fmt.Errorf("that cell is already occupied")