unison/unison-src/tests/tictactoe.u

-- board piece
structural type P = X | O | E

structural type Board = Board P P P P P P P P P

use Board Board
use P O X E
use Optional Some None

a |> f = f a

orElse a b =
  match a with
    None -> b
    a -> a

isWin : Board -> Optional P
isWin board =
  same : P -> P -> P -> Optional P
  same a b c = if (a == b) && (a == c) && (not (a == E))
               then Some a
               else None
  match board with
    -- vertical top/center/bottom
    -- horizontal left/center/right
    -- diagonal rising/falling
    Board a b c
          d e f
          g h i ->
      (same a b c |> orElse (same d e f) |> orElse (same g h i)
          |> orElse (same a d g) |> orElse (same b e h) |> orElse (same c f i)
          |> orElse (same a e i) |> orElse (same g e c))

> isWin (Board X O X
             O X X
             O E X)