Erlang Tic Tac Toe

2012 17 Oct

Everyone knows Tic Tac Toe, right?

Players choose either to be the Xs or the Os, then place their symbol on a 3x3 board one after another, trying to create a line of 3 of them.

Writing an algorithm to check for victory sounds easy, right? It's easily tested, considering there's only 8 possible winning rows (3 horizontal, 3 vertical and 2 diagonal).

In Erlang though, you probably wouldn't want an algorithm. Erlang has this cool feature called pattern matching which will allow us to completely avoid writing the algorithm by instead letting us match directly on the solutions.

Let's first create a board. A board is a list of 3 rows each containing 3 columns. It can also be thought of as a tuple containing 9 elements. A tuple is easier to manipulate so this is what we are going to use. Each position can either contain an x, an o, or be undefined.

new() ->
    {undefined, undefined, undefined,
     undefined, undefined, undefined,
     undefined, undefined, undefined}.

Now that we have a board, if we want to play, we need a function that will allow players to, you know, actually play their moves. Rows and columns are numbered 1 to 3 so we need a little math to correctly deduce the element's position.

play(Who, X, Y, Board) ->
    setelement((Y - 1) * 3 + X, Board, Who).

This function returns the board with the element modified. Of course, as you probably noticed, we aren't checking that the arguments are correct, or that the element was already set. This is left as an exercise to the reader.

After playing the move, we need to check whether someone won. That's where you'd write an algorithm, and that's where I wouldn't. Let's just pattern match all of them!

check(Board) ->
    case Board of
        {x, x, x,
         _, _, _,
         _, _, _} -> {victory, x};

        {x, _, _,
         _, x, _,
         _, _, x} -> {victory, x};

        {x, _, _,
         x, _, _,
         x, _, _} -> {victory, x};

        %% [snip]

        _ -> ok

Pattern matching allows us to simply draw the solutions directly inside our code, and if the board matches any of them, then we have a victory or a draw, otherwise the game can continue.

The _ variable is special in that it always matches, allowing us to focus strictly on the winning row. And because it's very graphical, if we were to have messed up somewhere, then we'd only need take a quick glance to be sure the winning solutions are the right ones.

Erlang allows us to transform algorithms into very graphical code thanks to its pattern matching feature, and let us focus on doing things instead of writing algorithms to do things.