Tic-Tac-Toe with F#, Azure Functions, HATEOAS and Property-Based Testing

This post describes a toy application that I’ve built with F# and Azure Functions in about 1 day of work. It shows a simple end-to-end implementation with some useful techniques applied, and can be used as a reference point for anyone interested in one of the topics mentioned in the title.

The requirements for my application are quite simple:

  • Implement the game of Tic-Tac-Toe for a human player to play against the computer
  • The field is 3x3, the player to have three-in-a-row wins
  • After the game, the score is calculated based on the number of moves combined with the duration of the game
  • The history of players' scores is persisted and presented as the leaderboard

Below I go through the code step by step. Feel free to jump to the part which interests you the most: Domain Modelling, Azure Functions, HATEOAS, Property-Based Testing.

The game is online, so you can play it here.

The full source code can be found in my github.

Modeling the Game with Types

I start with a domain model. The model is composed of immutable F# types (records and discriminated unions) and pure functions.

We have two players, so we need a type for them:

type Player = X | O

In addition, there is a useful function to return the other player based on the given one. Simple pattern-matching will do:

module Player =
  let other = function | X -> O | O -> X

The domain code is the most important part of the application, so I want it to be covered by unit tests. Of course, the above function doesn’t really warrant testing, but it’s a nice and simple way to try out Property-Based Testing. That is, instead of defining specific tests, we define properties which hold for any valid input.

For other function, I came up with two properties:

  • Other player is not equal to original player
  • Other player of other player is the player itself

Here is the code with Expecto and FsCheck:

testProperty "Other player is not equal to player" <| fun x ->
  Expect.notEqual x (Player.other x) "Other should be different from original"

testProperty "Other player of other player is the player itself" <| fun x ->
  Expect.equal x (Player.other (Player.other x)) "Other other should be equal to original"

Let’s move on to modelling the game. I decided to define a union type to be used for horizontal and vertical positions of the cells:

type Position = One | Two | Three

I could use the normal integers instead, but I don’t want to be worried about validating the ranges all the time.

My first record type models the move, or action done by a player: it has X and Y positions of the chosen cell, plus the player information:

type Move = {
  X: Position
  Y: Position
  By: Player

The following type RunningGame has just two properties, but its shape defines the design of the whole application:

type RunningGame = {
  MovesDone: Move list
  PossibleMoves: Move list

This type models any state of the game which is not finished yet.

MovesDone represents the ordered log of all moves, so we have the complete history of actions at any time. Event Sourcing in small.

Equally importantly, there is a list of all possible moves at this point of the game. I could get away without this property: in the end, it can always be derived from the history of done moves and 3x3 size of the field.

However, having the list of possible moves simplifies the design of all the decision maker (client) code:

  • Clients don’t have to search for remaining cells based on move log
  • Validation of a move received from clients gets trivial: just check that it’s in the list of possible moves
  • Bot implementation gets easy: it just needs to pick one of the valid moves. The most trivial bot is a one-liner: it picks a random move from the collection
  • Tests take advantage of this in a similar way, see Game Tests below
  • We build a nice bridge into HATEOAS-style API, where links provided in the response correspond to possible moves, see REST API below

Now, we can model a game which is already finished:

type GameOutcome = Won of Player | Tie

type FinishedGame = {
  MovesDone: Move list
  Outcome: GameOutcome

Each finished game has a list of moves and the outcome: either one player won, or there was a tie.

Each state of a game can be described by the union of the previous two states:

type GameState =
  | Finished of FinishedGame
  | InProgress of RunningGame

Modeling Game Flow

Now, when all the types are in place, we can model the game flow. The flow is a sequence of transitions between game states, implemented with pure functions.

First, each game starts at the same state, which is an empty field, and X turn. Here is the value which represents this initial state:

module Game =
  let initialState =
    let positions = [One; Two; Three]
    let cells = seq {
      for x in positions do
         for y in positions do
            yield { X = x; Y = y; By = X }
    { MovesDone = []; PossibleMoves = List.ofSeq cells }

After each move is made, we need a function to evaluate move outcome: whether current game is finished or is still in progress. I defined a function evaluate for that:

let private evaluate (history: Move list): GameOutcome option = ...

I don’t show the full body here, since it’s quite boring in evaluating rows, columns and diagonals for three-in-a-row. See the full code if you want to.

The following function is even more important: that’s the main domain function called makeMove. Its type is RunningGame -> Move -> GameState which perfectly communicates its intent: given a running game and a move, it returns the game state after the move. Note that

  • You can’t pass a finished game as an argument, because making a move on finished game doesn’t make sense
  • The result can be a finished game

Here is the function implementation:

let makeMove (game: RunningGame) (move: Move): GameState =
  let movesDone = move :: game.MovesDone
  match evaluate movesDone with
  | Some result -> Finished { MovesDone = movesDone; Outcome = result }
  | None ->
    let possibleMoves =
      List.except [move] game.PossibleMoves
      |> List.map (fun m -> { m with By = Player.other m.By })
    InProgress { MovesDone = movesDone; PossibleMoves = possibleMoves }

It works like this:

  • Prepend the new move to moves done
  • Evaluate the game result of these combined moves
  • If the result is known, return a Finished game with calculated outcome
  • If the result is not clear yet, return InProgress game with possible moves same as before, but excluding the move and assigned to the other player

Tic-Tic-Toe is two-player game, so I defined another function which runs a turn of 2 moves by 2 players, given the decision making functions of both players (so it’s a higher-order function):

let makeRound player1 player2 gameState =
  let newGameState = player1 gameState |> makeMove gameState
  match newGameState with
  | Finished _ -> newGameState
  | InProgress p -> player2 p |> makeMove p

Looks almost like monadic bind operation…

Property-Based Testing

I’ve already shown two simplistic properties for other function.

It’s a bit more challenging to come up with invariant properties when it comes to testing the game itself. After some brainstorming, I’ve made the following list:

  • The game is never finished after 4 moves
  • The game is always finished after 9 moves
  • X and 0 have to make moves in turns
  • Player wins by filling one column
  • Player wins by filling one row
  • Player wins by filling diagonal
  • Evaluate a known tie

Each property-based test should accept some input from the testing framework. It should then evaluate the test against this input and assert the invariants. If the property holds for any possible valid input, the test is green.

I decided to structure my property tests in the following way:

  • Each test accepts a list of non-negative integers
  • Each integer is interpreted as an index of a possible move to select at turn i. That means that the test receives a sequence which uniquely identifies the moves to be made
  • We can restrict this sequence to a scenario under test, e.g. make it less than 4 moves, or exactly 9 moves, or pick moves from a limited subset of all possible moves
  • We apply the moves to calculate the end result
  • We assert that the result confirms the property under test

Now, it’s the responsibility of property based testing framework to generate all kinds of input lists to try to break our property. If it succeeds, it will print the exact input which causes the test to fail.

Here is how one such test is implemented.

A helper function plays a sequence of indexes as moves:

let playSequence moves =
  let playOne s i =
    match s with
    | InProgress p -> Game.makeMove p (p.PossibleMoves.[i % p.PossibleMoves.Length])
    | _ -> s
  List.fold playOne (InProgress Game.initialState) moves

Then the property “The game is always finished after 9 moves” is simply:

testProp "The game is always finished after 9 moves" <| fun (Gen.ListOf9 xs) ->
  let result = playSequence xs
  Expect.isTrue (Game.isFinished result) "Game should be finished"

Note the restriction Gen.ListOf9 xs that we put on the input sequence. It’s a generator that I defined, so that the list always contains exactly 9 elements.

Other property tests follow a similar pattern, you can see them here.


Now, when I’m done with Game domain model, I want to define API that our HTTP service will expose to the clients. I define my API in REST model.

The main resource is /game resource. To start a new game, a client has to send POST command:

POST /game
Content-Type: application/json

{ "name": "Mikhail" }

And the response body is JSON which denotes a new game created:

    "id": "5d7b2261",
    "busyCells": [],
    "links": [
            "rel": "x1y1",
            "href": "/game/5d7b2261/move/0"
            "rel": "x1y2",
            "href": "/game/5d7b2261/move/1"
        // ... 7 more links follow

The response contains a game ID and the list of occupied cells, empty for now, because no moves have been made.

More importantly, it contains a list of links, each one of which has a rel field representing a cell, and a link. The client should POST to this link if it wants to make a move on the corresponding cell:

POST /game/5d7b2261/move/1

And the response is:

    "id": "5d7b2261",
    "result": null,
    "busyCells": [
            "name": "x2y3",
            "value": "O"
            "name": "x1y2",
            "value": "X"
    "links": [
            "rel": "x1y1",
            "href": "/game/5d7b2261/move/0"
            "rel": "x1y3",
            "href": "/game/5d7b2261/move/1"
        // ... 5 more links follow

It has the same structure as before, but now two cells are occupied: one X and one O. The list of links now has only 7 links, based on the count of free cells.

The client keeps navigating the links until it gets non-empty result field:

    "id": "5d7b2261",
    "result": "You Win!",
    "busyCells": [
            "name": "x3y1",
            "value": "X"
            "name": "x3y2",
            "value": "O"
        // ... more cells here
    "links": [],
    "score": 401

This denotes the end of the game. There’s no more links to navigate, so the client knows it has to stop playing.

This API is designed in HATEOAS-style (Hypermedia as the Engine of Application State). The clients only need to know the initial URL, while all the other URLs are received from the previous responses. It resembles the way a human navigates websites.

Azure Functions

I implemented the above API with Azure Functions. I used .NET Standard based v2 runtime with precompiled F# functions.

The initial POST /game request is handled by Start function:

type GameRequest = { Name: string }

type Cell = { Name: string; Value: string }
type Link = { Rel:  string; Href:  string }

type GameDTO = {
  Id: string
  Result: string
  BusyCells: Cell list
  Links: Link list
  Score: int

let start([<HttpTrigger(AuthorizationLevel.Anonymous, "POST", Route = "game")>] req: GameRequest,
          [<Table("TicTacToe")>] store: ICollector<GameEntity>) =
  let gameid = Guid.NewGuid().ToString()
  let state = InProgress Game.initialState
  let serializedState = JsonConvert.SerializeObject state
  store.Add(GameEntity(PartitionKey = "default", RowKey = gameid, Name = req.Name, State = serializedState))
  ObjectResult(Api.serialize gameid state 0)

The outline of this function:

  • It’s triggered by HTTP POST request, as configured for req parameter
  • The request body is parsed to GameRequest type containing player name
  • It generates a new game ID
  • It creates initial game state of empty field
  • It serializes the state and saves it to Table Storage with store output binding
  • It returns HTTP body with serialized game response of type GameDTO

The second Function Play handles the moves:

let play([<HttpTrigger(AuthorizationLevel.Anonymous, "POST", Route = "game/{gameid}/move/{index}")>]
         req: HttpRequest, gameid: string, index: int,
         [<Table("TicTacToe", "default", "{gameid}")>] entity: GameEntity) =
  let state = JsonConvert.DeserializeObject<GameState> entity.State
  match state with
  | Finished _ -> BadRequestResult() :> IActionResult
  | InProgress p when index < 0 || index >= p.PossibleMoves.Length -> BadRequestResult() :> IActionResult
  | InProgress p ->
    let result = Game.makeRound (fun _ -> p.PossibleMoves.[index]) Bot.pickMove p
    entity.State <- JsonConvert.SerializeObject result
    entity.Score <- Scoring.calculateScore (DateTime.UtcNow - entity.StartedAt).TotalMilliseconds result
    ObjectResult(Api.serialize gameid result entity.Score) :> IActionResult

The outline is very similar:

  • It’s triggered by a POST request with a URL template containing game ID and move index
  • It has an in/out Table Storage binding which reads the serialized state saved after previous game and move requests
  • It validates the state: if the game is already finished, or if the move index is not in the valid range, Bad Request HTTP status is returned
  • If the move is valid, it runs the round, including bot play
  • It also calculates the score, which is going to be non-zero only for finished games
  • The game state and the score are saved to Table Storage entity (that’s the only mutation in the whole application)


Azure Function above used a Bot.pickMove function which I haven’t described yet.

This function has the type RunningGame -> Move, exactly what is expected by makeRound game function. Its goal is to pick the O move for any given game-in-progress.

Obviously, 3x3 Tic-Tac-Toe is a very simple game and it’s quite easy to make a perfectly playing bot. This wasn’t the goal though: it’s more fun for a human to win.

So, actually, the only property test that I ended up implementing is the following:

testProp "Bot is able to play O at any possible position" <| fun (Gen.ListOfNonNegative xs) ->
  let human i p _ = p.PossibleMoves.[i % p.PossibleMoves.Length]
  let round s i =
    match s with
    | InProgress p -> Game.makeRound (human i p) Bot.pickMove p
    | _ -> s
  List.fold round (InProgress Game.initialState) xs |> ignore

It makes sure that for any possible sequence of human moves, bot is actually able to make any move of its own. Bot just shouldn’t crash :)

My very first implementation of the bot was just picking a random move. Such bot is fine, but it’s too boring to play against.

So, my current bot implementation has 3 rules:

  • If there is a move that immediately wins the game, do that move
  • If possible, don’t pick a move which leads to immediate loss after the next human move
  • Otherwise, pick a random move

I implemented the bot using the approach described in my Functional Fold as Decision Tree Pattern post:

let pickMove (game: RunningGame) =
  [winNow O; notLoseNow; pickRandom]
  |> Seq.ofList
  |> Seq.choose (fun x -> x game)
  |> Seq.head

So, there is a prioritized list of decision functions. The first one returning Some decision will be promoted to final decision.

And here are those functions:

let winNow player (game: RunningGame) =
  let isWin = function | Finished { Outcome = Won x } when x = player -> true | _ -> false
  |> List.tryFind (fun move -> Game.makeMove game move |> isWin)

let notLoseNow (game: RunningGame) =
  let canLose = function
    | InProgress p -> match winNow X p with | Some _ -> true | None -> false
    | _ -> false
  let notLosingMoves =
    |> List.filter (fun move -> Game.makeMove game move |> canLose |> not)
  if List.isEmpty notLosingMoves && notLosingMoves.Length < game.PossibleMoves.Length then None
  else Some (notLosingMoves.[random.Next notLosingMoves.Length])

let pickRandom (game: RunningGame) =
  Some (game.PossibleMoves.[random.Next game.PossibleMoves.Length])

Such rule-based setup is easy to extend, and also to test when it becomes needed.

Score Calculation

Last twist to the application is the scoring system. If a human player wins or gets a tie, they are assigned a numeric score, which can be then compared to the historic leaderboard of all players.

The score is calculated based on two principles: the less moves you make, and the faster you play, the higher the score is. Move count is more important than timing.

These principles are nicely expressed as property tests:

testProp "The score of faster game is not lower than slower game"
  <| fun (Gen.Positive duration1) (Gen.Positive duration2) game ->
  let (slower, faster) = maxmin id duration1 duration2
  let scoreFaster = Scoring.calculateScore faster game
  let scoreSlower = Scoring.calculateScore slower game
  Expect.isGreaterThanOrEqual scoreFaster scoreSlower "Bigger duration has lower score (or same)"

testProp "The score of won game in less moves is greater than game with more moves"
  <| fun (Gen.Positive duration1) (Gen.Positive duration2) game1 game2 ->
  let (slower, faster) = maxmin id duration1 duration2
  let (moreMoves, lessMoves) = maxmin List.length game1 game2
  let score1 = Scoring.calculateScore slower (Finished { Outcome = Won X; MovesDone = lessMoves })
  let score2 = Scoring.calculateScore faster (Finished { Outcome = Won X; MovesDone = moreMoves })
  if moreMoves.Length = lessMoves.Length then
    Expect.isGreaterThanOrEqual score1 score2 "Bigger duration has lower score (or same)"
    Expect.isGreaterThan score1 score2 "More moves have lower score"

Note that tests are parameterized for durations and game states. We don’t have to come up with specific scenarios: the framework should take care of those.

One of the possible implementations for scoring is:

let calculateScore duration (state: GameState) =
  let durationScore = (100.0 * (1.0 - duration / (duration + 10000.0))) |> int
  match state with
  | Finished { Outcome = Won X; MovesDone = ms } -> (11 - ms.Length) * 100 + durationScore
  | Finished { Outcome = Tie } -> durationScore
  | _ -> 0

Now, the leaderboard piece. You’ve already seen the bits of this functionality in Azure Functions: they store the game state into Azure Table Storage.

There is another Azure Function which handles GET requests to /leaderboard resource. It loads all the past games from Table Storage, and then passes them to leaderboard calculation function below:

let calculateLeaderboard top ns =
  |> Seq.filter (fun entity -> snd entity > 0 && not (String.IsNullOrEmpty (fst entity)))
  |> Seq.sortByDescending snd
  |> Seq.truncate top
  |> Seq.mapi (fun index entity -> { Index = index + 1; Name = fst entity; Score = snd entity })

Wrapping Up

Ok, the application is simple, but the blog post ended up being quite long. Thank you if you made it so far.

I touched base on several important concepts and tools, which can be useful apart or in combination.

Please leave a comment if such kind of articles is useful, and which part you found most inspirational or boring.

The full source code can be found in my github.

Happy coding!

Cloud developer and researcher.
Software engineer at Pulumi. Microsoft Azure MVP.

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