Johan Zietsman

Johan Zietsman

I'm Johan Zietsman, and welcome to my software blog, where I'm committed to sharing valuable insights, best practices, and in-depth tutorials.

Curious software developer, motorcycle enthusiast, rugby fanatic and an avid gamer. My code always works sometimes.



Johan Zietsman

Building Tic Tac Toe in Kotlin with Functional Programming

Explore the basics of functional programming in Kotlin as we dive into building a Tic Tac Toe game. Discover the power of pure functions, immutable state, and learn how to separate side effects from the functional core using an IO monad.

By following this blog post, developers will gain a solid understanding of functional programming principles in Kotlin and learn how to apply them effectively to build a Tic Tac Toe game.

The accompanying source code can be found on GitHub.


Kotlin is not a purely functional programming (FP) language, but offers plenty of features to support it. Immutable collections, data classes, higher order functions and pattern matching to name but a few.

Why use functional programming in the first place? It leads to increased code modularity through pure functions, which in turn, leads to better testability, reuse, parallelization and generalization.

Functional Programming Fundamentals

1. Pure Functions

A pure function can be defined as a function that has no observable side effects. A function is said to have a side effect if it does anything other that computing and returning a result.

2. Referential Transparency

A function is said to be referentially transparent if everything it does is represented by what it returns. Substitution can be used to determine if a function is referentially transparent. Consider the following:

The sum function is referentially transparent because its result can be replaced with the function call itself & vica versa without changing the program. On the other hand consider a function that is not referentially transparent:

3. Immutable Data

Immutable data means that once a value is assigned to a variable, it cannot be modified. Functional programming encourages creating new data structures with modifications instead of modifying existing data. Immutable data aligns with the principles of pure functions. Benefits of immutable data include but are not limited to:

Designing the Game State

Game state class diagram

The game state can be represented by a 2 dimensional immutable list containing game symbols. A symbol is an enum of either CROSS, NOUGHT or a BLANK.

The sealed Game class plus the 3 data constructors Won, Draw and InProgress captures the states that a Game can be in. Note that kotlin's pattern matching can be used with a sealed class hierarchy. All the class members are immutable.

Implementing Pure Functions

The heart of the game is the InProgress.make(move) pure function. It creates a copy of the board state with the move applied. The new state is then passed to a smart constructor that determines the state of the game (Won, Draw or InProgress).

Note the return type of the function Either<GameError, Game>. Throwing exceptions in functional programming is typically avoided because it breaks referential transaprency. Consider the following:

Reasoning about a referentially transparent function is local and does not require a broader context, the same does not hold for a non referentially transparent function (e.g is the expression in a try catch block).

The Either type and either DSL is provided by the arrow-kt library. The sealed Either type can either be a Left value representing an error that occurred or Right if it succeeded. The either together with raise simplifies using the Either type. More information about the types can be found on the arrow-kt website.

Once the new board state has been created, it is passed to a smart constructor that determines the state of the Game.

Managing Side Effects with the IO Monad

A side effect is an interaction a program has with the external world, like making a network request or reading/writing to a file. While side effects are unavoidable in most real world applications, functional programming seeks to minimize its impact on a functional program. This is achieved by separating side effects from the functional core.

It is always possible to refactor an impure function into 3 parts:

  1. A pure core function
  2. A side-effecting function that provides input to the pure function
  3. A side-effecting function that does something with the pure functions output.

A detour, what is a monad?

“The curse of the monad is that once you understand it - you lose the ability to explain it to anybody else.”— Douglas Crockford

A monad is a programming concept that provides a structured way to handle and compose computations or actions. It encapsulates operations, allows sequencing, and maintains a context for computations, enabling better control over effects, error handling, and composition of complex operations.

Monads typically contain 2 primitive monadic combinators, unit & flatMap. Many other useful combinators can be derived from these primitives. Let's look at a couple of examples:

From the examples above we can extract a few things. A monad is a type of container and you can wrap or lift any type into the monad using the unit function. Secondly, computations can be chained on the monad by calling the flatMap function. What is the difference between the IdMonad and the OptionMonad? In both cases we can see that a chain of flatMap calls is like an imperative program with statements that assign to variables, the monad implementation defines what happens at statement boundaries (inbetween flatMap statements) The IdMonad simply unwraps and rewraps the value, on the other hand the OptionMonad may terminate early if a None is returned.

A more formal definition of a monad:

A monad is an implementation of one of the minimal sets of monadic combinators, satisfiying the laws of associativity and identity.

The miminal sets of combinators are:

The associative law deals with ordering and guarantees the outcome will remain the same no matter how flatMap operations are nested.

The identity laws are left identity & right identity each dealing with the situation where unit is the subject or object of a flatMap expression.

IO Monad

One way functional programs achieve isolating side effects is by having the pure core of the application describe a set of side effects that need to occur, without actually performing the side effects. The IOmonad can then be used to chain these descriptions of side effects together. Outside of the functional core an Interpreter can be implemented to perform the described side effects.

A simple implementation of an IO monad could be:

The implementation allows us to represent the 2 side effects we need for the tic tac to game, namely stdin and stdout, as an IO monad. However there is one problem with the implementation, note how the flatMap implemntation nests function calls. Every call to flatMap will add a stack frame. A large number of flatMap calls will cause a stack overflow.

A technique called Trampolining converts recursive stack consuming operations into an iterative form. In Kotlin this makes use of tailrec functions. For a function to qualify for the tailrec optimization, the very last operation in the function has to be a recursive call to the function itself.

To implement trampolining on the IO Monad, 3 data constructors are introduced namely Return, Suspend & FlatMap. Return represents an IO action that has finished and has a value to return. Suspend represents an IO action needs to be executed to produce a result, and finally a FlatMap allows us to chain or continue computation by using the result of the 1st computation to produce the 2nd computation.

Note that the run method is moved out into an Interpreter that ensures that the trampolining technique is applied to the chain of io's.

The run method is marked as tailrec and every recursive run call is the last operation in the method. The 2nd implementation is superior based on how 2 consecutive flatMap's are combined in a way that allows the recursive call to run to be the last statement of the function.

Building the User Interface

Program flow

The basic flow of the program is a recursive loop from an Screen IO monad that renders the game state to an Input IO monad that parses user input before passing the move to the game to compute the new game state.

The Screen builds on top of stdout by first converting the ProgramState to a string that is then rendered to the console by stdout. The ProgramState is a type alias for Either<Pair<Game,GameError>,Game>.
Where the Pair on the Left represents the original game state plus the error generated by the last move if one occurred. On the other hand if the last move was successful, the Right value of the the Either is the new Game state.

Screen also takes care of flipping the error state (When the Either is of type Left) to a valid state after the screen output is generated via the map { programState.toMostRecentValidState() }. This is done because after the error associated with the last move is displayed, the state of the game is reverted to the last valid state.

After the screen is rendered to stdout control is passed to the Input which builds on top of stdin. The Input uses kotlin's pattern matching on the sealed Game class to determine if the game is still in InProgress. Should this be the case, the user input is parsed into a Move and passed to the Game to compute the new game state. A mapLeft on both parsing the move, and making the move ensures that if either of those functions fail (by returning a Left constructor on the Either) the original game state is preserved (mapLeft(errorToCurrentState)). Once the input has been processed, the program loops by peforming recursive call on the program function with the resulting game state.

Zooming out a little bit, it is worth noting that everything in the program is purely functional, the only time side effects occur is once the program is passed to This clearly demonstrates how side effects are separated from the functional core.

Testing the Functional Code

Among the benefits of functional programming is simplified testing. The output of a pure function depends entirely on it's input, therefore tests require no mocking, stubbing or any other common testing techniques. The project leverages the property based testing capabilities of the Kotest test framework. To test the functional core a set of custom move generators are used to simulate games with different outcomes. For example, to test winning condition:

The final game state is derived by making all the generated moves. Every cell is checked to ensure that it contains the correct symbol and then game state and winner is verified.

Testing side effects with the IO monad is more challenging. While the bulk of the Screen and Input classes were pure functions and could be tested directly, I chose to test these classes wrapped in their monad form. The challenge here is that you can only chain more computations onto the monad or run it via the Interpreter, there is no easy way to inject input or grab the output for verification. To make these classes more testable, I introduced a InterpreterContext which provides the dependencies required by the low level side effects, namely printMessage to write to stdout and readLineFromInput to read from input. With this abstraction in place it is possible to provide a TestInterpreterContext which writes output to memory and reads input from a predefined list. The tests can then be written by actually interpreting the side effect descriptions created by these monads.


This post covered some of the fundamental concepts in functional programming. We looked at pure functions which have no side effects. Referentially transparent functions whose output is entirely determined by its input and can be verified by substitution. Immutable data that is modified by making a copy. Finally we looked at Monads and how to separate side effects from the functional core with an IO Monad. The post also covered testing of a functional program using kotest and property based testing.

The concepts discussed in the post primarily comes from Functional programming in Kotlin by Marco Vermeulen, Rúnar Bjarnason, and Paul Chiusano and the arrow-kt library documentation.

To learn functional programming requires a paradigm shift, and the initial learning curve is quite steep. However I believe the benefits are worth the investment. As with anything, practicing will lead to improvement.

The source code used in this post is available on GitHub.

Curious software developer, motorcycle enthusiast, rugby fanatic and an avid gamer. My code always works sometimes.

View Comments