A recent Veritasium video sparked my interest in Game Theory, and I decided to try to implement some of the experiments in it.

When I was talking about this with a friend of mine, she incessantly pushed me to write an article about it in Marathi in a magazine. This is a post accompanying the article. I will only discuss the implementation in this post. I would highly recommend watching the Veritasium video before reading further. Derek has done a phenomenal job in this one.

The word “game” in game theory makes it seem that it is related to (possibly silly) computer or board games. But that is not true. In game theory, we create models which attempt to replicate real life situations in terms of rules and some scoring system to measure the outcomes. A “game” also has rules and a scoring system, hence the name.

Prisoner’s Dilemma is one of the famous games in game theory. You can read about it here.

We are going to implement an inverted version of this game. The rules of our game are as follows:

There are two players who can either cooperate with each other or defect. If both cooperate, they each get 3 points. If one cooperates and the other defects, the defector gets 5 points while the cooperator gets 0. If both defect, they each get 1 point.

  Player 1 cooperates Player 1 defects
Player 2 cooperates P1 - 3
P2 - 3
P1 - 5
P2 - 0
Player 2 deffects P1 - 0
P2 - 5
P1 - 1
P2 - 1

My plan is to implement a two player version first and then implement a multi-player version (which will be a new post).


This game will be played between two players over many rounds. Therefore, we need to store the moves that each player makes in all the rounds, as well as their scores. Let’s define the state of the game:

(def initial-state
  {:first-player-moves []
   :second-player-moves []
   :score {:first-player 0
           :second-player 0}})

Now let’s see how the players can make a move. For this, we need a function for each player which will return whether the player wants to cooperate or defect in the current round. Let’s represent cooperation with :co and defection with :de.

For the players to decide their next moves, we need to provide them with the history of their previous moves. So that they can devise a strategy which takes into account the previous moves made by them and their opponent.

(defn- tit-for-tat
  [player-moves opponent-moves]
  (or (last opponent-moves) :co))

(defn- devil
  [_player-moves _opponent-moves]

The tit-for-tat strategy replicates the other player’s previous move. If it is the first round, it cooperates. The devil strategy always defects.

Now that we know how to write strategy functions to determine moves, let’s compute the scores based on the moves made by both the players. Here, we will implement the scoring system as described by the table above.

(defn- compute-score
  "Returns the score in this format:
   [<first-player-score> <second-player-score>]"
  [first-player-move second-player-move]
      ;; Both cooperate
      (and (= :co first-player-move)
           (= :co second-player-move))
      [3 3]

      ;; Both defect
      (and (= :de first-player-move)
           (= :de second-player-move))
      [1 1]

      ;; One cooperates, other defects
      (and (= :co first-player-move)
           (= :de second-player-move))
      [0 5]

      (and (= :de first-player-move)
           (= :co second-player-move))
      [5 0]))

The compute-score function, as the name suggests, calculates the score based on the moves of the first and second players.

Now to play this game over and over again for many rounds, we have to update the state of the game after each round to keep the track of the moves history and the total score. Let’s do that:

(defn- simulate-game
  [rounds-number first-player-stragey second-player-stragey]
  (loop [state initial-state
         n rounds-number]
    (let [first-player-moves (:first-player-moves state)
          second-player-moves (:second-player-moves state)

          first-player-move (first-player-stragey first-player-moves
          second-player-move (second-player-stragey second-player-moves

          [first-player-score second-player-score]
          (compute-score first-player-move

          new-state (-> state
                        (update :first-player-moves conj first-player-move)
                        (update :second-player-moves conj second-player-move)
                        (update-in [:score :first-player] + first-player-score)
                        (update-in [:score :second-player] + second-player-score))

          n (dec n)]
      (if (pos? n)
        (recur new-state n)

This is the complete implementation of the two-player version of this game. Now we can simulate the game between two strategies:

(simulate-game 100 devil tit-for-tat)

;; Or if you just want to see the score

(-> 2000
    (simulate-game tit-for-tat devil)

I have written some more strategies here.

Play around and find out which is the best strategy! :-)

I have also written a multi player version here. I will soon write a post about it.

Feel free to make a PR if you think anything in the code needs to be improved.

Happy learning! :-)