Nomyx tutorial, part 2: the Language

Learn how to code rules in Nomyx

Published on September 23, 2014

Welcome to this tutorial on the Nomyx Language! Don’t forget to first watch the introductory video and GUI presentation. The rules are based on Haskell, which you can learn some basics here. During this tutorial we’ll go through the examples in the file Examples.hs, that will allow us to introduce smoothly all the concepts of Nomyx. You can test and submit all the rules proposed here in the online game, by creating a new game of your own.

Our first rule is:

nothing :: Rule
nothing = return ()

The type of this function is Rule, which is a synonym for Nomex (). We will come back on this type in the section on effects. As its name hints, this function does nothing! However if you submit it, it will trigger an event called Rule Proposed. This event will wake up the handlers of the active rules that are subscribed on this event. Thus, event if this rule nothing doesn’t do anything on its own, submitting it can trigger a lot of things! When you submit it, the votation rule (usually rule #1) is woken up and will display the forms for voting. If the vote comes out as positive, rule #1 will activate and effectively run the incoming rule.

Only an existing active rule can activate a proposed rule: that’s what makes the beauty of the game. The logic for voting (democratic, monarchic…) is provided by the players and can be changed at any time. A rule can also suppress and modify an existing rule.


Here is our Hello World:

helloWorld :: Rule
helloWorld = outputAll_ "hello, world!"

We use the output system of Nomyx, that you can find in the file Outputs.hs. The output system is not merely a logging system but is dynamic: a given output can be automatically updated by the system at any time. For example, you can create an output that displays the current number of players:

displayNbPlayers :: Rule
displayNbPlayers = void $ newOutput (Just 1) $ do
   pls <- getAllPlayerNumbers
   return $ "There are " ++ (show $ length pls) ++ " players"

As shown in this example, newOutput takes a player number to whom display the message to (or Nothing for all players). The second argument of newOutput is a small program, that will return the current number of players. If new players are arriving or leaving, the message will update automatically with the current number of players. You can create dynamic messages for pretty much anything. Check for example how to display a bank account balance, the current time, the status of a vote. Other functions allows you to manage the outputs: check for example getOutput, updateOuput and delOutput.


An important feature of a Nomic game is to be able to change the way how to win the game. This is performed by this instruction:

setVictory :: NomexNE [PlayerNumber] -> Nomex ()

setVictory allows to set the victory condition. Similarly to the output system, the victory is dynamic: it’s a small program with type NomexNE [PlayerNumber] that returns the list of victorious people. As soon as this NomexNE returns a non-null list, the victory will be announced on the GUI to all players. The rule victoryXEcu in the example file show how to change this condition: you will win if you have more than a given amount of ecus in your bank account. As soon as your bank account reaches that number: bingo!

You could also be bold and propose directly your own victory:

iWin :: Rule
iWin = do
   me <- liftEffect getProposerNumber 
   setVictory $ return [me]

This function takes the player number of the proposer of the rule (so, you) and sets the victory with it. Good luck to have this rule accepted by other players!!


Let’s now create a bank account to store our money! This is the following rule:

createBankAccount :: Rule
createBankAccount = void $ newVar_ "account" (0::Int)

We are creating a variable with the name “account”, the type “Int” and the initial value 0. All the variable related functions are located in the file Variables.hs. Once accepted, you will be able to see this variable appear in the section “Details” at the bottom of the game screen. This variable can now be accessed by any other rules.

As shown in the example file, it is useful to create a constant value to hold your variable name and type, so that each function acting on the variable can easily refer to it:

accounts :: MsgVar [(PlayerNumber, Int)]
accounts = msgVar "Accounts"

A MsgVar is a refinement of a variable: it is a variable with a message attached. Each time the variable will be updated or deleted, a Message event will be sent. Other rules are able to subscribe on those events and will be waken up in case of any changes on the variable.

A rule can also delete our variable easily:

delAccounts :: Rule
delAccounts = delMsgVar accounts


This is where it gets exciting: a rule can create fields on the screen of any players such as buttons, text boxes or radio buttons to collect inputs. In the file Inputs.hs you can find a lot of helpers to create such fields and their callbacks. For example:

enterHaiku :: Rule
enterHaiku = void $ onInputTextarea_ "Enter a haiku:" outputAll_ 1

This rule will display a text area on the screen of player 1, and prompt him to enter a Haiku. Once he validates his entry, the poem will be displayed on the screen of each player. The display is performed by the callback of onInputTextarea_:

onInputTextarea_ :: String -> (String -> Nomex ()) -> PlayerNumber -> Nomex EventNumber

The callback takes the string entered by the player as an input, and can perform any effect on the game (thus the Nomex type). There are similar functions for creating text boxes, buttons, check boxes and radio buttons.


Now that we overviewed some of the functions available to create the rules, it’s time to come back to their common point: They all have type Nomex. Nomex is a DSL, defined in the file Expression.hs. It allows you to modify the game state. It is an instance of Monad, so the traditional “do” notation is available:

displayActivateTime :: Nomex ()
displayActivateTime = do
   time <- liftEffect getCurrentTime
   outputAll_ $ "This rule was executed at: " ++ (show time)

This rule will display the time at which it has been activated and executed. The type of getCurrentTime is not a Nomex but a NomexNE:

getCurrentTime :: NomexNE UTCTime

We thus need to use the function liftEffect to use it in our function displayActivateTime:

liftEffect :: NomexNE a -> Nomex a

We then use the function outputAll_ to create a static display on the screen of all players:

outputAll_ :: String -> Nomex ()

But, what if we wanted to display the current time instead of the activation time? Remember that the rules are usually activated and executed only once: at the moment the vote is passed. The display provided by displayActivateTime would then be a fixed time. Instead, to display the current time we need to use the dynamic version of the outputs:

outputAll :: NomexNE String -> Nomex OutputNumber

Our function becomes:

displayCurrentTime :: Rule
displayCurrentTime = void $ outputAll $ do
   t <- getCurrentTime
   return $ "The current time is: " ++ (show t)

Here, instead of displaying a simple string, we are registering a small program that will yield the current time each time it is called. The display system of Nomyx will then call this function each time you refresh the screen (pressing F5 for example), thus always displaying the current time.

Of course, the small program passed in parameter of newOuput or outputAll should not have any effect on the game state! Otherwise very strange things could happen, such as increasing your bank account each time you press F5. That’s why its type is NomexNE: “NE” stands for “No Effect”. NomexNE is a sub-DSL of Nomex, collecting all instructions that doesn’t have any effect on the game state. By comparison, Nomex collects all instructions that can have an effect. Of course, a rule can have an effect on the game (but doesn’t return anything), that’s why its type is Nomex ().


Wow! With all what we learned so far, we are now able to construct more interresting rules. Let’s now change our political system: it’s the revolution! We want to go from the current democracy to a monarchy. The democracy correspond to the initial voting system: everybody votes. But in a monarchy, only the king (hopefully you) decides! This is demonstrated in the rule revolution:

revolution :: PlayerNumber -> Rule
revolution pn = do
   suppressRule 1
   rNum <- addRule' "Monarchy" (monarchy pn) ("monarchy " ++ (show pn)) "Monarchy: only the king can vote on new rules"
   activateRule_ rNum

revolution is a sort of management rule: it will be executed once (if accepted), change some things, and then be deleted. Indeed, once executed, it doesn’t need to stay in the active rules: it will not do anything more. That’s why it contains the instruction autodelete. So, this rule first suppresses the rule number 1, which is usually the democracy. Then, revolution adds directly another rule in the pending rules, using addRule, which can be found in the file Rules.hs. We pass the title of the added rule, its code, a string representing the code and a description. Finally, we activate this new rule and delete ourself.

You are now the King! Only you will be prompted for any incoming rules. Be wise… The kind of rules such as revolution is really what makes a Nomic game: the ability for a rule to manipulate other rules, including itself. As a reminder, it’s always another already active rule that can activate a proposed rule. This is done with the function:

activateRule :: RuleNumber -> Nomex Bool

When called with a rule number, this function will change the status of the rule from “Pending” to “Active”, as can be seen at the top of the game screen. Furthermore, it will execute the rule. So, to go from democracy to monarchy, you have to repect the democracy rules that are active one last time, and then you can change the political system.


In the section on Inputs, we showed how to create single form fields such as a text field, and how to attach a callback to them. But what if we want several input fields combined together? What if we want to combine the form results with other events happening in the game, such as a player arriving or a specific time is reached? This is done through the event system of Nomyx. Here is the complete list of events (in file Events.hs):

An event yielding a value a is represented by the type Event a. Event is an instance of the following typeclasses: Functor, Applicative, Alternative, Monad, MonadPlus and finally Shortcutable. This allows to combine the events in many ways, in an FRP style.

##Event Applicative interface

For example, if you need to fill a record with two fields containing the name and surname of your player, you can do everything in one go:

data NameSurname = NameSurname {name :: String, surname :: String} deriving (Show, Typeable)

eventNameSurname :: Event NameSurname
eventNameSurname = NameSurname <$> (inputText 1 "Name:") <*> inputText 1 "Surname:"

Using the Applicative interface, you can fill up your record NameSurname with the result of the two text fields easily. When submitting this event, two input fields will appear on the screen of player number 1, and their results will be combined into the NameSurname record. You can register your event with the event system of Nomyx this way:

displayNameSurname :: Nomex ()
displayNameSurname = do
   let displayNameSurname ns = void $ newOutput_ Nothing ("Name: " ++ (name ns) ++ " Surname: " ++ (surname ns))
   void $ onEvent_ eventNameSurname displayNameSurname

Once the event is registered in Nomyx with the function onEvent_, the inputs that are found will be automatically displayed to the players, in the “Inputs/Outputs” zone of the screen. Remember that in Nomyx, an Event and an Input are the same thing! In Nomyx we are merging in the same representation two different activities that GUIs are traditionaly performing separately:

Here everything is performed in the same language. The players don’t control where the forms are displayed and how they are styled: this is performed by the engine and GUI of Nomyx. This allows to let the players concentrating on the logic of the rules. Indeed the rules have to be read by other players and voted on, so they should be kept the most simple possible.

##Event Alternative interface

Now, what if, instead of a “product” datatype, such as the NameSurname record, you need a “sum” datatype, such as a Boolean? This is performed with the Alternative interface:

eventBool :: Event Boolean
eventBool = True <$ inputButton 1 "click here for True" <|> False <$ inputButton 1 "click here for False"

This event creates two buttons, both on the screen of player 1 (but they could also appear on the screen of different players). The first button clicked makes the whole event returns True, the second False. Here is the type of inputButton:

inputButton :: PlayerNumber -> String -> Event ()

We use the Functor operator <$, which is short for const True <$>. Finally, the whole expression True <$ inputButton 1 "click here for True" is typed Event Bool. Using the Alternative operator <|>, we introduce a choice between the two Event Bool: the first that fires wins.

With both Applicative and Alternative, we can create any ADT from complex forms. The forms can be distributed to several players. This is specially useful for voting systems, where each player has to complete a vote form, and all the results are collected to form a single “vote” event.

##Event Monad interface

Event is also an instance of Monad. We can create events that react on the result of previous events:

eventName :: Event String
eventName = do
   myBool <- eventBool
   if myBool 
      then inputText 1 "Enter your Name:"
      else return "No name"

This event creates the two buttons from eventBool defined above. If the “True” button is pressed, another input text field is created asking for a name. Otherwise, the string “No name” is directly returned.

Other types of events can be combined. For example, a timer can be embedded:

eventQuickName :: UTCTime -> Event String
eventQuickName timeOut = inputText 1 "Your name, quick!" <|> "Too late!" <$ timeEvent timeOut

This event will ask you for you name, but after a certain time the text field will disappear and a string “Too late” will be returned instead.

An Event can also access the content of the game state, and make decisions based on it. This is performed by the function liftEvent:

liftEvent :: NomexNE a -> Event a

It is demonstrated in the example function moneyTransfer:

moneyTransfer :: Rule
moneyTransfer = do
   let askAmount :: PlayerNumber -> Event (PlayerNumber, Int)
       askAmount src = do
          pls <- liftEvent getAllPlayerNumbers
          guard (length pls >= 2)
          dst <- inputRadio' src "Transfer money to player: " (delete src $ sort pls)
          amount <- inputText src ("Select Amount to transfert to player " ++ show dst ++ ": ")
          return (dst, readDef 0 amount)
   void $ forEachPlayer_ (\pn -> void $ onEvent_ (askAmount pn) (transfer pn))

In this rule we create an Event askAmount. askAmount will first check if the number of player is more than 2. If that’s the case, it will display successively two fields, one for asking to which player you would like to transfer funds to, and the second for the amount. Finally this event askAmount is registered for each player, with an appropriate callback transfer (defined in the example file), so that everybody can transfer money to whom they want.

The objective of all this is to let the players create complex events while keeping a simple language. This simplified version of Functional Reactive Programming allow us to do exactly that. Without FRP (so Event not being an instance of anything), you would be obliged to attach a callback to each single event, and combine the results yourself, asynchronously: this would be very tedious for big events.

Have fun with Nomyx!!