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Sep 20 2017

CX Tutorial: Using Affordances to Build a Small Text-based Adventure


This tutorial presents a text-based “game” (the user does not interact with the program, and can not influence the character’s decisions) that uses a challenge-response architecture to determine what are the possible actions the game’s character can do. The full source-code can be found in CX’s repository, in the file examples/

The game describes the adventure of a traveler that is escaping from a monster (Halloween is coming next month, after all). If the traveler survives certain number of hours (well, these are just iterations in a for loop), the monster will stop chasing the traveler. An example of a session is below:

The traveler keeps following the lane, making sure to ignore any pain.
Howling and growling, the monster is coming.
Bravery comes into sight, in the hope of living for another night.
Naive, and even dumb, but the traveler's act leaves the monster numb.
North, east, west, south. Any direction is good,
as long as no monster can be found.
Howling and growling, the monster is coming.
The traveler runs away, and cowardice lets him live for another day.

You survived.

If the traveler decides to fight the monster and his heroic attempt fails, the game ends. An example of a game ending is:

North, east, west, south. Any direction is good,
as long as no monster can be found.
Howling and growling, the monster is coming.
Bravery comes into sight, in the hope of living for another night.
But failure describes this fend and, suddenly, this adventure comes to an end.

You died.

Call's State:
flag:			true
nonAssign_32:		""

halt() Arguments:
0: "You died."

65: call to halt

As you can see, an error is raised if you die (this is suitable, as it’s a scary situation for a programmer).

Challenge-response Architecture

In this architecture, a question is raised and different agents (in this case, functions) must answer to that question. A simple question that can be asked is “Who can be executed at this moment?” and those functions that are allowed to execute will do so.

The following function prototypes represent the possible actions that can occur during the traveler’s adventure.

func walk (flag bool) () {}
func noise (flag bool) () {}
func consider (flag bool) () {}
func chance (flag bool) () {}
func fightResult (flag bool) () {}
func theEnd (flag bool) () {}

Affordance System

Another function must coordinate the function calls. In this case, CX’s affordance system is used to determine if an action is allowed to run or not.

yes := true
no := false

affExpr("walk", "yes|no", 0)
:tag walk;

In the code above, remArg() looks for an expression with the “walk” tag and removes its argument. This is done in order to make the affordance system list the arguments that can be sent to the expression’s operator. Next, affExpr() is telling CX “among all the arguments that can be sent to walk, tell me if yes or no no can be used as arguments, and apply the 0th option from the affordance list that you return.”

The previous procedure is applied to all the actions that can happen during the traveler’s adventure. For each of these actions, the following rules are queried to determine if the action should be allowed or not:

          aff(walk, yes, X, R) :- X = monster, R = false.
          aff(noise, yes, X, R) :- X = monster, R = false.

          aff(consider, yes, X, R) :- R = false.
          aff(chance, yes, X, R) :- R = false.
          aff(fightResult, yes, X, R) :- R = false.
          aff(theEnd, yes, X, R) :- R = false.

          aff(consider, yes, X, R) :- X = monster, R = true.
          aff(chance, yes, X, R) :- X = fight, R = true.
          aff(fightResult, yes, X, R) :- X = fight, R = true.
          aff(theEnd, yes, X, R) :- X = died, R = true.

The first rule can be read as “I will be queried if you’re considering to send the yes argument to the walk action. If the object monster is present, then this argument is not an option.”

The rules in the second block (the 4 rules after the first empty line) tell the affordance system to “never” accept a yes argument. We do this because we want this to be the default behaviour, but we can later declare rules that override this behaviour. This override process happens with the last 4 rules. Basically, this block of rules is telling CX to accept yes as arguments if a particular object is present in the object stack.


Some of the actions add or remove objects from the object stack. For example, whenever the noise action decides to make the monster appear, addObject(“monster”) is executed. If the traveler decides to run away from the fight, the “monster” object is removed from the stack.

In the case of the chance action, the monster can decide to spare the traveler a few more seconds to see what he will decide to do next. To do this, the “fight” object is removed (as the monster does not want to start a fight yet), but the “monster” object remains.


CX’s affordance system uses objects and rules to make complex decisions about how affordances are going to be filtered.

By using objects, we can decide what actions will be activated or deactivated. For this example, a small amount of actions are being considered for this activation process, and the benefit of using this architecture could seem worthless at first sight. Nevertheless, more complex rules involving more objects could be defined, and a single rule could be in charge of activating several nodes in a big network of actions. Also, in this example only two possible arguments are considered: yes and no; we could have more arguments, and actions that accept different types of arguments other than booleans.