An Adventure Game – Part 4

And yet again I display exceptional prowess in the (un)holy art of procrastination. This time I blame lack of coffee, Sweden’s gloomy weather and the Illuminati. This post will concern two of the three remaining obstacles. First we’re going to add enough properties so that we can create a map consisting of a few rooms together with some entities. Second, we’re going to translate the parsed commands from the user into a suitable set of action commands and execute them with respect to the game world and its entities. As always it’s best to start with a use-case scenario and extrapolate some requirements.

> look

You see a small, rusty key and a door.

> pick up the key and open the door.

> go through the door.

This room is almost identical to the previous one. How eerie. You see a banana and a door.

> eat the door.


> eat the banana


> take the banana and eat it.

Congratulations! You solved mystery of the missing banana!

Let’s don our declarative thinking cap. This particular game consists of two rooms and a player. The player starts in the first room, picks up the key, unlocks the door and enters the second room where he/she in a moment of absolute intellectual clarity manages to deduce that the only way to beat the game is to eat the banana. Just like in real life. We can summarize the requirements as:

  • The player must be able to move from one location to another.
  • The player must be able to reference entities by their names.
  • The door must be linked to the second room.
  • The key must be able to lock/unlock the door.
  • The banana must have the property that the game stops when the player eats it.

Fortunately we can already handle some of these. The most fundamental requirement is that of referencing entities. Since each entity is just a list of properties we’re currently unable to distinguish them in any sensible way. The most obvious solution is to add another property, identity\_property, which describes the property of having an identity. For example, the door in the scenario would be represented by the list:

[openable\_property-State_1, printable\_property-State_2, ..., identity\_property-State_n]

For simplicity the state of identity\_property is just going to be an atom, e.g. door. In a real implementation it would of course be preferable to use a more complex data structure so that entities can be referenced by attributes as well, e.g. “wooden door”, but the basic idea is the same. Of course, just because this is a nice, declarative solution doesn’t mean that it’s good. To quote Richard O’Keefe from The Craft of Prolog:

The nasty thing about declarative programming is that some clear specifications make incredibly bad programs.

It’s not to hard to see that storing the identity of an entity as a property is hopelessly inefficient. To find an entity in a container we have to iterate through the whole container and check whether or not every element satisfies the identity property in question. Ouch. It would be a much better idea to demand that all entities have an identity and then store the container as a tree where nodes are entities ordered by their identities. Of course, it doesn’t mean that I’m going to use this solution just because it’s better! Sometimes it’s enough to be aware that something is a potential bottleneck and fix it if/when it becomes a real problem. Or buy a faster computer.

Next up is the problem of the game world. We shall introduce a new property by the name map\_property and stipulate that it consists of a list of rooms and a player. Why not add the player as an item in the current room instead? Just for simplicity; it’s slightly easier to move the player from room to room if we don’t have to explicitly remove him/her from the first room and add him/her to the new one. Since we have removed the player from the rooms we’re going to need another property, that of having a position/being movable, so that it’s always possible to find the current room.

:- object(movable_property,


    action(move, [New], Owner, _, Owner, New).

    action(get_location, [Room], Owner, Room, Owner, Room).
:- end_object.

The state of movable\_property is simply an identity property. When someone issues the move command the current location is changed. Exactly how this works should become clearer later on. For now, let’s concentrate on implementing map\_property. Its state will be a tuple of a list of rooms and the player, and it’ll have commands to add/remove rooms, get the current room, update the rooms and so on.

:- object(map_property,


    update(E0, E) :-
        entity::update_property(map_property, E0, Rooms0-P0, Rooms-P, E),
        entity::update(P0, P),
        update_rooms(Rooms0, Rooms).

    update_rooms([], []).
    update_rooms([R0|R0s], [R|Rs]) :-
        entity::update(R0, R),
        update_rooms(R0s, Rs).

    action(add_rooms, [Rooms1], Owner, Rooms2-P, Owner, Rooms-P) :-

    action(get_room, [Property, R], Owner, Rooms-P, Owner, Rooms-P) :-

    action(select_room, [Property, R], Owner, Rooms-P, Owner, Rooms1-P) :-

    action(print, [], Owner, Rooms-P, Owner, Rooms-P) :-
        action(current_room, [Room], Owner, Rooms-P, _, _),
        entity::action(print, [], Room, _).

    action(current_room, [Current], Owner, Rooms-P, Owner, Rooms-P) :-
        entity::action(get_location, [Id], P, _),
        list::member(Current, Rooms),
        entity::get_property(Current, Id).

    action(get_player, [P], Owner, Rooms-P, Owner, Rooms-P).

:- end_object.

It’s not necessary to study the details of this particular implementation, but some of the predicates demand an explanation. update/2 uses update\_property/5 in entity to update map\_property with the state obtained by updating the list of rooms and the player. To put it more simply: it just calls update/2 on the rooms and the player. The print command extracts the current room and prints it (it would not be very interesting to print anything else). The current\_room command gets the current location of the player and then uses member/2 to find the room with that particular identity.

Then there are doors. To have a text-game without lots of doors would simply be madness. Given the representation of the game world, each door must contain the identity of the area which it leads to. It just stores the identity, not the area itself (since all areas are stored in the map\_property). We shall store this identity in entrance\_property:

:- object(entrance_property,


    action(get_location, [Location], Owner, Location, Owner, Location).
:- end_object.

So when we create a door we plug in an entrance\_property with a suitable identity of a room.

Translating commands to entity actions

Before we begin the translation process we’re going to define some useful action commands in container\_property and map\_property. The most frequently used action will be that of extracting  an entity according to some property (e.g. the identity), perform some state-changing operation and then adding it anew. This procedure is necessary since we don’t have explicit state: whenever we extract an entity we get a copy of it, and changes to this copy won’t affect the original. For this purpose we’re going to define two additional action commands in container\_property and map\_property that takes three arguments:

  • P – the property of the entity that shall be updated.
  • Old – will be unified with the old entity.
  • New – the new entity.
:- object(container_property,

     . % As before.

    action(update_item, [P, Old, New], Owner, Items, Owner, [New|Items1]) :-
        list::select(Old, Items, Items1),
        entity::get_property(Old, P).

    . % As before.
:- end_object.

The neat thing about this definition is that we can extract an entity with Old and simply pass a variable as New, and unify this variable to the updated entity later on. The definition in map\_property is similar, but works for the player and the current room.

:- object(map_property,

    . % As before.

    action(update_current_room, [Current0, Current],
           Owner, Rooms-P, Owner, [Current|Rooms1]-P) :-
        entity::action(get_location, [Id], P, _),
        list::select(Current0, Rooms, Rooms1),
        entity::get_property(Current0, Id).

    action(update_player, [P0, P], Owner, Rooms-P0, Owner, Rooms-P).

    . % As before.

:- end_object.

Now we can finally begin with the translation process. Since we know that the input from the user will be a list of commands (remember that conjunctions are allowed) we will execute them one by one and thread the state of the game world.

eval_commands([], World, World).
eval_commands([C|Cs], World0, World) :-
    write('The command is: '), write(C), nl,
    eval_command(C, World0, World1),
    eval_commands(Cs, World1, World).

Each eval\_command/3 rule changes the state of World0 to World1 according to the command in question. The simplest one is the look-command with no arguments, that just prints the current room:

    eval_command(look-[], World, World) :-
        entity::action(current_room, [Room], World, _),
        write('You see: '), nl,
        entity::action(print_children, [], Room, _).

It asks the game world for its current room and then issues the print\_children command. The take-command is slightly more convoluted. It takes an identity as argument, tries to find the entity in question and asks the player to pick it up.

eval_command(take-[Id], World0, World) :-
    entity::action(update_current_room, [R0, R], World0, World1),
    entity::action(update_player, [P0, P], World1, World),
    entity::action(select_item, [identity_property-Id, Item], R0, R),
    entity::action(add_item, [Item], P0, P).

The move-command is perhaps the most complex of the bunch, but follows the same basic structure:

    eval_command(move-[Id], World0, World) :-
        entity::action(current_room, [Room], World0, _),
        entity::action(update_player, [P0, P], World0, World),
        entity::action(get_item, [identity_property-Id, Entrance],
                       Room, _),
        entity::action(open, [], Entrance, _),
        entity::action(get_location, [Location], Entrance, _),
        entity::action(move, [Location], P0, P).

It tries to find and open the entrance in the current room, asks it where it leads and finally asks the player to move to that location. The lock/unlock and open/close commands are implemented in the same way. One problem remains though: it’s possible to take the key, unlock the door, open it and go through it, but no way to actually finish the game. Just like everything else this functionality can be implemented in a number of ways. It might be tempting to somehow augment the top-loop and in every iteration check whether or not the final state have been reached, but this is needlessly complicated. Instead we’re going to introduce a special entity with only two properties, that of having the identity final\_state and that of being printable. It’s constructed as:

    build_win_screen(Screen) :-
        Screen = [printable_property - State, identity_property-final_state],
        State = "Congratulations! A winner is you!\n (No, you can't quit. Stop trying.)\n".

Then we need an object that has the effect that it asks the player to move itself to the final state whenever it is used, for instance a banana.

:- object(fruit_property,

    action(dissolve, [E0, E], Owner, State, Owner, State) :-
        entity::action(move, [identity_property-final_state], E0, E).

:- end_object.

The banana entity is created by combining an identity, a printable, a carriable and a fruit property:

    build_test_banana(Banana) :-
        Banana = [fruit_property - State1, printable_property-State2,
                  identity_property-banana, carriable_property-State3],

Then we of course need an eat-command, but this is straightforward to implement. So what happens when the player eats the banana is that the current location changes to final\_state. This room doesn’t have any entities and doesn’t support any operations besides being printed, which means that the player can’t return to the rest of the game world and have completed the game.

Putting everything together

We shall use the top-loop from part 2, but with some modifications. The input will be parsed into commands that are executed with respect to the current game world. The loop then calls itself recursively with the new state and asks for new input.

    init :-
         write('Welcome to Bacchus-Bosch!'), nl,
         repl(S, [], World).

    repl(S, History, World0) :-
        entity::update(World0, World1),
        entity::action(print, [], World1, _),
        write('> '),
        nlp::parse_line(S, Atoms),
        write('The input is: '),
        meta::map([X] >> (write(X), write(' ')), Atoms), nl,
        nlp::tag_atoms(Atoms, AtomTags),
        write('The tagged input is: '),
        meta::map([X] >> (write(X), write(' ')), AtomTags), nl,
        (   eval(History, AtomTags, World1, World) ->
        ;   write('no.'), % This is Prolog after all.
            World = World1
        write('-------------------'), nl,
        repl(S, AtomTags, World).

    eval(History, AtomTags, World, World1) :-
        nlp::resolve_pronouns(History, AtomTags, AtomTags1),
        nlp::parse_atoms(AtomTags1, _, Commands),
        eval_commands(Commands, World, World1).

Like a professional TV-chef I prepared a small test world and got the following result (with some of the debug output omitted):

Welcome to Bacchus-Bosch!
A rather unremarkable room.
> look

You see:
A slightly bent key.
A wooden door with a small and rusty lock.
A rather unremarkable room.
> open the door

A rather unremarkable room.
> take the key and unlock the door with it

A rather unremarkable room.
> go through the door
A room almost identical to the previous one. What on earth is going on!?
> look

You see:
A yellow banana. Hot diggity dog!
A wooden door with a small and rusty lock.
A room almost identical to the previous one. What on earth is going on!?
> take the banana
A room almost identical to the previous one. What on earth is going on!?
> eat it
Congratulations! A winner is you!
(No, you can’t quit. Stop trying.)

The final herculean task, the creation of a script-language, is saved for the next entry!

Source code

The source code is available at


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