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The game of Traveller

The basic predicates are square, building, joins, in, loop, sells_fuel, buys and sells, plus three ones derived from them: next, clockwise and distance.

Together, these describe a simple street layout which you can view here. It has squares containing shops and fuel stations. Unlike Monopoly, the squares are not confined to the outside of the board, but can form streets, some looped back on themselves, wandering over its interior.

The design of the game is discussed here.

More specifically, the predicates have the meanings given below:

If you care to look at the diagram of the board, it should become clear what these facts say.

The buildings

In this era of recession - I wrote that nine months ago, and it is still true! - it is important that we all be good consumers and keep the economy moving. So, bearing this in mind, the buildings all concern buying and selling, as you may have guessed from their names.

There are three kinds of building: wholesalers, retailers, and fuel-stations. Each retailer sells one (and one only) of five types of good to the public: coal, diamonds, glasses (tableware, not eyewear), peaches, and televisions. The retailers buy their stocks from traders, who in turn buy from wholesalers. By buying cheap at a wholesaler and selling dear to a retailer, a smart trader can make a hefty profit. Wholesalers, retailers, and fuel-stations are described by the facts below:


To play Traveller, you have to move around the board, making as much profit as possible. It takes some skill to keep going: for example, you must not allow yourself to run out of fuel. But there is a twist. You yourself are not playing Traveller - or not directly. It is your task to program an automatic trader in Prolog, and to give it rules that determine when and where to buy, sell, or move. Once you start your trader off, it must run without any help from you. The only thing you can do is to watch the output: sympathise if it runs out of fuel or money; applaud if it makes a huge profit.

Before showing you how to program a trader, I need to say some more about prices. Goods are bought and sold in ``units''. So a trader may tell Prolog that it wants to buy 20 units of coal, or to sell 100 units of diamonds. To make Traveller more realistic, you can think of these units as being sacks (of coal); individual boxes (for diamonds and televisions); boxes of six (glasses); boxes of twenty-four (peaches). However, it doesn't matter how many things are inside; you can only buy in whole units.

The buying and selling prices given by buys and sells are all quoted as the price per unit.

Products have volumes, measured in cubic feet. These are given by the only fact I haven't yet mentioned: unit_volume(Good,V). V is the volume occupied by one unit of Good (all goods of the same type have the same unit volume - containerisation). This matters because as a trader, you drive a lorry whose size is strictly limited. It has a capacity of 1000 cubic feet. You can always fill it up to this amount, provided you have the money, but you cannot cram in any more above the limit.

When you start off, you are given £ 5000 in cash, to spend as you will. There is no upper limit on your cash - you can accumulate indefinitely - but you are not allowed to go below zero. There are no credit facilities in this game.

Your lorry has a fuel tank whose capacity is twenty units. When you start off, it is full. At each stage in the game, you are allowed to do one of four things:

Thus, when you move, you can only move to an adjacent square, and then only if it is connected to your current square. Each move costs one unit of fuel, so after twenty moves you will be out of the game unless you remember to refuel.

To buy or sell, you must be on the same square as the person you're dealing with. You can only buy (fuel or tradeable goods) if you have enough money; otherwise you will be thrown out of the game for fraud. If you try to buy more fuel than you have room for in your tank, you will certainly spend the money; your tank will be filled, and any excess will just be spilt. However, if you try to buy more of a good than you have room for, the transaction will be forbidden for safety reasons, and you will be kicked out as an unsafe driver.

You can only sell a good if you have enough of it in your lorry. If you haven't, you will again be thrown out for fraud.

Running a trader

Each trader begins with a full tank. Initial cash is £ 5000, and initial loads are all zero. The total load is displayed in cubic feet, and the stock of each good is displayed both in units and in cubic feet, using unit_volume to provide conversion factors.

trader is on square 67.
  Fuel 20 in tank size 20.
  Cash 5000.
  Total load 0 cu ft in lorry size 1000.
    Stock of televisions = 0 units (0 cu ft).
    Stock of peaches = 0 units (0 cu ft).
    Stock of glasses = 0 units (0 cu ft).
    Stock of diamonds = 0 units (0.0 cu ft).
    Stock of coal = 0 units (0 cu ft).

As the game continues, you will see a summary of the actions the trader takes at each turn, and the result on his fuel, cash, position, and stocks. Wait until the trader makes his way to The Hub, and has gone through a few cycles of buying and selling; when you have seen enough, you can interrupt the game.

Now, how can you write a trader of your own? Well, when you call run, it starts by asserting some clauses which describe the trader's state. These are:

The name T of the trader is the first argument to run. So for run(trader,67), you get clauses

Note that, apart from carries, there is only one clause of each kind.

Having done this, run then tries to find out what the trader's first action is to be. Will he move to a new square, buy something, or sell something? It does so by calling (i.e. by asking itself the question) act( T, Action, Arg1, Arg2 ) where T is the name of the trader. The file TRADER contained clauses I have written for act: to play Traveller, you must write some for your trader.

When act is called, it must set Action to one of move, buy, or sell. If Action is move, Arg1 must become the next square to move to, and Arg2 must become the atom dummy. If Action is buy or sell, Arg1 must become the name of a good, and Arg2 must become the quantity in units that is to be bought or sold. Finally, if Action is buy, Arg1 can also be fuel. In this case, the trader is buying fuel, not a good for resale.

1st November 2008.