PopBeast lives in a microworld called Eden, designed to provide an environment which is reasonably challenging, whilst being easy to compute. Unfortunately, the Department is not providing funds to improve this course, and we are still limited to a design more suited to the computer terminals of ten years ago. Hence, there are severe limits on how much of the environment we can display, and these have dictated its design.
Given these constraints, what does Eden look like? Strictly speaking, you can have many different worlds, but I'll refer to them all as ``Eden''. Figure 1 shows one possibility, with an explanation of the symbols underneath.
Eden is two-dimensional, and divided into a rectangular grid of squares. Each square can hold one object, and may also be able to accomodate PopBeast. However, whether this is possible depends on the object's ``size''. Boulders, rocks, and doors are big enough to block a PopBeast from moving into the square they occupy; food, hammers, and keys are smaller, and can be in the same square as a PopBeast.
How does a PopBeast interact with its environment? A PopBeast has a
``retina'' and a ``sense of smell''. Both these are automatically
updated by the simulator so that they always reflect the current state
of the world. The retina is a 7 
5 array representing PopBeast's
immediate environment. The sense of smell gives the general direction of
the food, as one of forward, back, right,
left. In the world shown above, PopBeast is facing east (right):
it smells the food as being forward, and it sees its retina as
shown in Figure 2. PopBeast's position is represented by a
B.
These are severe sensory limitations, which represent a compromise. PopBeast needs some senses: it is difficult to build intelligent behaviour into a being with no sensory apparatus whatsoever. On the other hand, the senses must not provide information that's too precise, because one of Eden's objectives is to let people experiment with the problems of perceptual decoding. Given the size of the retina, if PopBeast is to do any more than react to its immediate surroundings, it somehow needs to move around the world and combine successive retinal images into a consistent and correct world model.
Incidentally, it is an increasing trend that not everyone agrees on the need for a world-model; some people believe that trying to construct and maintain one leads to inefficient and non-adaptable agents. The contrary view is put by R A Brooks in Challenges for Complete Creature Architectures, from From Animals to Animats, edited by Meyer and Wilson (MIT 1991; PSY DB:M 057).
PopBeast also faces the problem of integrating sight and smell. This is harder than it might seem, because the senses are so different. If PopBeast can see the food at all, then it knows exactly where it is. However, its sense of smell only gives a general direction. So the world model must be able to accomodate imprecise data: ``hints'' about the state of the world. The problem of merging information from different sources is an important one in robotics, and is known as sensory fusion.
Having got this sensory information, how does PopBeast use it? The simulator works in a continuous cycle, rather like the turns in a game. At the start of each turn, it updates PopBeast's senses so they're consistent with the current state of the world. It then activates PopBeast's ``brain''. This is a chunk of computer program which takes the sense data, ``thinks'', and eventually comes up with a ``motor command''. The simulator then takes this action, modifies the world accordingly, and then starts the next turn. Warning: these ``motor commands'' are single symbols, and have very little in common with motor commands in real animals.
At the end of each turn, PopBeast can perform one of eight actions. Four are moves: it can move one step forward or one step back, or turn 90 degrees left or right. It can't move diagonally, or more than one square at a time. Every time PopBeast moves, it loses one unit of ``energy'': starting with 500 units, if the energy gets down to zero, PopBeast will ``die''. This, plus the restricted range of movements, make it vital to plan efficient paths when travelling. You also have to avoid obstacles. You can only move forward or back on a quicksand; if you turn, you'll fall in. You can't move through a boulder, rock or door. Boulders represent fixed obstacles; rocks and doors are not so stubborn, because you can smash or open them as described below.
Of the other four actions, two are to do with carrying objects. PopBeast can pick up an object (provided that its ``hand'' is empty), or put one down (if there's room for it in the square). It can't hold more than one object at a time, however. Again, this restriction enforces efficient planning. Some objects - boulders, rocks, doors - can't be picked up; food, keys and hammers can.
PopBeast can also ``use'' the object it's carrying. What effect this has depends on what the object is. Using food is equivalent to eating it. Using a key will open a door, if PopBeast is standing next to the door. Using a hammer will smash a rock, if PopBeast is standing next to the rock. Keys and doors are not quite the same as hammers and rocks; if you use a key, it disappears, but if you use a hammer, it remains for you to use again. Using the other objects has no effect.
Eden was designed by Simon Perkins for the AI Society, as a setting for members to write and test AI programs. He has done a very good job of providing a challenging, yet conceptually simple, world within the constraints of Oxford's computers. You might like to think about what the PopBeast in Figure 1 has to do in order to get at the food. Note that it has to open the door in the top middle of the world, but it needs a key to do this, and the only accessible key is hidden in a nest of quicksands.
Note: by ``challenging'', I mean ``puzzle-like'' or intellectually challenging, in the way in which so many problems posed to classical AI systems have been. Eden contains nothing equivalent to the challenge of running the London Marathon, or playing a complicated piece of Liszt.