Section: Robots, Real and Imagined
...Subsection: Shakey's Language

Shakey understands the following six commands:

1. step: When Shakey executes a step instruction, he moves forward one square on his grid, continuing to face in the same direction as before. Shakey's program will terminate with a run-time error if he is ever directed to step into a wall. He may occupy the same square as an item.

2. left: Shakey executes a left instruction by pivoting 90 degrees to the left, remaining on the same square.

3. right: Shakey executes a right instruction by pivoting 90 degrees to the right, remaining on the same square.

4. pickup: Shakey picks up an item from the square he is standing on. If he executes a pickup instruction and the square he is standing on does not contain an item, then his program terminates with a run-time error.

5. drop: Shakey places one item on the currently occupied square. If he is not carrying any items, or if the current square already contains an item, then his program terminates with a run-time error.

6. home: Shakey moves directly to his home position. He does not pass go... he does not collect 200 dollars. (Ordinarily, Shakey's home position is in the upper left hand corner of his grid, facing north.)

There is no limit to the number of items that Shakey can carry at one time.

These six commands constitute the core of Shakey's language. Later, you will see how to extend the language by using these six commands as building blocks. Because they form the foundation of the language, they are sometimes called primitives.