Walking Around in Our World
Ok, now that we can see our world, let's write some code that lets us walk around in it. The function `walk-direction` (not in the Functional style) takes a direction and lets us walk there:
      (defun walk-direction (direction)
        (let ((next (assoc direction (cddr (assoc location map)))))
          (cond (next (setf location (third next)) (look))
    	    (t '(you cannot go that way -)))))
The special command `let` allows us to create the local variable `next`, which we set to the path descriptor for the direction the player wants to walk in - `cdr` just chops the first item off of a list. If the user types in a bogus direction, `next` will be `nil`. The `cond` command is like a chain of if-then commands in Lisp: Each row in a `cond` has a value to check and an action to do. In this case, if the next location is not `nil`, it will `setf` the player's location to the third item in the path descriptor, which holds the symbol describing the new direction, then gives the user a look of the new place. If the next location is `nil`, it falls through to the next line and admonishes the user. Let's try it:
      (walk-direction 'west)
      ==> (you are in a beautiful garden -
           there is a well in front of you -
           there is a door going east from here -
           you see a frog on the floor -
           you see a chain on the floor -)
Now, we were able to simplify our description functions by creating a `look` command that is easy for our player to type. Similarly, it would be nice to adjust the `walk-direction` command so that it doesn't have an annoying quote mark in the command that the player has to type in. But, as we have learned, when the interpreter reads a form in Code mode, it will read all its parameters in Code mode, unless a quote tells it not to. Is there anything we can do to tell the interpreter that `west` is just a piece of data without the quote?
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