Now that we can see things in our world, let’s write some code so that we can walk around. The walk function (not in the functional style) takes a direction and lets us walk there:

(defun walk (direction)
  (let ((next (find direction
                    (cdr (assoc *location* *edges*))
                    :key #'cadr)))
  (if next
      (progn (setf *location* (car next))
             (look))
      '(you cannot go that way.))))

First, this function looks up the available walking paths in the *edges* table, using the current location . This is used by the find function to locate the path marked with the appropriate direction . (find searches a list for an item, then returns that found item.) The direction (such as west, upstairs, and so on) will be in the cadr of each path, so we need to tell find to match the direction against the cadr of all the paths in the list.

We can do this by passing find a keyword parameter . In Common Lisp, many functions (such as find) have built-in features that can be accessed by passing in special parameters at the end of the function call. For instance, the following code finds the first item in a list that has the symbol y in the cadr location:

> (find 'y '((5 x) (3 y) (7 z)) :key #'cadr)
(3 Y)

A keyword parameter has two parts:

We use keyword parameters the same way in our walk function to find the proper path based on the given direction.

Once we have the correct path, we store the result in the variable next . The if expression then checks whether next has a value (the next variable isn’t nil). If next has a value, if adjusts the player’s position because this is a valid direction . The call to look retrieves the description for the new location and returns it as a value. If the player chooses an invalid direction, look will generate an admonishment instead of a new description .

Here’s what our walk function looks like now:

> (walk '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 CHAIN ON THE FLOOR.
YOU SEE A FROG ON THE FLOOR.)

There’s a quote in front of the direction, since the direction name needs to be written in data mode. It’s kind of awkward to force a player to put a quote in a game command, but the interface we are creating now is intended for easy debugging and development. Heck, it’s almost not even worth calling an “interface,” since we just enter the game commands directly into the REPL. In the next chapter, we’ll create a much nicer interface using a custom REPL designed for playing text games that will take care of this wart.