Exits

One thing that our describe-location function doesn't do is tell us about the exits in and out of the current location to other locations. Let's write a function that describes these exits, starting with one exit:

(defun describe-exit
  (((match-exit object obj direction dir))
    (++ "There is a " obj " going " dir " from here.")))

Let's try it out first and then figure out how to use it. To try it, we're going to need some testing data, though:

lfe> (set test-exit (car (place-exits
                        (car (state-places state)))))

#(exit "west" "door" garden)

There: that gives us the first exit in the list of exits we got back from the place-exits call. Let's try these out:

lfe> (describe-exit test-exit)

"There is a door going west from here."

Now we can describe an exit, but see what we had to do to get our exit data? (place-exits ...) returns a list (and we called car to get the first element of that list). All of the place records have a list of exits (even if there's only one of them). As such, we need a function what will describe one or many exits. Sounding familiar? Good! You're catching on!

In fact, it's so familiar, we can use the same get-here function that we used before!

(defun describe-exits (game-state)
  (string:join
    (lists:map
      #'describe-exit/1
      (place-exits (get-here game-state)))
    " "))

What our describe-exits function does is build a list of strings and then joins them together with a space.

But we skipped over something. Remember how we used lists:map before? By passing it a lambda? Well here, we're passing it a named function, not an anonymous function. To pass a function, you need to put #' in front of the function name, and then it's arity after the function name. So, we get a list of the exits records, pass each record to describe-exit/1, and with the resulting list of exit descriptions, join them together using a single space.

Let's try this new function:

lfe> (describe-exits state)

"There is a door going west from here. There is a stairway going upstairs from here."

Beautiful!

Next, we'll want to find things ...