(objects-at ()) returning NIL in the REPL

[Konstantinos PAPADOPOULOS]

I've recently picked up LoL without any previous programming experience. I had an issue at chapter 7 with the graph creation (apparently, the instructions are incorrect, I remember not finding the same commands in the Graphviz documentation), kinda got discouraged and came back again to give it a fresh shot. I started from the beginning and now I have come across an issue I may have had before too, but it could be that I didn't give it much thought.

The objects-at function returns a NIL result regardless of the inputs I use. I can't see what I'm doing wrong here. And to be honest, though, I don't understand how the function actually works yet, I'm kind of just following along, hoping to understand along the way.

It's not as if there is a bad function "definition", I would get a REPL warning about that, telling me to abort mission (on a side note, I haven't figured what those :Rn ABORT etc. mean and how to "use" them).

Any ideas? What might I be doing wrong? Thanks.

[Win Treese]

Hi Konstantinos. Welcome to Lisp!

How did you invoke objects-at?

I copied the relevant code from the book

(defparameter *objects* '(whiskey bucket frog chain))

(defparameter *object-locations* '((whiskey living-room)
(bucket living-room)
(chain garden)
(frog garden)))

(defun objects-at (loc objs obj-locs)
(labels ((at-loc-p (obj)
(eq (cadr (assoc obj obj-locs)) loc)))
(remove-if-not #'at-loc-p objs)))

Note: in general I recommend typing in code yourself when you are learning new stuff, and you may be doing that, but I wanted to make sure I got it right quickly and just copied it.

Then I ran:

(objects-at 'garden *objects* *object-locations*)
=> (FROG CHAIN)

So, two questions:

1. How are you invoking objects-at?
2. Did you double-check all the code?

We can also talk about how it works, if you want.

I’ve confused myself with both of those in previous learning experiences….

Best,

Win

Win Treese
tre...@acm.org

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