We've been using Lisp in my AI course. The assignments I've received have involved searching and generating tree-like structures. For each assignment, I've ended up writing something like:

(defun initial-state ()
  (list 
    0    ; score
    nil  ; children
    0    ; value
    0))  ; something else

and building my functions around these "states", which are really just nested lists with some loosely defined structure.

To make the structure more rigid, I've tried to write accessors, such as:

(defun state-score ( state )
  (nth 2 state))

This works for reading the value (which should be all I need to do in a nicely functional world. However, as time crunches, and I start to madly hack, sometimes I want a mutable structure). I don't seem to be able to SETF the returned ...thing (place? value? pointer?).

I get an error with something like:

(setf (state-score *state*) 10)

Sometimes I seem to have a little more luck writing the accessor/mutator as a macro:

(defmacro state-score ( state )
  `(nth 2 ,state))

However I don't know why this should be a macro, so I certainly shouldn't write it as a macro (except that sometimes it works. Programming by coincidence is bad).

What is an appropriate strategy to build up such structures?

More importantly, where can I learn about whats going on here (what operations affect the memory in what way)?