Question

I am doing Exercise 14.11 in “A Gentle Introduction to Symbolic Computation,” and wrote the following function:

(defmacro compile-machine (nodes)
  `(progn ,@(mapcar #'compile-node nodes)))

When calling (compile-machine *nodes*), where *nodes* is a list of NODE structures, I get the following error:

Error: *NODES* is not of type LIST.

So I see what *nodes* is, and

CL-USER 116 > *nodes*
(#<Node START> #<Node HAVE-5> #<Node HAVE-10> #<Node HAVE-15> #<Node HAVE-20> #<Node HAVE-25> #<Node END>)

CL-USER 117 > (type-of *nodes*)
CONS

CL-USER 118 > (listp *nodes*)
T

It seems as if *nodes* is indeed a list. What am I doing wrong here?

EDIT: More code that can perhaps clarify

(defun compile-arc (arc)
  `((equal this-input ',(arc-label arc))
    (format t "~&~A" ,(arc-action arc))
    (,(node-name (arc-to arc)) (rest input-syms))))

(defun compile-node (node)
  `(defun ,(node-name node) (input-syms &aux (this-input (first input-syms)))
     (cond ((null input-syms) ',(node-name node))
           ,@(mapcar #'compile-arc (node-outputs node)) ;isn't this basically the same?
           (t (error "No arc from ~A with label ~A."    ;and yet Lisp doesn't complain
                     ',(node-name node) this-input)))))

Answer 1

The value of *NODES* is a list, but itself it is a symbol.