Common LISP 中出现奇怪的“EVAL：未定义 function T”错误

Question

I'm writing a function in Common Lisp, but I'm getting this error I mentioned in the title.

(defun sample (graph-id vertices)
    (cond
        ((null vertices) nil)
        (t 
            (and
                (setf 
                    (gethash (list graph-id (first vertices)) *keys*)
                    MOST-POSITIVE-DOUBLE-FLOAT)
                (sample graph-id (rest vertices))))))

In particular, when the compiler reaches the (gethash (list graph-id (first vertices)) *vertex-keys*) line, if I swap graph-id (first vertices) with (first vertices) graph-id , the error disappears. It also disappears if I use second , third or any other nth functions rather than first , I can't understand why it happens. vertices is a list like (AB C DEF)

Answer 1

I feel like your problem is related to how you construct the hash table *key*. The way gethash works is, it uses the hash table's designated:test function (given during make-hash-table call).

Now, given that you're setting a value to a key that's a list each time in the recursion, I doubt you'll add anything to your hash table if it uses default make-hash-table (which uses eql for testing), because each time your (gethash (list graph-id (first vertices)) *keys*) runs, the (list..) call creates a list that's never eql to any previous list.

If you haven't already, change your possibly:

(defvar *key* (make-hash-table)) 

line to something like

(defvar *key* (make-hash-table :test #'equal)) 

and things should work better.