Question

What does the syntax, colons preceding variable in Common Lisp, mean? I’ve seen programs with such, and I’ll present some sample code here, out of a large set of functions.

(defun expand (successorf node)
    (mapcar (lambda (action-state-cost)
          (let ((action (car action-state-cost))
                (state (cadr action-state-cost))
                (cost (caddr action-state-cost)))
            (make-node :state state :parent node
                       :action action :path-cost (+ (node-path-cost node) cost)
                       :depth (1+ (node-depth node)))
            ))
      (funcall successorf (node-state node))
      ))

Answer 1

They’re not variables, actually; those are keywords. They’re a special kind of efficient token, similar to “atoms” in other languages. It’s a convenient, built-in way to pass named (and, almost always, optional) parameters into a function call.

http://www.gigamonkeys.com/book/functions.html describes the syntax of function calls.