Lexical closures (bug?)

[Stephen Robbins <Stever@ivory.s4cc.symbolics.com>]

    Date: Mon, 15 Aug 88 14:58:32 pdt
    From: lbaum@BOEING.COM (Larry Baum)

    We were working thru the exercises in Charniak, Riesbeck & McDermot, 
    Artificial Intelligence Programming, and tried the following:

    (defun compose (&rest functions)
      #'(lambda (x)
          (do ((fns functions (cdr fns))
               (value x (funcall (car fns) value)))
              ((null fns) value))))

    However, when I try this on my 36xx, with either Genera 7.1 or 7.2, the
    control stack overflows.

On the 36xx machines, &REST arguments to a function are CONSed on the stack.
They go away when the function returns.  When COMPOSE returns, FUNCTIONS, which
the closure uses, now looks into the stack somewhere.

Yes, this is a violation of the Common Lisp spec.  &REST lists are supposed to
be freshly consed, with indefinite extent.

You can work around the problem by consing up a "real" list of functions:

(defun compose (&rest functions)
  #+LISPM (setf functions (copy-list functions))
  #'(lambda (x)
      (do ((fns functions (cdr fns))
           (value x (funcall (car fns) value)))
          ((null fns) value))))

-- Stephen