Re: [Gcl-devel] (random tester) Error in FUNCALL [or a callee]: Caught fatal error [memory may be damaged]

[Paul F. Dietz]

Camm Maguire wrote:

> One day I'd really like your and others advice on some lisp
> programming dos and donts.  In particular, I frequently don't
> understand why global special variables are used in place of
> explicitly passed arguments.  My understanding of the rules governing
> lexical and dynamic binding are still a bit primitive, with the latter
> in my mind basically being equivalent to protecting a body of code
> with an assignment to a C global and a save of the original value on
> entry, and a restore of the old value on exit.


Special variables get used when the value of the variable rarely
changes.  Parameters are used when the values change frequently.

One can combine the two styles by making a function's lambda
parameter be a special variable:

(defun foo (x *y* z)
   (declare (special *x*))  ;; a prior toplevel (declaim (special *x*))
                            ;; would also work
   ;; The 2nd value passed to FOO will become the value of a binding
   ;; to the special variable *y* for the life of this call.
   ...)

        Paul