Re: How does letf work?

[Michael Heerdegen <michael_heerdegen@web.de>]

Nicolas Richard <theonewiththeevillook@yahoo.fr> writes:

> How I understand it is : what is returned by the letf *is* the value of
> the symbol, but since it is a cons cell it can be modified by setf
> (using setcar and setcdr). So part of the job of letf is to *change* the
> cons cell upon exiting. The box is the same (and is what is returned),
> but the content was changed at the moment the last closing paren of letf
> is crossed. IOW:

A good summary, IMHO.

In LISP, lists are passed by reference.  When a list is bound to a
variable, the list is not copied.  The value referenced by the variable
will be the very same object.

> The object is the same, but its content changed. Similar to :
> (let ((foo (cons 'a 'b))
>       (bar))
>   (setq bar foo) ;; bar and foo have the same object in their value cell.
>   (setcdr foo 'c) ;; change the cdr of that cons cell
>   (eq bar foo)) ;; they're still the same.
> => t
>
> > But that's not because outside the letf the object created
> > inside it is necessarily gone. It's gone because letf doesn't return a
> > pointer to it
>
> I guess it's not gone per se until the garbage collector does its
> work.

(a b c d) is garbage collected.  The object equal to (KEY a b c d)
inside letf is never destroyed or garbage collected, since this list is
referenced by test-x and the return value of the function.  But a part
of the object has been changed (restored) by letf.

Regards,

Michael.