Re: Elisp flet construct

[Ken Raeburn]

On Jul 23, 2009, at 16:46, Andy Wingo wrote:
> On Tue 21 Jul 2009 15:10, Daniel Kraft <address@hidden> writes:
> Just a little addition to the subject of extensions:  I'd very much  
> like
> > to add lexical-let and lexical-let* as another set of extensions,
> > because this gives the possibility to use "fast" lexical variables
> > without the dynamic-scoping-fluid-pain.
>
> Yes, yes. I totally agree. This allows stack allocation of the  
> variables
> as well, which can be a significant speed win (because of not making  
> so
> much garbage).

In some cases, but not all.  Consider the Lisp version of an example I  
remember from my Scheme intro class:

  (defun make-counter ()
    (lexical-let ((count 0))
      (lambda ()
         (setq count (+ 1 count))
         count)))

In the Scheme version, and in Emacs with the cl package, a separate  
binding of "count" exists for each generated counter "object", so you  
can create and use several counters independently.  If we want this to  
work in Guile-Elisp too, I don't think "count" can live on the stack  
in this case, since it has to survive past the return from make- 
counter.  The dynamic-scoping-fluid-pain is replaced by lambda- 
environment-tracking-pain. :)

Without lambda or defun forms inside the lexical-let, yes, I think  
stack slots can be used.  (Well, I'm kind of fuzzy on the whole call/ 
cc thing, but I assume that's already addressed elsewhere....)

Ken