Re: %nil once again

[Ken Raeburn]

On Jul 20, 2009, at 04:12, Daniel Kraft wrote:
> > > No, I don't think there is.  `let' can't operate on function slots.
> > Not in the main emacs lisp implementation.  However, cl-macs.el  
> > provides an "flet" macro that does work on function slots; it uses  
> > the "letf" macro which expands to include a use of unwind-protect  
> > to do its dirty work.
>
> But that's not part of the real "elisp semantics", and so if we  
> don't want to include this as a genuine extension into Guile's  
> compiler, we don't have to use the fluids for function slots.

True.

> With unwind-protect and friends one can, just like in emacs,  
> implement this functionality, even without a fluid implementation.

Yes... though it means the elisp implementation wouldn't be able to  
cleanly support a multithreaded implementation that makes "flet"  
possible to implement safely.  If we want multiple threads, we  
probably do want thread-safe flet, I would guess.

> However, if you agree that this would be a useful feature (dynamic  
> scoping of functions) I can of course implement this as an extension  
> to elisp...?  I agree that it might be of use sometimes, but am not  
> convinced it is really "necessary".

I haven't looked into what it's needed for in great detail, but it  
does seem to get used in some places (gnus, mh-e, org, vc-rcs, ...).   
I don't know if we'll get to multithreaded elisp (I kind of hope so)  
or if we'll care about having a thread-safe flet (???), or thread-safe  
versions of whatever other interesting things unwind-protect might be  
getting used for.

Ken