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Re: %nil once again

From: Ken Raeburn
Subject: Re: %nil once again
Date: Mon, 20 Jul 2009 05:15:44 -0400

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.


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.


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