Re: multiple-value binding let and let*

Subject: Re: multiple-value binding let and let*
From: Erik Naggum <erik@naggum.no>
Date: 1999/08/18
Newsgroups: comp.lang.lisp
Message-ID: <3143984828088049@naggum.no>

* Dave Bakhash <cadet@bu.edu>
| (let* ((x 3)
|        (y 4)
|        ((q r) (truncate x y)))
|   <body>)

  I prefer

(let ((x 3) (y 4) q r)
  (setf (values q r) (truncate x y)))

  although I can see the value of these binding forms, we should not forget
  that DESTRUCTURING-BIND and MULTIPLE-VALUE-BIND go through a lot more
  work than they appear to be and that it's a lot harder to specify exactly
  how these things are supposed to interact with missing values than one
  might believe from watching the trivial cases.

  I note in passing that the generated machine code for the following two
  forms are identical in Allegro CL 5.0.1 for all the machines I have
  tested it on.  (Duane?)

(let (a b) (setf (values a b) (truncate pi 3)) (list a b))
(multiple-value-bind (a b) (truncate pi 3) (list a b))

| Personally, I'd like LET handle both MVs and destructuring.

  I think MULTIPLE-VALUE-BIND and DESTRUCTURING-BIND abuse indentation and
  clutter up the binding/setting distinction, and I see no reason why we
  should now fix that non-problem by cluttering up the meaning of binding
  so much we don't know what we're getting, anymore.

  when I use MULTIPLE-VALUE-BIND and DESTRUCTURING-BIND, it is around short
  pieces of code because I frequently need to rearrange the returned values
  before using them as arguments for another function call, or even to
  avoid the overhead of MULTIPLE-VALUE-CALL, which I would much prefer to
  be implemented at least as efficiently as APPLY (and perhaps with the
  same underlying machinery, even possibly exposed as an argument sequence).

#:Erik
-- 
  (defun pringles (chips)
    (loop (pop chips)))