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MULTPLE-VALUE-OR &rest args
- To: common-lisp@SAIL.STANFORD.EDU
- Subject: MULTPLE-VALUE-OR &rest args
- From: MURRAY%cs.umass.edu@RELAY.CS.NET
- Date: Tue, 21 Apr 87 15:45 EDT
>I think more to the heart of the matter is that Common LISP needs a way to grab,
>inspect, and pass on an unknown number of multiple values without consing
> (and yes, avoiding consing can be crucial for real applications
> give the current state of the art for garbage collection
> on conventional hardware). It's possible that, using
>multiple-value-call, simply having some way of
>doing non-consing &rest arguments may be sufficient. ...
I whole-heartly agree that A) Unneccessary consing should always be avoided,
B) it would be great to have &rest args that don't cons.
I currently have plans to have a STACK-LIST declaration that declares that
a &Rest is stack-allocated (as well as WITH-STACK-LIST, ala Lisp Machines)
in my system. I would suggest that others consider this as well, but
of course, it is only a declaration that could be ignored.
(I already avoid consing for (&Rest ignore), which should be easy).
My point is that MULTIPLE-VALUE-OR can only be done efficiently by low-level
implementation-specific code, and thats why it should be in Common Lisp.
>I don't think this conclusion follows from your previous paragraph.
>If the proposed MULTIPLE-VALUE-OR can easily be implemented in terms of
>existing primitives, such as MULTIPLE-VALUE-CALL, and would be efficient
>if there were non-consing &rest arguments, and non-consing &rest arguments
>are desirable for other reasons, then I think a more logical conclusion is
>"because non-consing &rest arguments can only be done efficiently by low-level
>implementation-specific code, they should be in Common Lisp."
Are you just pointing out a flaw in my logic, or are you suggesting that
Common Lisp REQUIRE certain &rest args to not cons? The last sentence
was not meant to logically follow from the previous -- I was merely
re-iterating my point in requesting a MULTIPLE-VALUE-OR form, which is it
can be efficiently implemented WITHOUT non-consing rest args. I might
also add that using MULTIPLE-VALUE-CALL is going to require both
a Lambda and a Block, in which the Return-From will end up being a Throw.
It might not Cons, but I wouldn't call that "Efficient" (Maybe Symbolics does
function calls and throws more efficiently than a couple tests and a branch?
If so, that is exactly why it should be in Common Lisp -- You do it your way,
I'll do it mine).
>By the way, I don't think a declaration for non-consing &rest arguments
>should be necessary. It should be simple enough for any compiler to
>recognize special cases such as the following, once implementors'
>attention has been drawn to the desirability of such optimizations. Another
>way of saying the same thing is that I think consing is an implementation
>issue, not a language issue, just as much as whether integer multiplication
>takes 1 microsecond or 10 microseconds.
> (lambda (&rest values)
> (when (first values)
> (return-from block (values-list values))))
I guess you are saying Common Lisp shouldn't REQUIRE any &rest args not to cons.
I certainly agree - it is properly in the domain of Optimizations. A declaration
may not be always be necessary, but this is just about in the same category
as determining whether a function has side-effects. You can sometimes prove
it doesn't, but why go to all the bother if a very knowledgable and intelligent
(compared to a compiler) programmer can simply tell you?
- Kelly Murray