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Integer Shift Function(s)
- To: Scott E. Fahlman <Fahlman@C.CS.CMU.EDU>, David Singer <DSinger@SRI-KL.ARPA>
- Subject: Integer Shift Function(s)
- From: David C. Plummer <DCP@SCRC-QUABBIN.ARPA>
- Date: Thu, 17 Apr 86 12:26 EST
- Cc: common-lisp@SU-AI.ARPA
- In-reply-to: <FAHLMAN.12199433363.BABYL@C.CS.CMU.EDU>
Date: Wed, 16 Apr 1986 22:28 EST
From: "Scott E. Fahlman" <Fahlman@C.CS.CMU.EDU>
[This looks like a reply.]
All of the logical operations are defined in terms of an abstract
integer type that is infinitely sign-extended to the left. That way,
the machine's word-length, or the length of the data part of its fixnum
representation, is hidden. What would a logical shift operation (or a
rotate, though you don't ask about that) do in this context?
I agree, it can't do anything.
An implementation may want to define LSH and ROT operations for its own
peculiar fixnum length, but such a thing makes no sense in portable
JonL, at one point, suggested abstract LSH and ROT operations that take
a third argument which is the pretend-I'm-a-fixnum-length. Thus,
(rot 1 -1 5) == (ash -1 4) == -16.
(lsh -1 -1 6) == #2r011111 == 31.
Personally, I think this is cute, but probably not extrememly practical.
Also note: I think you will find most uses of LSH and ROT (for those
extended Common Lisps that implement them) are almost exclusively inside
operating system code.