Issue: BIT-ARRAY-FUNCTIONS (version 6)

[Barry Margolin <barmar@Think.COM>]

    Date: Wed, 21 Jun 89 14:25 EDT
    From: David A. Moon <Moon@stony-brook.scrc.symbolics.com>

	Date: Mon, 19 Jun 89 18:39 EDT
	From: Barry Margolin <barmar@Think.COM>

	    Date: Mon, 19 Jun 89 13:44 EDT
	    From: David A. Moon <Moon@stony-brook.scrc.symbolics.com>

		Date: Mon, 19 Jun 89 13:31 EDT
		From: Barry Margolin <barmar@Think.COM>

		I'd like to suggest an additional change, which seems to be consistent
		with the attitude about use of bit vectors expressed in the proposal.
		The BIT and SBIT functions should return 0 if asked to access outside
		the bit array.  This would maintain the tautology

			(bit (bit-XXX v1 v2) n) == (logXXX (bit v1 n) (bit v2 n))

		If slowing down these functions (they'd be the only array accessors
		REQUIRED to check the dimensions) is considered unacceptable, then a new
		accessor should be added.

	    I don't like this idea.

	Could you elaborate?  Why is it that you like the idea of assuming 0
	elements on the end when combining bit vectors, but not when accessing
	them?  Either you think of them as being infinitely padded with zeros,
	or you don't.

    I disagree with the last sentence.  I don't think the BIT function is in
    the same category as the BIT-AND function, I think of it as being at a
    different conceptual level.

That was why I suggested the possibility of inventing a new function for
this purpose (although the reason I gave wasn't complete).

I just can't think of a good name that would make it obvious that one is
just a pre-optimized bit array accessor and the other is for accessing
conceptually infinite bit vectors (or bit arrays, if the other part of
the proposal passes).

                                                barmar