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Date: Tue, 6 Jun 89 11:46 EDT
From: David A. Moon <Moon@STONY-BROOK.SCRC.Symbolics.COM>
Subject: miscellaneous questions
To: chapman%aitg.DEC@decwrl.dec.com
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Message-ID: <19890606154646.3.MOON@KENNETH-WILLIAMS.SCRC.Symbolics.COM>

    Date: 5 Jun 89 14:03
    From: chapman%aitg.DEC@decwrl.dec.com

    re: COPY-SEQ
    Did you receive any comments about which of these interpretations
    is the preferred?

There have been no replies to that message I sent on 30 May.

    %% Moon's suggested interpretation follows:

    For {\function nreverse\/}, if 
    {\arg sequence\/} is a {\datatype vector\/}, the result is a
    {\datatype vector\/} that has the same
    {\function array-element-type\/} as {\arg sequence\/}.
    The result might or might not be simple, and 
    might or might not be {\function eq\/}
    to {\arg sequence\/}.

    - or - 

    %% Another possible interpretation:
    %% The result might or might not be {\function eq\/} to {\arg sequence\/}.
    %% If the result is  a {\datatype vector\/} that is not {\function eq\/}
    %% to {\arg sequence\/}, the result is a freshly-allocated {\datatype
    %% simple-array\/} of rank one.
    If {\arg sequence\/} is a {\datatype list\/}, the result is a 
    {\datatype list\/}. 
    %% End Moon's suggested interpretation.

I now think the alternate interpretation is better, because it specifies
more, and what it specifies is not harmful.  Specifically it requires that
if the result is a vector, it is either eq to the argument or a freshly
allocated simple vector.  The only problem with this is that if the result
is a list, we cannot say that, since the result could also be eq to any
cons of the argument, or could be a freshly allocated cons whose cdr chain
shares structure with the argument in a complicated way.

Thus:

For {\function nreverse\/}, if 
{\arg sequence\/} is a {\datatype vector\/}, the result is a
{\datatype vector\/} that has the same
{\function array-element-type\/} as {\arg sequence\/}.
If {\arg sequence\/} is a {\datatype list\/}, the result is a 
{\datatype list\/}.
The result might or might not be {\function eq\/} to {\arg sequence\/}.
If the result is  a {\datatype vector\/} that is not {\function eq\/}
to {\arg sequence\/}, the result is a freshly-allocated {\datatype
simple-array\/} of rank one.