Vectors versus Arrays

[Dick Gabriel <RPG at SU-AI>]

Watching the progress of the Common Lisp committee on the issue
of vectors over the past year I have come to the conclusion that
things are on the verge of being out of control. There isn't an
outstanding issue with regard to vectors versus arrays that
disturbs me especially as much as the trend of things - and almost
to the extent that I would consider removing S-1 Lisp from Common Lisp.

When we first started out there were vectors and arrays; strings and bit
vectors were vectors, and we had the situation where a useful data
structure - derivable from others, though it is - had a distinct name and
a set of facts about them that a novice user could understand without too
much trouble. At last November's meeting the Symbolics crowd convinced us
that changing things were too hard for them, so strings became
1-dimensional arrays. Now, after the most recent meeting, vectors have
been canned and we are left with `quick arrays' or `simple arrays' or
something (I guess they are 1-dimensional arrays, are named `simple
arrays', and are called `vectors'?).

Of course it is trivial to understand that `vectors' are a specialization
of n-dimensional arrays, but the other day McCarthy said something that
made me wonder about the idea of generalizing too far along these lines.
He said that mathematicians proceed by inventing a perfectly simple,
understandable object and then writing it up. Invariably someone comes
along a year later and says `you weren't thinking straight; your idea is
just a special case of x.' Things go on like this until we have things
like category theory that no one can really understand, but which have the
effect of being the most general generalization of everything.

There are two questions: one regarding where the generalization about vectors
and arrays should be, and one regarding how things have gone politically.

Perhaps in terms of pure programming language theory there is nothing
wrong with making vectors a special case of arrays, even to the extent of
making vector operations macros on array operations. However, imagine
explaining to a beginner, or a clear thinker, or your grandchildren, that
to get a `vector' you really make a `simple array' with all sorts of
bizarre options that simply inform the system that you want a streamlined
data structure. Imagine what you say when they ask you why you didn't just
include vectors to begin with.

Well, you can then go on to explain the joys of generalizations, how
n-dimensional arrays are `the right thing,' and then imagine how you
answer the question:  `why, then, is the minimum maximum for n, 63?' I
guess that's 9 times easier to answer than if the minimum maximum were 7.

Clearly one can make this generalization and people can live with it. 
We could make the generalization that LIST can take some other options,
perhaps stating that we want a CDR-coded list, and it can define some
accessor functions, and some auxilliary storage, and make arrays a 
specialization of CONS cells, but that would be silly (wouldn't it??).

The point is that vectors are a useful enough concept to not need to suffer
being a specialization of something else.

The political point I will not make, but will leave to your imagination.

			-rpg-