Re: character objects & representation issues

[Daniel L. Weinreb <DLW at MIT-AI>]

I have heard a lot of discussion here, and I think some of it arises
 from people's not understanding what is being proposed here.  For
example, of course converting a character into a fixnum, adding a
constant, and converting it back is not a character-set-independent
way to uppercase something.  Nobody ever said it was; the proposal was
to have a CHAR-UPCASE function.  On the other hand, you can certainly
have such a function even if there is no distinction made between
fixnums and characters.

When this discussion first started, I thought RWK was advocating the
virtues of having character objects be a distinct data type so taht it
might be possible to check at runtime whether a given Lisp object was a
character object as opposed to a fixnum, and so that functions like
CHAR-UPCASE might be able to detect fixnum arguments as errors, thus
helping out a user who is mistakenly trying to uppercasify the number
of or something.

However, in recent messages, GLS seems to be saying that his idea of
character objects does not require that it be possible to distinguish
characters from fixnums at runtime.  I am not sure what this is all
about.  Perhsps the idea is that one can write code that will work in
two different Lisps, one in which charcter objects are really a
diffrent data type, and one in which they are not; but that any
functions that distinguished betwen the data types at runtime could NOT
be used in such code, as they would work only in the first dialect but
not in the second.  Then users of the first dialect would gain the
advantages and disadvantages of having separate character types, and
users of the second would not, but it would still be possible to write
portable code.  Existing Lisp Machine code would have to be modified only
if someone wanted to make it portable, and the modifications would be to
add some coercion functions around arithmetic uses of characters,
or to change those arithmetic uses to functions like UNMETA and
CHAR-UPCASE and such.  (The latter is not a bad idea anyway.)

Maybe I misunderstood one of them, or maybe they are not expounding the
same proposal. The proposal that I am construing from GLS's remarks
sounds pretty reasonable to me.  Lisp Machine people should note that
my construal requires no change in any code UNLESS you are interested
in being portable.

More than one person has said "this won't really solve portability
anyway".  While it is certainly not a panacea, it does solve some
problems.  (1) When you debug your portable code in NIL, you get NIL's
extra error checking -- but the code works on the LispM too.  (2) You
can uppercasify characters and it will still work in EBCDIC.  (As I
said, this could also be done without character objects, by providing
CHAR-UPCASE for fixnums; but the important thing is to CLEARLY DEFINE a
portable set of allowable operations, so that it is possible to write
code and be reasonably sure it will stand a good chance of running in
other character sets.)