Re: Issue FIXNUM-NON-PORTABLE, v4

[masinter.pa@Xerox.COM]

The portable use of FIXNUMs that convinced me came from applications that
wanted to use, say, a list of fixnums instead of a bit vector or a bignum
as an efficient representation of an arbitrary length sequence of bits.
For such an application, the FIXNUM type is ideal: it is the largest
"efficient" set of integers, and any explicit ranged integer type specifier
would not have the right properties. 

I thought it was a legitimate portable use of the FIXNUM tpe. It is really
only a portable use of FIXNUM if FIXNUM is constrained to be a "reasonable"
size, however; e.g., it doesn't work if MOST-POSITIVE-FIXNUM is negative!

Constraining FIXNUM to be a reasonable size also makes other uses of FIXNUM
more legitimate. Perhaps we could make FIXNUM more useful if we required
MOST-POSITIVE-FIXNUM to be at least as big as ARRAY-DIMENSION-LIMIT or
CHAR-CODE-LIMIT or could be guaranteed to hold the largest "count" of
objects, e.g., that LENGTH always returns a FIXNUM. An alternative would be
to invent new types, e.g., 

(deftype array-dimension () `(integer 0 ,array-dimension-limit))

What should the name of "an integer big enough to count objects" be?