From: Ken Dickey <xxxxxx@allvantage.com>
   Date: Wed, 31 Dec 2003 09:22:57 +0100

   On Wednesday 31 December 2003 03:51 pm, Richard Kelsey wrote:
   > Are there any implementations of Scheme in use where SCHEME_FALSE,
   > SCHEME_TRUE, SCHEME_NULL, or SCHEME_UNSPECIFIC should be freshly
   > allocated?  Or where EQ? and == are not equivalent?  (These are not
   > rhetorical questions; I really would like to know -- is this an
   > existing or only potential problem for portability?)

   This is not exhaustive, but a quick look at a few Schemes on my disk reveals
   that quite a few Schemes do use "constant objects":

Constant objects aren't the issue.  I am curious as to whether their
are implementations where:
  - It is not okay to reuse the same value for multiple
    instances of one of these constants?
  - EQ? is not equivalent to comparing two fixed-sized chunks
    of bits?  (Because that is what C's == does.)
These two are related; if you use C's (or Java's) == for EQ? then
you can't have multiple instances of #t, #f, or ().

	   Kawa  [boolean? is a test for instance of Boolean class]
Kawa uses Java's == for EQ?, so it isn't what I am looking for
(there can be only one instance of #T, #F, and ()).

	   Stklos
	   SXM
	   TinyScheme
	   Gauche

Stklos, SXM, TinyScheme, and Gauche all implement EQ? as C's ==.
So they are standard as well.

(By the way, the current version of Stklos doesn't use
 pointers for these constants; I didn't check any earlier
 versions.

 From stklos.h (version 0.56):

  #define AS_SCM(x)		((SCM) ((unsigned long) (x)))
  ...
  #define MAKE_SCONST(n)   (AS_SCM(n << 2 | 3))
  ...
  #define STk_nil		((SCM) MAKE_SCONST(0))
  #define STk_false	((SCM) MAKE_SCONST(1))
  #define STk_true	((SCM) MAKE_SCONST(2))
  #define STk_eof		((SCM) MAKE_SCONST(3))
  #define STk_void	((SCM) MAKE_SCONST(4))
)

                                         -Richard