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Re: no constants please
From: Ken Dickey <Ken.Dickey@xxxxxxxxxxxxxx>
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, 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
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))