[plt-scheme] Bit manipulation in PLT Scheme

On Mar 16, Matthew Flatt wrote:
> At Thu, 16 Mar 2006 12:23:32 -0500, Eli Barzilay wrote:
> > On Mar 16, Matthew Flatt wrote:
> > > At Wed, 15 Mar 2006 19:53:58 +0000, "Paulo J. Matos" wrote:
> > > > How can I know when I have a 30 or 62 bit fixnum then?
> > > 
> > > There's no simple way to tell, currently.
> > 
> > Wouldn't
> > 
> >   (eq? (expt 2 35) (expt 2 35))
> > 
> > work?
> 
> This is fragile. MzScheme will compile the above expression to the
> constant #t or #f.

Ah...  (There's always the `eval' back door... except that you have to
bend low to use it...)


> This sort of problem is why I'm reluctant to add `fixnum?' to the
> language. Right now, I think that "fixnum" is just an implementation
> and performance concept, instead of being part of the language.
> 
> > And related to that -- it seems that something like a `fixnum?' or
> > `most-positive-fixnum' would be useful for bit-manipulation cases
> > and foreign interaction...
> 
> Foreign interaction is a good idea: using `(lib "foreign.ss")',
> `(compiler-sizeof 'long)' will return 4 for a 32-bit platform or 8
> for a 64-bit platform.

I think that what's missing here is the set of assumptions that
MzScheme does, like the fact that a `long' is what's used as a single
fixnum/pointer.

-- 
          ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
                  http://www.barzilay.org/                 Maze is Life!