ratio implementation

[Bill Schottstaedt]

There is an implementation of ratios for Guile (based on CVS version
of 27-Jul-03) at ccrma-ftp.stanford.edu:/pub/Lisp/gratio.tar.gz.
Rather than send a huge diff, I placed the new versions of the changed
files (all from the libguile directory) in the tarball along with
*.diff showing the changes.  I added numerator, denominator,
rationalize and ratio?, and at the C level scm_make_ratio and
scm_i_ratio2real (should it be scm_ratio2dbl?).  "ratio?" is needed
because "rational?" returns #t if passed a real -- there has to be
some way to distiguish a ratio from a real. An alternative would be to
make "rational?"  rational.

I don't know how the FSF/GPL copyright stuff works, but I did this
work on my own time, did not look at any other implementation, and
hereby donate the code to you.  I'd be happy to "sign the papers".

I use longs for the numerator and denominator, so if bignums are
encountered, I fallback on the old method using scm_divide.  I didn't
try to provide the standard set of C-level type conversions -- not
sure these are needed in this case. I decided to follow Common Lisp
and reduce ratios, and return an integer if the ratio denominator is 1
(or if the numerator is 0).  One ugly change is the SCM_NUMP macro in
numbers.h -- the original of this macro strikes me as a kludge; the
new version is worse.  A questionable change (I followed Dybvig's book
p 126): (/ 17) -> 1/17, (/ 2 3) -> 2/3.  I notice that r5rs seems to
imply that (inexact->exact .3) should return 3/10 (see the rationalize
example which is assuming this) -- should I implement this?  What is
"reasonably close" in this case?  Another gray area involves functions
like round -- in r5rs (round 7/2) returns 4, whereas Guile returns 4.0
(I left it this way because (round 7) currently returns 7.0 rather
than 7) -- I decided to make minimal changes, but handling of
exact/inexact distinctions in Guile could use some work (I am willing
do this, if others approve).  Also, should random accept ratios?
And currently (format #f "~F" 2/3) hangs, but so does
(format #f "~B" 1.5) (in Guile 1.6.4 you get an error, but
in the CVS Guile it hangs in mutex_lock).


The changed files are:

smob.c
hash.c
eq.c
goops.c
gc-card.c
objects.c
numbers.c (nearly all the changes are in this file)
eval.c

numbers.h
tags.h
objects.h

Also in the tarball: ratio-tests.scm.