Kill the SCM_NIMPs

[Dirk Herrmann <dirk at ida dot ing dot tu-bs dot de>]

Hi.

After Greg's famous patch, it has been possible to get rid of a thousand
calls to SCM_NIMP.  My believe is, that SCM_NIMP and also SCM_IMP are
extremely low level structures that should be very rarely used.  However,
I realized that there are still quite a lot of them.  Looking closer, I
figured out that many of these are just used as a replacement for
SCM_NNULLP (or SCM_NULLP), for example when cdr'ing along a variable that
is known to be a list.

The funny thing (after having that infamous cycle-counting debate) is,
that when you look at the implementation of SCM_NIMP and compare that
with SCM_NNULLP you realize, that SCM_NNULLP is only a comparison of a SCM
value with the constant SCM_EOL, while SCM_NIMP requires to extract a
single bit from a SCM value.

It's just a guess, but I assume that using SCM_NIMP instead of SCM_NNULLP
(in places where it is appropriate) is not only obfuscating the
semantics of the code, but also might lead to a performance penalty.

Example:  This code is extracted from backtrace.c

-------------------------------------------------------------------------
SCM_DEFINE (scm_set_print_params_x, "set-print-params!", 1, 0, 0,
           (SCM params),
"")
#define FUNC_NAME s_scm_set_print_params_x
{
  int i, n = scm_ilength (params);
  SCM ls;
  print_params_t *new_params;
  SCM_ASSERT (n >= 1, params, SCM_ARG2, FUNC_NAME);
  for (ls = params; SCM_NIMP (ls); ls = SCM_CDR (ls))
    SCM_ASSERT (scm_ilength (SCM_CAR (params)) == 2
...
-------------------------------------------------------------------------

First, the assertion makes sure that 'params' is a proper list and has at
least one element.  From then on, SCM_NIMP is used to test whether the end
of that list is reached.  Obviously, the use of SCM_NNULLP would much
closer reflect the semantics.

I will be glad to help 'kill the SCM_NIMPs', but this will not be starting
before January, 15th:  Now I'm heading off to prepare for my skiing
vacation next week.

So long,
Dirk Herrmann