Re: scheme max/min in c++

[David Kastrup]

Mark Knoop <address@hidden> writes:

> At 13:56 on 19 Aug 2016, David Kastrup wrote:
>>Mark Knoop <address@hidden> writes:
>>> Just trying out some more ideas for the keep-alive-together-engraver.
>>> How do I use the Guile max/min functions on a scheme list in C++? I
>>> have tried this:
>>
>>Well...
>>
>>    5.5.2.11 Arithmetic Functions
>>    .............................
>>
>>    The C arithmetic functions below always takes two arguments, while
>> the Scheme functions can take an arbitrary number.  When you need to
>> invoke them with just one argument, for example to compute the
>> equivalent od `(- x)', pass `SCM_UNDEFINED' as the second one:
>> `scm_difference (x, SCM_UNDEFINED)'.
>>
>>[...]
>>
>>     -- Scheme Procedure: max x1 x2 ...
>>     -- C Function: scm_max (x1, x2)
>>         Return the maximum of all parameter values.
>>
>>Most notable, scm_max is a C function, not a Scheme function (which
>>would be of type SCM itself).  You cannot pass it to scm_apply_0.
>
> Thanks David for the pointers.
>
>>If you really, really want to use the Scheme function, however, ...
>
> I have no particular desire to use the Scheme function, but was just
> looking for the most elegant way to do this. Would it perhaps be better
> to convert to C types?

The usual thing is to open-code the min/max for arbitrary numbers of
arguments.  Like

SCM max;
if (scm_is_pair (x))
  {
    max = scm_car (x);
    for (SCM n = scm_cdr (x); scm_is_pair (n); n = scm_cdr (n))
      max = scm_max (max, scm_car (n));
  }
else
  max = default_value;

Maybe one should write a C version of `reduce' for automating this.  But
then it's not all that complicated wither.

-- 
David Kastrup