Re: passing flags to a function

[Ludovic Courtès]

Hi,

Dan McMahill <address@hidden> writes:

> I can do something like
>
> SCM scm_myfn(SCM flags)
> {
>
>   myfn (scm_num2int (flags, SCM_ARG1, "myfn"));
>
>   return SCM_BOOLEAN_T;
>
> }
>
> but I'm not sure of the best way to define the flags in scheme.  Or
> maybe this is not "the scheme way".

What you describe above is doable and is an approach sometimes taken.
See for instance the POSIX functions in Guile, e.g., `popen'.

Personally, I prefer to pass a list of symbols rather than a single
number in such situations.  This might require some more work if you
need to convert those flags to a C or'ed integer, but not so much
because (i) symbols can be compared with `eq' which is fast, (ii)
the list of flags may usually be small, and (iii) the number of values
that can be taken by the flags is small as well.

  SCM_DEFINE (scm_func, "func", 0, 0, 1,
              (SCM flags),
              "My function takes any number of symbols.")
  #define FUNC_NAME "func"
  {
    int c_flags;

    for (c_flags = 0;
         scm_is_pair (flags);
         flags = SCM_CDR (flags))
      {
        SCM f = SCM_CAR (flags);

        if (scm_is_eq (f, my_first_flag_sym))
          c_flags |= MY_FIRST_FLAG;
        else if (scm_is_eq (f, my_second_flag_sym))
          c_flags |= MY_SECOND_FLAG;
        else
          scm_wrong_type_arg (FUNC_NAME, 1, f);
      }

    /* ... */
  }
  #undef FUNC_NAME

But this is debatable and it also depends on the context.

Thanks,
Ludovic.