passing flags to a function

passing flags to a function

[Dan McMahill]

Hello,

I'm sure this is a very basic question about passing "or-able" flags to 
a function.  In C I might do something like:

#define FLAG_FOO 1
#define FLAG_BAR 2
#define FLAG_BAZ 4

....

myfn(int flags)
{

    if (flags & FLAG_BAR)
      printf ("bar flag is set\n");

...

}

and then call myfn with things like

myfn(FLAG_FOO | FLAG_BAZ);


now suppose I want to wrap up myfn and make it available from scheme. 
What is the normal scheme way for passing in flags to a function?

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".

Thanks for any suggestions.

-Dan


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Re: passing flags to a function

[Ludovic Courtès]

Hi,

Dan McMahill <mcmahill@mtl.mit.edu> 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.


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Re: passing flags to a function

[Dan McMahill]

Ludovic Courtès wrote:
> Hi,
> 
> Dan McMahill <mcmahill@mtl.mit.edu> 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.
> 


Hi Ludovic,

Thanks for the reply. Could you show a short example of what the scheme 
function call might look like?

Are you meaning

(foo (list 'flag1 'flag2 'someotherflag))

I don't care much about speed in this particular case.

Thanks
-Dan


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Re: passing flags to a function

[Ludovic Courtès]

Hi,

Dan McMahill <mcmahill@mtl.mit.edu> writes:

> Thanks for the reply. Could you show a short example of what the
> scheme function call might look like?
>
> Are you meaning
>
> (foo (list 'flag1 'flag2 'someotherflag))

Rather, it would look like:

  (foo 'flag1 'flag2 'someotherflag)

This is because my example uses "rest" arguments, but you could also do
the same as what you describe using a non-rest list argument.

Thanks,
Ludovic.


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Re: passing flags to a function

[Dan McMahill]

Ludovic Courtès wrote:
> Hi,
> 
> Dan McMahill <mcmahill@mtl.mit.edu> writes:
> 
> 
>>Thanks for the reply. Could you show a short example of what the
>>scheme function call might look like?
>>
>>Are you meaning
>>
>>(foo (list 'flag1 'flag2 'someotherflag))
> 
> 
> Rather, it would look like:
> 
>   (foo 'flag1 'flag2 'someotherflag)
> 
> This is because my example uses "rest" arguments, but you could also do
> the same as what you describe using a non-rest list argument.
> 
> Thanks,
> Ludovic.
> 

Thanks!  I think this should be enough to get me pointed in the right 
direction.

-Dan



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Re: passing flags to a function

[Marius Vollmer]

Dan McMahill <mcmahill@mtl.mit.edu> writes:

> Are you meaning
> 
> (foo (list 'flag1 'flag2 'someotherflag))

To make this look nicer, you can also write

    (foo '(flag1 flag2 someotherflag))

Another option are keyword arguments, like

    (foo :flag1 #t :flag2 #t :someotherflag #t)

There is no C-level support for parsing argument lists like this,
unfortunately; you have to do it manually.

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Re: passing flags to a function

[Clinton Ebadi]

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On Tue, 2006-05-02 at 08:25 -0400, Dan McMahill wrote:
> Hello,
> 
> I'm sure this is a very basic question about passing "or-able" flags to 
> a function.  In C I might do something like:
...snip...

You can make the flags available to scheme as numbers, and then the
Scheme users can use Guile's bitwise operations (logand, longior,
logxor, etc. [see Simple Data Types -> Numbers -> Bitwise Operations in
the manual]).

> but I'm not sure of the best way to define the flags in scheme.  Or 
> maybe this is not "the scheme way".

This isn't really the Scheme way, but it works if you want to get
bindings done quickly. Ludovic already recommended the more schemey way,
but this isn't such a bad thing for an extension to a C program.

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Re: passing flags to a function

[Kevin Ryde]

Dan McMahill <mcmahill@mtl.mit.edu> writes:
>
> SCM scm_myfn(SCM flags)
> {
>   myfn (scm_num2int (flags, SCM_ARG1, "myfn"));

Various things in the guile core are done like that, stuff like O_RDWR
for `open'.  The list of symbols Ludovic described is done in the
guile-gtk interface and works nicely too.

If you're entirely in scheme then the ice-9 optargs module to take
keywords is good, especially if you want actual values, not just
yes/no flags.

	(foo #:error-check #t
	     #:verbose #f
	     ...)


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