Using (defined? foo) from C.

Using (defined? foo) from C.

[Thamer Al-Harbash]

I'm attempting to do binding reflection with libguile from within
C. However, I cannot find a way to create a symbol without
defining it. There is no way to pass a C string to an equivalent
of scm_definedp() without using scm_define() to create the
symbol in the first place. A bit of a paradox :)

Is there anyway to create a symbol SCM in C without defining it
in the current environment? If not is there a way of passing a C
string as a symbol name to do reflection with? I'm looking for
something as simple as evaluating (defined? foo) where foo is
variable and receive the boolean value back.

-- 
Thamer Al-Harbash            http://www.whitefang.com/
	(if (> pressure too-much-pressure)
		'play-ac2 'work)


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Re: Using (defined? foo) from C.

[Marius Vollmer]

Thamer Al-Harbash <tmh@whitefang.com> writes:

> Is there anyway to create a symbol SCM in C without defining it
> in the current environment?

Yes.  Symbols are a data type of their own, independent of bindings,
environments and modules.  You can use

    scm_str2symbol ("foo")

in C to get the symbol with the name "foo".  Symbols in turn are used
to name things in environments and modules, but they can be used for
other things as well.

> If not is there a way of passing a C string as a symbol name to do
> reflection with? I'm looking for something as simple as evaluating
> (defined? foo) where foo is variable and receive the boolean value
> back.

Try this (with a big comment that Guile should provide it itself,
damnit):

    static SCM
    false (void *unused1, SCM unused2, SCM unused3)
    {
      return SCM_BOOL_F;
    }

    static SCM
    wrapped_scm_c_lookup (void *data)
    {
      return scm_c_lookup ((char *)data);
    }

    int
    defined_p (char *str)
    {
      return !SCM_FALSEP (scm_internal_catch (SCM_BOOL_T,
                                              wrapped_scm_c_lookup, str,
                                              false, NULL));
    }

The reason for this longwinded code is that scm_c_lookup either
returns a valid variable or throws an error.  We did this to have a
simple way to lookup variables and at the same time handle unbound
variables in a consistent manner.

Clearly, there should also be a function that return SCM_BOOL_F when a
name is unbound, but we don't have that yet.  It will appear when the
module system API as a whole, ahem, 'stabilizes'.

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Re: Using (defined? foo) from C.

[Thamer Al-Harbash]

On Sat, 29 Mar 2003, Marius Vollmer wrote:

> Thamer Al-Harbash <tmh@whitefang.com> writes:
>
> > Is there anyway to create a symbol SCM in C without defining it
> > in the current environment?
>
> Yes.  Symbols are a data type of their own, independent of bindings,
> environments and modules.  You can use
>
>     scm_str2symbol ("foo")

Thanks, I couldn't find this in the procedure index of the manual
but it's mentioned in section 19.14 (transitioning to the scm
interface). That's exactly what I need.

-- 
Thamer Al-Harbash            http://www.whitefang.com/
	(if (> pressure too-much-pressure)
		'play-ac2 'work)


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Re: Using (defined? foo) from C.

[Neil Jerram]

>>>>> "Thamer" == Thamer Al-Harbash <tmh@whitefang.com> writes:

    Thamer> On Sat, 29 Mar 2003, Marius Vollmer wrote:
    >> Thamer Al-Harbash <tmh@whitefang.com> writes:
    >> 
    >> > Is there anyway to create a symbol SCM in C without defining it
    >> > in the current environment?
    >> 
    >> Yes.  Symbols are a data type of their own, independent of bindings,
    >> environments and modules.  You can use
    >> 
    >> scm_str2symbol ("foo")

    Thamer> Thanks, I couldn't find this in the procedure index of the
    Thamer> manual but it's mentioned in section 19.14 (transitioning
    Thamer> to the scm interface). That's exactly what I need.

If you have time, please could you work out the texinfo needed to get
this into the procedure index, and send a patch?

Many thanks,
        Neil



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