Mark H Weaver <mhw@netris.org> writes:

> Hi Alex,
>
> Alex Vong <alexvong1995@gmail.com> writes:
>
>> Oh, I didn't really think too much about this. I always thought immediate
>> objects like SCM_EOL, SCM_BOOL_T, ..., SCM_UNDEFINED can be compared
>> using '=='. Is this an implementation detail that I should not depend
>> on?
>
> That's right.  You should not rely on SCM being represented in any
> particular way.  In C, '==' works only on pointers and values of
> arithmetic type, not on structs or unions.  Therefore, in general, you
> cannot use '==' on a value of an abstract type without some knowledge of
> the underlying type.
>
> Guile's API is designed to avoid propagating assumptions about the SCM
> type.  Please use only the functions and macros in Guile's public API to
> inspect, compare, or manipulate values of type SCM.  This will enable
> Guile implementors and other hackers to experiment with other SCM data
> representations.  We reserve the right to change the SCM type in future
> versions of Guile (but not within a stable release series e.g. 2.2.x).
>
> I should also mention that Guile provides a mechanism to check for such
> mistakes.  If you test compiling your libguile-using code with
> -DSCM_DEBUG_TYPING_STRICTNESS=2, it will change SCM to be a struct type,
> and you'll get a compile error if you attempt to use '==' on values of
> type SCM.  (Note that this will produce code that is ABI-incompatible
> with libguile compiled using the default SCM_DEBUG_TYPING_STRICTNESS=1
> setting, so typically you would set SCM_DEBUG_TYPING_STRICTNESS=2 only
> as a compile-time check.)
>
> For details, see the description of SCM_DEBUG_TYPING_STRICTNESS in
> __scm.h, and also the top few pages of tags.h, which describes the
> fundamental concepts of how Guile Scheme objects are represented in C,
> including the definitions of the SCM type and the 'scm_is_eq' macro.
>
>      Regards,
>        Mark

Thanks for your informative post!