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Re: Accessing multiple values from C

From: Ludovic Courtès
Subject: Re: Accessing multiple values from C
Date: Wed, 18 Jan 2012 22:44:39 +0100
User-agent: Gnus/5.110018 (No Gnus v0.18) Emacs/24.0.90 (gnu/linux)


Mark H Weaver <address@hidden> skribis:

> address@hidden (Ludovic Courtès) writes:
>> Mark H Weaver <address@hidden> skribis:
>>> Julian Graham <address@hidden> writes:
>>>> +SCM
>>>> +scm_c_value_ref (SCM obj, size_t idx)
>>>> +{
>>>> +  SCM values = scm_struct_ref (obj, SCM_INUM0);
>>>> +  return scm_list_ref (values, SCM_I_MAKINUM (idx));
>>>> +}
>>>> +
>>> Should this verify that `obj' is a values object?
>>> Should it verify that `idx' can fit in an inum?
>> Yes, good point.  Using an ‘SCM_VALIDATE’, which will throw an exception
>> upon error, right?
> Well, there's a complication with using an `SCM_VALIDATE' to check for
> values objects.  If `idx' is 0 (make sure it's exact!), then we must
> _not_ throw an exception if `obj' is not a values object.  However, in
> that case we still need to know whether `obj' is a values object or not.

Yes, I already agreed on this one.  ;-)

> Also, a fine point about desirable error messages: asking for `idx' 5 of
> a values object with only 3 elements is, conceptually, the same kind of
> error as asking for `idx' 5 of something that is _not_ a values object.
> Therefore, I think in both cases the error should be something to the
> effect of "too few values".



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