[cdr-discuss] What about CDR 3, then?

[Pascal Costanza]

On 6 Dec 2006, at 11:12, Christophe Rhodes wrote:

> [ I think I basically agree with you; some comments below ]
>
> Pascal Costanza <pc at p-cos.net> writes:
>
>>> I think that an implementation providing a non-list non-vector
>>> sequence would have to document its behaviour, so it falls in the
>>> realm of language extension or implementation-defined behaviour.
>>
>> OK, it's still the case that you are only relaxing the requirements,
>> but you are not making it explicit what it means when some code it
>> outside of the domain of lists and vectors. With the current ANSI CL
>> spec, a conforming program can rely on the fact that the functions in
>> the Sequence Dictionary always work as expected when the objects
>> passed are of type sequence (!).
>
> Up to a point.  Depending on how carefully you read, you might find
> that in fact some or most of them are implicitly defined only on lists
> and vectors, in wording that is new since CLtL2 (and apparently
> editorial rather than decided upon in an issue).

OK, apparently my wording wasn't very clear. What I meant is this:  
Actually just because these functions are defined only on lists and  
vectors _and_ the only kinds of sequences you can create in ANSI CL  
are lists and vectors, everything is well-specified.

>> With your current formulation, it's not clear what can and cannot be
>> relied on.
>
> This I agree with.
>
>> My suggestion to resolve this is as follows: Instead of just removing
>> the "must signal an error of type type-error" requirement, it should
>> be made more specific what the consequences are when the types are
>> not list or vector, roughly as follows.
>>
>> - If the given type is not a recognizable subtype of sequence, then
>> an error of type type-error is signaled.
>> - If the given type is a recognizable subtype of sequence, but
>> neither a recognizable subtype of list, nor a recognizable subtype of
>> vector, the consequences are unspecified.
>
> I think this is probably close to my intent, and would make no current
> implementation non-conforming; if I may, I will use wording similar to
> this in a revision of CDR 3.

Please feel free to do so.

I am not sure whether implementations that adopt CDR 3 would still be  
conforming, though. The error terminology in ANSI CL implies that  
conforming programs can rely on the fact that sequence functions  
signal errors when they are passed what are neither lists nor  
vectors. With CDR 3, they cannot rely on this anymore.

But that's actually the whole point of CDR 3, as far as I understand it.



Pascal

-- 
Pascal Costanza, mailto:pc at p-cos.net, http://p-cos.net
Vrije Universiteit Brussel, Programming Technology Lab
Pleinlaan 2, B-1050 Brussel, Belgium