Re: Is (VECTOR NIL) a valid type specifier? (Re: [Sbcl-devel] Re: STRING is too hairy for sequence functions.)

[Marco A. <ma...@cs...>]

On Sep 28, 2004, at 10:03 AM, Paul F. Dietz wrote:

> Marco Antoniotti wrote:
>> On Sep 27, 2004, at 10:31 PM, Paul F. Dietz wrote:
>>> Marco Antoniotti wrote:
>>>
>>>> The above should always generate an error as (VECTOR NIL) is not a 
>>>> valid type specifier.
>>>
>>>
>>> It's not? :)
>> I took it back yesterday, but I have been having nightmares about it 
>> tonight (I should get a life, shouldn't I ? :) )
>> I checked with LW (which I find usually very good at compliance) and 
>> it gives an error.
>> The reasoning why (ARRAY NIL) , hence (VECTOR NIL) should not be 
>> valid type specifiers is because they would violate the disjointness 
>> requirement on the array type hierarchy.  Since NIL is the bottom of 
>> the type hierarchy, all disjointness requirements break down there.  
>> I.e.
>> REAL----+------- FLOAT--------------+--------NIL
>>                 +------- RATIONAL--------+
>> where I depicted the "disjoint" relation, which is fine.
>> However, for compound types (I know there are tons of Ph.D. theses on 
>> the subject: I am not an expert) if we allowed (ARRAY NIL) to be a 
>> valid type specifier, then we should allow the following
>> ARRAY -----+---- (ARRAY CHARACTER) ------+---- (ARRAY NIL) -------- 
>> NIL
>>                      +---- (ARRAY T) ------------------------+
>> which would be incorrect as we would "rejoin" the ARRAY hierarchy 
>> above NIL, while this is expressively forbidden in the ARRAY and 
>> VECTOR definitions (CLHS 15.2: System Class ARRAY and System Class 
>> VECTOR.)
>> Since the CLHS is weak on typing issues, and I do not recall any 
>> place where (ARRAY NIL) is explicitly forbidden,  I would say that 
>> disallowing it is ok and it is a fair work around the CLHS 
>> incompleteness (or inconsistencies :) )
>> Please correct me if you think this is bogus.
>
>
> I think this is bogus. :)
>
> I think you are imagining that if T1 is a subtype of T2, then (array 
> T1)
> must be a subtype of (array T2).  That's not the case.

You are right.  My bogosity score just went up :)

However, then I do not understand why in SBCL you need the (OR (VECTOR 
NIL) (VECTOR CHARACTER)) thingy when dealing with STRING.

Cheers

--
Marco Antoniotti					http://bioinformatics.nyu.edu
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