[closer-devel] using a custom unbound slot marker

[Pascal Costanza]

On 30 Oct 2006, at 15:37, Gary King wrote:

> Hi Pascal,
>
> I thinking adding such a thing is a _good_ idea (we can add it to  
> the new "standard")
>
> On Oct 30, 2006, at 7:40 AM, Pascal Costanza wrote:
>
>> I think it's a pity that the CLOS MOP doesn't just specify a  
>> constant for this. Maybe I should add this to Closer to MOP...
>>
>

Hm, I have thought a little bit about this, and maybe it's not a good  
idea after all.

It could be that the low-level machinery to detect unbound slots is  
actually part of the MMU, or some other element at the machine level.  
For example, in a machine that uses tagged values, unbound could be a  
configuration of tag bits. In that case, "the" unbound value would  
probably not exist as such. Something like slot-makunbound is then  
indeed a better abstraction for this.

I am by far not an expert in hardware / cpu / mmu matters, so maybe  
what I am saying here is pure nonsense. On the other hand, I wouldn't  
be surprised if, say, Lisp Machines had some better way to deal with  
unbound variables than using a constant value to represent them. And  
I think we should leave the door open for such possible  
implementations...

Having said that, almost all the implementations that I have checked  
seem to use some constant for unbound slots. The only one that seems  
to behave somewhat mysteriously is MCL.

An alternative to defining a constant for unbound slots would be to  
add explicit functions for reading and writing them. In the end, we  
should probably have the following set of functions:

standard-instance-access
(setf standard-instance-access)
standard-instance-boundp
standard-instance-makunbound

standard-class-access
(setf standard-class-access)
standard-class-boundp
standard-class-makunbound

funcallable-standard-instance-access
(setf funcallable-standard-instance-access)
funcallable-standard-instance-boundp
funcallable-standard-instance-makunbound

funcallable-standard-class-access
(setf funcallable-standard-class-access)
funcallable-standard-class-boundp
funcallable-standard-class-makunbound

The -class- versions would be there for slots  
with :allocation :class. (I don't know what parameters they would  
take, though. Probably just whatever slot-location returns, without  
specifying further what it is that slot-location returns for slots  
with :allocation :class...)

Any comments?


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




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