Re: Optional attributes



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From: Daniel Jackson (dnj@mit.edu)
Date: Thu 20 May 2004 - 02:42:46 BST


joaquin,

hehner's theory is actually more complicated, and results in a 4-valued 
logic. our approach is closer in spirit to tarski's relational 
calculus, or Codd's relational model.

/daniel



On May 19, 2004, at 7:52 PM, Joaquin Miller wrote:

> For those interested, there is a theoretical foundation for Daniel's 
> approach, worked out by Hehner.  See chapter 2 of 
> http://www.cs.toronto.edu/~hehner/aPToP/aPToPtrans2.pdf
>
> (In this pleasant theory, 'null' is the name of an item (the name of a 
> value, if you like), to wit: the empty bunch.  Clean as a whistle.  
> And uniform, a quality much to be desired.)
>
>
> Daniel Jackson wrote:
>> john,
>>
>> we've done quite a lot of work on this in the context of Alloy, an 
>> object modelling language very similar to OCL. Alloy takes a more 
>> minimalist approach than OCL, and it's not as expressive in some 
>> respects, so it may be that Alloy's solution won't work for you. but 
>> you may want to look at what we've done as it may give you some 
>> useful ideas.
>>
>> see, for example, the now rather dated alloy overview at:
>>
>>         http://alloy.mit.edu/reference-manual.pdf
>>
>> in short, Alloy has no null or undefined value, and treats an 
>> optional attribute as one that either has an empty value, or a set 
>> containing a single element. this has nice syntactic and semantic 
>> consequences, and seems to be easier for people to understand than 3 
>> valued logics or special values.
>>
>> /daniel

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