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From: aaron@grad2.cis.upenn.edu (Aaron Watters)
Newsgroups: comp.databases
Subject: Re: errors and 4 valued logic
Keywords: logic, error, domain
Message-ID: <33148@netnews.upenn.edu>
Date: 19 Nov 90 19:28:34 GMT
References: <720@creatures.cs.vt.edu>
Sender: news@netnews.upenn.edu
Reply-To: aaron@grad1.cis.upenn.edu (Aaron Watters)
Organization: University of Pennsylvania
Lines: 58


 Once again we are concerned with the truth values
 for the following statements
 	B: `the king of north america is Bald.'
 	H: `the king of north america has Hair.'
 	G: `I am Green years old.'
I claimed B = not H, B=H=error, G=false.

In article <720@creatures.cs.vt.edu> holliday@csgrad.cs.vt.edu (Glenn Holliday) writes:
...
>  Chuck Phillips (Chuck.Phillips@FtCollins.NCR.COM) also took my
>path when he broke [B and H] down into
>
>>B1: There exists exactly one king of North America.
>>-and-
>>B2: The king of North America is bald.
>
>>H1: There exists exactly one king of North America.
>>-and-
>>H2: The king of North America is not bald.
>
>Aaron Waters argues [Watters]
>>... the noun `the king of north america' does not correspond to
>>any existential statement it corresponds
>>	in logic		to a constant
>>	in pascal (ml)		to a pointer (reference)
>>	in O-O dbs's		to an object.
>
>Exactly!  Each of your statements H B and G are not constants.  They are
>intended to model more than one chunk of information about the objects
>"king", "north america", "hair", etc.  If you _do_ treat them as logical
>constants, then your argument is correct.  But I believe they have lost
>all meaning if you do that.

Well, I didn't say that, I said that `the king of north america'
behaved like a logical constant -- NOT like an existential assertion
 `the king is bald' != `there is a king who is bald'
the two statements simply have differing intuitive behaviors.  The
only way you can enforce the restatement you suggest on a database
user is to disallow the first sort of statement...

Imagine, if you will, a `fourth generation object oriented database
language' where the user writes:
	let K be any king of north america: (K is an object)
           if K is not bald print `the king has hair'
           if K is does not have hair print `the king is bald'
        endlet.
Now I admit that the program is awkwardly written, but I think
you would agree that the output
	`the king is has hair the king is bald'
defies the ordinary interpretation of the above pseudocode.  I claim
that since the code assumes the existence of such a king the let
statement should result in a `null' object (no reason to raise an
error if it is never used) and the evaluation of `K is not bald'
should result in an error. such as
>	Answer: No [king] is defined on this domain.
As you suggest.
		-aaron