Re: more closed-world chatter
Date: Sun, 06 May 2007 09:40:46 -0300
Message-ID: <463dcc9e$0$4027$9a566e8b_at_news.aliant.net>
Gene Wirchenko wrote:
> Marshall <marshall.spight_at_gmail.com> wrote:
>
>
>>On May 5, 10:04 am, "David Cressey" <cresse..._at_verizon.net> wrote:
>>
>>>"David BL" <davi..._at_iinet.net.au> wrote in message
>>>
>>>news:1178377225.112732.324630_at_e65g2000hsc.googlegroups.com...
>>>
>>>
>>>>On May 5, 3:16 am, Marshall <marshall.spi..._at_gmail.com> wrote:
>>>
>>>>[snip]
>>>
>>>>>def pricedomain = set {1, 2};
>>>>>def anotherdomain = set {3};
>>>
>>>>>def R1 = set(price) [price : pricedomain] {(1)};
>>>>>def R2 = set(price) [price : anotherdomain] {(3)};
>>>
>>>>>def R = R1 & R2;
>>>>>---
>>>
>>>>>At this point, what is the value of R?
>>>
>>>>> R = set(price) [price:pricedomain; price:anotherdomain] {};
>>>
>>>>>Since the union of pricedomain and anotherdomain is empty:
>>>
>>>>IMO it's confusing to talk about taking a "union of the constraints".
>>>>In a sense you are aggregating them. I would call it an ANDing of
>>>>constraints as boolean valued expressions, which leads to set
>>>>intersections not unions.
>>>
>>>Sorry to add to the confusion, but I will: the union of the constraints
>>>yields the intersection of the data. Clear as mud?
>>
>>Yes, exactly.
>>
>>All the constraints that apply in a given context could be thought of
>>as a set, or they could be thought of as a single constraint, with
>>the terms ANDed together. Furthermore, a single constraint can be put
>>in Conjunctive Normal Form and each term can then be separated.
>>
>>I remember reading about Prolog some years ago, and seeing:
>>"the comma can be read as AND" and thinking WTF?! But
>>if you have an expression like
>>
>> x > 0, x < 10
>>
>>then the comma certainly means AND.
>
> Not in C.
Since when did Prolog become C ? ;) Received on Sun May 06 2007 - 14:40:46 CEST