Re: deductive databases

From: Simon Taylor <stayl_at_cs.mu.oz.au>
Date: Wed, 18 May 2005 00:54:54 GMT
Message-ID: <428a925c$1@news.unimelb.edu.au>

In article <30pie.93684$yc.5551208_at_phobos.telenet-ops.be>, Jan Hidders wrote:

> Simon Taylor wrote:
>> In article <1116283418.903814.273940_at_f14g2000cwb.googlegroups.com>, Mikito Harakiri wrote:

>>>Jan Hidders wrote:
>>>>
>>>>The way that function symbols are interpreted in Prolog and what gives
>>>>them their expressive power in combination with recursion is more like
>>>>what you would call a tuple constructor. So a term like f(x,y)
>>>>represents a binary tuple with fields x and y and a label f that
>>>>distinguishes it from g(x,y). So a better analogue would be user-defined
>>>>record types where the type system allows arbitrary deep nesting or
>>>>recursive types.
>>>
>>>I'm not sure I see the significance of nesting, although I seem to get
>>>a feeling why genericity of record type is a big deal.

>>  
>> Nesting is not significant; it's allowing data structures to express
>> choice that increases the expressive power.
> 
> I tend to disagree. Choice by itself can always be flattened, but 
> arbitrary deep nesting cannot, so it does matter for the expressive 
> power of the query language.

How can you have arbitrarily deep nesting without choice? At some point you have to choose to stop nesting.

Simon. Received on Tue May 17 2005 - 19:54:54 CDT