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Re: Aggregation (with GBY) is Relational Division

From: Frank Hamersley <terabitemightbe_at_bigpond.com>
Date: Wed, 07 Jun 2006 04:44:31 GMT
Message-ID: <PQshg.4775$ap3.560@news-server.bigpond.net.au>


vc wrote:
> Frank Hamersley wrote:

[..]

>> I know its just an intro web page ... but does anyone know how foldr
>> disperses the start point?
>>
>>    eg> fold1 (+) 4 [1..3]   produces a computational sequence 4+1+2+3
>>

> No, foldl (+) 4 [1..3] => ((4+1)+2)+3 = 10

Per your subsequent post - our expressions are the "same".

>> so does
>>
>>    eg> foldr (+) 4 [1..3]   produce 4+3+2+1 or 3+2+1+4?

>
> foldr (+) 4 [1..3] => 1 + (2 + (3 +4)) = 10

or

   foldr (+) 4 [1..3] => 1 + (2 + (4 + 3)) = 10

or

   foldr (+) 4 [1..3] => (((4 + 3) + 2) + 1) = 10

???

The question becomes does the leftness/rightness persist in the construction of the expression prior to evaluation. I must confess my past exposure to stack oriented processing of expressions is influencing my interest.

Cheers, Frank. Received on Tue Jun 06 2006 - 23:44:31 CDT

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