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Home -> Community -> Usenet -> comp.databases.theory -> Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]

Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]

From: VC <boston103_at_hotmail.com>
Date: Thu, 23 Jun 2005 06:11:50 -0400
Message-ID: <kJadnYB99Oh2FyffRVn-oA@comcast.com>

"Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message news:0ssue.127522$jg2.7110724_at_phobos.telenet-ops.be...

> VC wrote:

>> "Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message
>> news:UGiue.127257$mY2.7061611_at_phobos.telenet-ops.be...
>>
>>>vc wrote:
>>
>> [...]
>>
>>>>But let's not sidetrack to XQuery, rather tell me what 'print' is in
>>>>Daplex without drawing analogies with XQuery. Just give a definition
>>>>if you please.
>>>
>>>The identity function. The print is only there to indicate that you
>>>produce a value, so you can simply translate it to the expression that
>>>produces that value if you makes sure that they are concatenated for each
>>>value that you iterate over.
>>
>> I do not understand this. I tried different interpretation but none
>> fits. The closest translation might be:
>>
>> "' for each' denotes a list comprehension; 'print' is a monadic unit
>> (identity ?) presumably applied to the constructed list. Is that so ?
>> Or you meant something different by 'identity' ?
>
> The expression "for each x in e1 print e2" denotes "[ e2 | x <- e1 ]". Why 
> do you think that apart from this there is some need to give a separate 
> definition of "what print is in DAPLEX"?

OK, let's assume you are right and the expression means what you are saying it means, namely e2 is just a copy of e1, nothing more. A simple experiment would show there is somethng else going in addition to merely mapping the original list to its copy. Running the original statement (for each x in e1 print e2) would create some symbols on your screen or paper. So your hypothesis is wrong because it does not explain the observed phenomenon. My suspicion is that the culprit is 'print' (which magically disappeared from the list comprehension). What's your explanation ?

>
> -- Jan Hidders Received on Thu Jun 23 2005 - 05:11:50 CDT

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