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Re: Backus-Naur descriptions for STAR and CIF

  • Subject: Re: Backus-Naur descriptions for STAR and CIF
  • From: Nick Spadaccini <nick@xxxxxxxxxxxxx>
  • Date: Wed, 17 May 2000 16:03:40 +0100 (BST)
On Wed, 17 May 2000, Brian McMahon wrote:

> Hence for CIF data files - as originally envisaged - the save_ and global_
> restrictions should apply; and Nick's BNF is erroneous.

Yes, this and many other CIF restrictions are things I have not come to 
grips with yet - perhaps it is why I stick to STAR. The global_ structure
was a feature of CIF dictionaries, but I had never really thought of CIF
dictionaries not being themselves CIFs. But you are correct, and certainly
the mmCIF dictionary is not a CIF, because of the save frames. 

I am happy to update the CIF BNF accordingly, if you give me the go ahead
Brian. If I read you correctly the DDL1 based dictionaries will follow the
same BNF except that one can explicitly enumerate the allowed datanames.
 
> So where does this leave us? Do we need one BNF, for STAR, and then handle
> all other cases as special, with different exceptions hard-coded? Or do we
> need multiple BNFs (STAR, CIF, dictionaries - even DDL1 and DDL2
> dictionaries)? There was some debate about this on COMCIFS a while back,
> and we didn't really reach a conclusion. I think the answer will be a
> pragmatic one, based on the needs of the software community. So this is an
> opportunity to poll the list on just that point.

I am not a fan of one STAR BNF and then defining the other formats as
exceptions, because these are usually described in a loose fashion. A BNF
can at least be specific about syntax - semantics are a different matter.


On Wed, 17 May 2000, Bollinger, John Clayton wrote:

> Thanks for the clarification.  What about designating the end of looped
> data with "stop_"?  Without nested loops that is unnecessary, of
> course, but it does not appear to actually be excluded.  (And
> Spadaccini's CIF BNF provides for it.)

It is provided for within the BNF but not mandatory. Therefore its
presence or otherwise still makes for a consistent CIF.

> Well, an appropriate question would seem to be: "what is the purpose
> of having a BNF representation?"  I would argue that the principal
> reason is to have a concrete, formal, precise definition of the
> language(s) for reference purposes.  Once we have a final BNF
> representation for STAR, then, do we need additional BNFs for
> restricted dialects of STAR?  Is it less precise or concrete if we say
> that a CIF, for example, is a STAR file that also satisfies certain
> (specified) structural and syntactic constraints?  In the cases of CIF,
> DDL1, and DDL2, I don't think so.  In my opinion, the nature of the
> relevant constraints is such that specific BNF representations of those
> languages wouldn't help us much more than the STAR BNF and the relevant
> set of restrictions.

Again my problem is in what form do you define these restrictions. English
prose is expressive but unfortunately easily open to misinterpretation. A
BNF on the other hand gives you a strong idea of the allowed syntactic
constructs. By analogy we don't have one BNF describing imperative
procedural programming languages, and then describe C, Fortran etc as a
set of restrictions to that BNF. It is far easier to have a BNF for each
language.

The trick is, REDUCING the number of languages we are dealing with.

cheers

Nick

--------------------------------
Dr Nick Spadaccini
Department of Computer Science              voice: +(61 8) 9380 3452
University of Western Australia               fax: +(61 8) 9380 1089
Nedlands, Perth,  WA  6907                 email: nick@cs.uwa.edu.au
AUSTRALIA                        web: http://www.cs.uwa.edu.au/~nick





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