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[ddlm-group] Semantics of whitespace-delimited values

  • To: Group finalising DDLm and associated dictionaries <ddlm-group@iucr.org>
  • Subject: [ddlm-group] Semantics of whitespace-delimited values
  • From: SIMON WESTRIP <simonwestrip@btinternet.com>
  • Date: Fri, 24 Jul 2015 12:26:38 +0000 (UTC)
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I agree - indeed the 'less-tolerant' applications in my little survey used third-party APIs to read the CIF so it is probably the case that the API is being 'intolerant' rather than the application.

I'd also be happy to see period and question mark in the EBNF - afterall these tokens when white-space delimited should never be interpreted as the string values "?" or ".", so fundamentally they could be regarded as structural tokens, regardless of any other semantics associated with them.

In general, with respect to the imminent introduction of CIF2, the assumption will likely be that the common semantic features of CIF1 will apply to CIF2, which is fair enough. However, personally I would prefer that such semantics were kept distinctly separate from the specification. For example, the CIF1 line-folding 'semantics' are now part of the specification that a parser is expected to be aware of, while the CIF1 character encoding semantics are purely conventions that may be useful in certain domains (the parser really doesn't need to be aware of them). So if the CIF1 convention with respect to period and question marks is generally thought to be an inherent part of CIF, then it would be better placed in the specification that parsers should be aware of (i.e. parsers should be aware of these 'null' tokens and not simply return a "." or "?" with no context)?

The same applies to numbers - if a parser is expected to unequivocally identify numbers from the syntax, then this is no longer a 'common semantic feature'. I believe that a parser needs minimally to identify a 'value', which can be interpreted further down the line.

So perhaps the question boils down to: which (if any) of the semantic features of CIF1 would we expect a CIF2 parser to be aware of?

Cheers

Simon




From: James Hester <jamesrhester@gmail.com>
To: SIMON WESTRIP <simonwestrip@btinternet.com>; Group finalising DDLm and associated dictionaries <ddlm-group@iucr.org>
Sent: Friday, 24 July 2015, 6:37
Subject: Re: [ddlm-group] Semantics of whitespace-delimited values

Let me take up one of Simon's comments:

"...we could suggest that CIF applications started to turn to the dictionary rather than syntax to determine the exact nature of a data item".

What is not perhaps appreciated is that the application programmer accessing the CIF file searching for a numeric value for a particular dataname has already consulted the dictionary when writing the program (with the minor exception of e.g. pretty-printers as noted before).  Consulting the very same dictionary at runtime is pointless as these meanings are never supposed to change. So I would suggest to Simon that there is no problem nudging *application* programmers to accept the dictionary definitions as they already have done so in order to write correct calculations, the only problem will be nudging CIF APIs to do their best to return a number if asked (and your survey would suggest that the majority already do).  My position is very strongly pro application programmer - if they are asking my API for a number, I am not going to second-guess them unless they have asked me to by providing a dictionary as well.

I'd be happy to see period and question mark added to the EBNF as primitive productions, this is a simple change.




On 9 July 2015 at 07:46, SIMON WESTRIP <simonwestrip@btinternet.com> wrote:
Dear all

I extended the mini survey of current applications a little and looked  closer at some of the less-liberal parsers:

one of the applications I've looked at did not complain when I included some non-ASCII text in the CIF, while another complained about a data value constructed as '''z''' (valid CIF1), and one displayed rather quirky behaviour with regard to semicolon-delimited strings, rejecting the contained 'value' if it had a leading newline but not if it had a leading space -
all of these particular applications complained about delimited numbers to the extent that the application stopped processing.

Based on this (albeit limited) survey of some current well- known CIF applications, regarding the introduction of CIF2 it would definitely be prudent to indicate that 'yes indeed' the interpretation of CIF1.1 wihitespace-delimited values retains significance in CIF2. However, if possible I think it would be in the interests of flexibility and unambiguity if somehow we could suggest that CIF applications started to turn to the dictionary rather than syntax to determine the exact nature of a data item (afterall, as I see it, that's one very strong  motivation for developing CIF2 in the first place - and is the preferred approach in CIF1 too).  Thankfully (from my point of view) this isn't even an issue for the majority of applications I have looked at (they simply grab the data however they've found it and make use of it if they can, or they carefully validate the data against the dictionary). So what is challenging me is how we achieve this - i.e. nudging some applications to be a little more flexible (which in my experience is what many  'users' would most appreciate)., while at the same time maintaining the convention that numbers especially are still presented in an undelimited (uncluttered) fashion. I've no convincing answer to this yet.

Regarding the ? and . 'null' values I hesitate to suggest that we could take these out of the issue altogether by making them CIF key tokens - I hesitate because I suspect that some applications simply ignore their significance anyway and incorrect usage rarely presents a real problem (and also I haven't yet attempted to see if its actually possible to define them in this way in any case:-) - so its probably unnecessary and may even seem like a new complication to applications that were not particularly aware of, or bothered by, the significance of these tokens in the first place.

Cheers

Simon







From: "Bollinger, John C" <John.Bollinger@STJUDE.ORG>
To: Group finalising DDLm and associated dictionaries <ddlm-group@iucr.org>; SIMON WESTRIP <simonwestrip@btinternet.com>
Sent: Wednesday, 8 July 2015, 16:21
Subject: RE: [ddlm-group] Semantics of whitespace-delimited values

 
Thanks Simon, James, and John.  I am uncertain how many distinct parsers are represented by the reports so far, but it seems there must be at least five.
 
I think we agree that parsers and applications should be permitted, if not required, to distinguish between the values . and '.', and between the values ? and '?'.  We also seem to agree that it is not useful to insist that parsers or applications refuse to interpret quoted values as numbers, although some CIF 1.1 parsers in fact do so at their own discretion, and some warn instead of refusing (with even that relying on taking the position that numbers are not supposed to be quoted).
 
Not being enamored of special cases, and not wanting CIF 2.0 to rule out CIF interpretation practice that is accepted and common in CIF 1.1 applications, I find myself favoring CIF 2.0 taking the position that in general, it is permitted but not required to interpret any string value differently when it is presented in whitespace-delimited form than when it is presented in any of the other forms.  The conventions for the special values . and ? could then be taken to apply on a domain-wide basis, whereas the convention for the form of numbers could be taken to apply on a more selective basis (per-DDL, per-dictionary, or even per-definition).  An implication of this position, however, is that whether or not a value is presented whitespace-delimited becomes a property of that value that a fully general CIF 2.0 parser must make available to its clients.  Moreover, for better or for worse, future dictionaries could establish additional items or data types whose values are required to be presented unquoted.
 
We could perhaps characterize that more specifically, maybe by saying that the exact form of values presented in any of the quoted forms is significant, or something along those lines, whereas values presented in whitespace-delimited form may afford equivalent alternative expressions.  That doesn’t exactly fit . and ?, but perhaps some similar statement could do so better.
 
 
John
 
--
John C. Bollinger, Ph.D.
Computing and X-Ray Scientist
Department of Structural Biology
St. Jude Children's Research Hospital
 



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