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I have no issue with question mark or period, as these are necessary for semantic completeness.
What I would like to discuss for CIF2.0 is the following:
(i) The interpretation of a data value as numeric is determined solely by the dictionary with no regard to the particular delimiters used in the CIF file;
(ii) A convention is encouraged for CIF writers whereby numeric values are not enclosed by delimiters.
The advantage of this simpler scheme is a clean separation between syntax and human-relevant semantics. The only CIF applications that can have a use for the CIF1 scheme are those that are written without reference to a dictionary, most obviously pretty-printers that might want to tabulate numbers by lining up decimal points instead of left-justifying. Even if such formatting applications get it wrong, they will not change the meaning of the file and so I would view point (ii) as sufficient support for such applications. Conversely, any application that wishes to operate on a number as opposed to operating on the textual representation of the number will of necessity need to know what this number means and will therefore be written with reference to a dictionary, making it unnecessary to signal "numericness" using whitespace deliimited datavalues.
What do others think? If there is a body of CIF1 applications out there that have been designed to raise errors when values expected to be numeric are enclosed by delimiters, this proposal would represent a further annoying change from CIF1, and it would be good to have some idea of how many such applications there are. I speculate that many applications ignore the delimiter status, for reasons both of laziness, the authority of the dictionary definitions, and the philosophy of writing liberal parsers.
all the best,
James.
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[ddlm-group] Semantics of whitespace-delimited values
- To: ddlm-group <ddlm-group@iucr.org>
- Subject: [ddlm-group] Semantics of whitespace-delimited values
- From: James Hester <jamesrhester@gmail.com>
- Date: Wed, 8 Jul 2015 00:17:32 +1000
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Dear All,
One issue that has not been discussed in the context of the CIF2 syntax is the special interpretation of whitespace-delimited values. In CIF1.1 as recorded in Volume G, a whitespace-delimited question mark and a whitespace-delimited period have a special interpretation as "unknown" and "default/not applicable/null" respectively. Furthermore, only a whitespace-delimited value matching a specified syntax (which includes optional appended esd values) may be interpreted as a numeric value, and it would strictly speaking be a semantic error for a CIF processor to interpret as a number a numeric value enclosed in delimiters.(iii) The precise construction of numeric values is moved into the DDLm attribute dictionary.
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