A managed phase-out of DDL1 dictionaries

[Matthew Towler]

Dear James

Thanks for the more detailed explanation.  I agree with seeing what the state of the community is in two years rather than deciding now.

I remain slightly worried about the DDLm matching changes to DDL1 – as code that reads historic CIF already has to cope with quite a few different ways of specifying space groups (for example) and adding further improved methods will increase complexity.  We may reach a point where parsing a CIF is simple, but writing code that will reliably interpret what is intended by the values in a majority of extant CIF requires quite a steep learning curve involving many previous versions of the dictionaries.

I am interested in understanding the benefits in back porting DDLm changes to DDL1, and the trade-off of these against the cost of change.  What I am wondering is whether it would be better to have DDL1 remain as-is, and keep the better representations only in DDLm; the advantage being that if DDLm support is being added to existing code, then that would also be a good point to add support for improvements in semantics.  Back porting to DDL1 risks imposes an otherwise unrelated change ahead of the need to add DDLm support, which might actually detract from the effort required to add DDLm support.

Best wishes,

Matthew
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