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daffodil-trailing-sep branch - status of work on trailing separator sequences


I branched off Josh Adams' daffodil-trailing-sep branch a while back. Starting at hash

9f8173d4a962d9373d29f85d5a92435afe622bba


I have since made substantial changes, and just successfully rebased onto the 2.2.0 development branch.


That is, for some definition of "success". I had 44 failures in daffodil-test before, and after rebase, seems to be same 44 failures. These are all related to separated sequences in some way.


I did this rebase because I'm doing enough refactoring of the grammar stuff that I didn't want to too far without the changes to the grammar that were done for the layering feature.


Unfortunately, it looks like the whole way sequences and separators are dealt with has to change.


The grammar package of the Daffodil schema compiler has been "barely working" for  awhile now, is overdue for a revamping, and I've tried quite a few smaller fixes to it without success.


I'm at the point now where I'm planning to re-implement much of it. There's no way the existing grammar stuff was going to be just-tweeked and achieve all of:


  *   proper trailing-separator suppression behavior
  *   good diagnostic messaging about searches for delimiters
  *   performance
  *   code clarity - apparent correctness

A naive assumption in the grammar design was the idea that this aspect of the schema compiler could, in some way, match the data grammar of the Daffodil specification document. The intention here was that the grammar code would then be spec-compliant by inspection to some degree. I don't see this as feasible anymore at all. The grammar in the spec was created without an implementation to refer back to, and is really pretty poorly structured even for just a grammar in the specification document, and of course diagnostic behavior - which I firmly believe is 1/2 the complexity of any parser - was not a consideration when the DFDL spec grammar was formulated, and the projection of that into the Daffodil schema compiler's grammar module is similarly challenged.

Anyway, that's the status for those trying to follow it. I am going to try to get this put back together for review before I depart for a vacation on June 13.