Title: Re: exceptions

Author:jenglish@crl2.crl.com (Joe English)

Date:18 Aug 1997 11:35:59 -0700

------------------------------------------------------ Lars Marius Garshol <larsga@ifi.uio.no> wrote: >matzen@a.cs.okstate.edu (MATZEN RICHARD WAL) writes: [ Re: inclusion and exclusion exceptions ] >> 2.b. Does it add significant expressive power for authors? > >In theory, no, as you can always write an equivalent DTD without exceptions. Actually, it's _not_ always possible to write an equivalent DTD without exceptions. It is always possible to convert a DTD with exclusion exceptions into a "superset" DTD (which accepts a larger set of documents), simply by removing the exclusions. However, it's not possible to create an _exactly_ equivalent exclusion-free DTD. One possible approach is to create a new element type (with a restricted content model) for each original element type that's affected by an exclusion; for example: <!DOCTYPE MYDOC [ <!ELEMENT P - - (#PCDATA|X|Y|Z)* > <!ELEMENT STUFF - - (P+) > <!ELEMENT BOTHER - - (P+) -(X) > <!ELEMENT MYDOC - - (STUFF|BOTHER)* > ]> becomes: <!DOCTYPE MYDOC2 [ <!ELEMENT P - - (#PCDATA|X|Y|Z)* > <!ELEMENT PinBOTHER - - (#PCDATA|Y|Z)* > <!ELEMENT STUFF - - (P+) > <!ELEMENT BOTHER - - (PinBOTHER)+ > <!ELEMENT MYDOC - - (STUFF|BOTHER)* > ]> but this is *not* equivalent to the first DTD. You could I suppose follow it up with an architectural transformation: <!ATTLIST PinBOTHER MYDOC NAME #FIXED "P" > Then the set of documents conforming to MYDOC2 will, after architectural processing, be equivalent to the set of documents conforming to MYDOC. >However, that may be so clumsy that it makes a difference in real life. Definitely :-) In the worst case, the new DTD can end up with N*M element types, where N is the number of element types in the original DTD and M is the number of different potentially applicable exclusion exceptions. When it comes to inclusion exceptions, strictly speaking it's not even possible to create a superset DTD because of the interaction between inclusion exceptions and record ends. However, in the common case where those special-case semantics are not essential (or even desired), it is possible to create a superset DTD by adding the included subelements to the content models of all potentially affected element types: <!DOCTYPE MYDOC3 [ <!ELEMENT P - - (#PCDATA|Y|Z)* > <!ELEMENT STUFF - - (P+) +(X) > <!ELEMENT BOTHER - - (P+) > <!ELEMENT MYDOC - - (STUFF|BOTHER)* > ]> becomes: <!DOCTYPE MYDOC4 [ <!ELEMENT P - - (#PCDATA|Y|Z | X)* > <!ELEMENT STUFF - - (P+) > <!ELEMENT BOTHER - - (P+) > <!ELEMENT MYDOC - - (STUFF|BOTHER)* > ]> Then every document that's legal under MYDOC3 is also legal under MYDOC4, but the converse is not true since MYDOC4 allows X in P in BOTHER but MYDOC3 does not. Beware, though, that this transformation -- adding included elements to all affected content models -- is not always straightforward. SEQ and AND groups are tricky, and if an inclusion is also legal as a proper subelement you have to be particularly careful. >> 2.c. Are there document types that can be defined by a DTD with exceptions, >> but not by a DTD without exceptions? > >No. This is absolutely correct, with the caveat that in the DTD without exceptions some constraints must be expressed informally. --Joe English jenglish@crl.com