Framing the 2.1 ABNF discussion -- the separation of abstract vs concrete syntaxes

["Steven Churchill" <steven.churchill@xdi.org>]

All, 

It is to the TC's disadvantage that the discussion around the 2.1 ABNF has
not been framed in terms of abstract vs. concrete syntaxes. 

XRI *does* have an abstract syntax. It is (essentially and informally): "an
XRI is made up of a sequence of segments, where each segment has zero or
more subsegments. Each subsegment can be either a terminal or a cross
reference." (Fragments, queries, persistence, and other minutiae have been
omitted in this description.)

This abstract syntax is simple, elegant, and powerful. (It is, presumably,
one reason that we are all in this space.)

Up through XRI Syntax 2.0, the concrete syntax--reflected in the 2.0
ABNF--has had a high degree of fidelity with the abstract syntax. For
example, the concrete syntax has explicit delimiters between the
subsegments, and cross references are delimited by surrounding parens. (Note
that it did not have full fidelity, for example, because the first
subsegment delimiters could be omitted between a left-most GCS character and
the next subsegment. That is a good thing.)

Requirements of late have deemed that the concrete syntax should differ
further from the abstract syntax, conceivably, because a fuller-fidelity
concrete syntax is neither readable nor writable for common readers and
writers of the concrete syntax.

I do not believe that anyone is proposing that 2.1 be a change the
*abstract* XRI syntax. However, because the TC has failed to frame the
discussion as such, it is difficult for me to tell.

I think that by framing this discussion improperly, the TC is wasting lots
of time and effort. Here's how I would frame it:

o   Propose the abstract syntax for XRI 2.1. Discuss whether or not it
differs from the abstract syntax of 2.0.

o   Provide motivations (the requirements) for why the 2.1 concrete syntax
(reflected by the 2.1 ABNF) should have lower fidelity with the abstract
syntax. Conceivably, these are around read and write-ability. (Does the TC
agree with these requirements?)

o   Provide some analysis of alternative concrete syntaxes (give some
example XRIs) that may meet these requirements.

o   Once the "look" of the concrete syntax is decided, then start discussing
the nuances of the ABNF that will implement this concrete syntax.

For discussion around this "separation of abstract and concrete syntaxes",
see section 1.2 of 
<http://www.cs.chalmers.se/~bringert/publ/exjobb/embedded-grammars.pdf> or
<http://winnie.kuis.kyoto-u.ac.jp/foldoc/foldoc.cgi?abstract+syntax>

Perhaps ASN.1 or something should be used to formalize the XRI abstract
syntax.

~ Steve