XRIs and canonical form

["Dave McAlpin" <dave.mcalpin@epokinc.com>]

I've been asked to draft text specifying a "canonical form" for XRIs. I
wanted to start by understanding what canonical form meant for URIs in
general, and in searching the web I came across the following exchange. The
initial question is from Terence Spielman of Visa, followed by Gabe's and my
responses. Just interesting that we've considered this question before.

Dave

>>>In addition, aside from unresolvable references, is it possible
>>> to canonicalize XRIs?  This is a highly desireable feature
>>> (for equivalence, at a minimum).

>>We talked quite a bit about this. The decision was made to be silent on
>>canonicalization because equivalence is actually unambigious given the
>>rules stated. Now, that doesn't mean that its at all obvious.
>>
>>I do think giving names to the escaped vs. unescpaed forms of XRI, at
>>least, would be useful.  Canonicalization would then just be transforming
>>an identifier into one of those forms. We didn't want to mandate a single
>>canonical form because different environments would need XRIs in different
>>levels of escaping and it would be unfortunate to require a specific
>>canonicalization form that would require otherwise-unneeded
transformation.
>>
>>Again, Dave McAlpin probably has better input on this. 

>A canonical representation might be useful for comparison, but it would
>involve a formal definition of things like "minimally escaped", which would
>be fairly difficult to nail down. It would also depend on the existence of
>a canonical form for URIs used as cross-references. In other words, an XRI
>wouldn't have a canonical form if it contained cross-references that didn't
>define a canonical form.
>
>Note that equivalence rules are generally problematic. The IRI proposal,
>for example, completely dodges the question of equivalence when it says,
>"There is no general rule or procedure to decide whether two arbitrary IRIs
>are equivalent or not... Each specification or application that uses IRIs
>has to decide on the appropriate criterion for IRI equivalence." 2396bis
>notes that even terms like "different" and "equivalent" are fuzzy in the
>general spec and ultimately application dependent.