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Subject: RE: [xri] Solution for GCS ! ambiguity in ABNF
I'd point out that while this may make the ABNF less ambiguous, it now
makes parsing XRIs more complicated. Now, we don't just check the first
character to see if it's a GCS character, we also have to check the
second character (when the GCS char is !) to see if it is ! or something
else.
Just something to consider. I don't really have a strong opinion either
way - both have their negatives.
-Gabe
> -----Original Message-----
> From: Drummond Reed [mailto:drummond.reed@cordance.net]
> Sent: Tuesday, March 08, 2005 9:55 AM
> To: xri@lists.oasis-open.org
> Subject: [xri] Solution for GCS ! ambiguity in ABNF
>
> In doing the final review of the 05 Syntax spec, a solution to the
> longstanding issue about the ambiguity of "!" was identified.
> This email
> briefly summarizes the issue and the solution. The proposed
> change to the
> ABNF that fixes it has been closely reviewed by Dave, Chetan,
> and myself, so
> we plan to roll this change into the 06 draft unless anyone has an
> objection.
>
> SUMMARY OF THE ISSUE
>
> For simplicities sake, the current ABNF treats "!" the same
> as the other GCS
> characters, when in fact the intent all along has been that
> unlike the other
> GCS characters, "!" should only be followed by a persistent
> subsegment,
> i.e., that as an absolute XRI, you could only have "!!123"
> and never "!123".
> It should be unambiguous that the latter is a relative persistent XRI
> subsegment.
>
> However the current ABNF does not enforce this rule, so a relative XRI
> starting with "!" is currently ambiguous. As a result in the
> 05 draft Dave
> needed to add a section (2.4.7) that explained that you can't
> just write
> "!123" as a relative subsegment, but instead you need to
> prefix it with
> "./!123" to make sure it's not ambiguous.
>
> Because this is ugly, and because it requires treating relative
> !sub-segments differently than relative *sub-segments, the following
> solution was identified.
>
> PROPOSED SOLUTION
>
> The solution involves changing six lines of the ABNF, with
> five of these
> changes being just to make room for the one new production
> that actually
> eliminates the ambiguity. Following is a complete analysis.
>
> First, below is the current ABNF from the authority and path sections:
>
> ****CURRENT ABNF****
>
> xri-authority = gcs-authority / xref-authority
>
> gcs-authority = gcs-char xri-segment
>
> xref-authority = xref *xri-subseg
>
> gcs-char = "=" / "@" / "+" / "$" / "!"
>
> xri-path = xri-path-absolute
> / xri-path-noscheme
> / ipath-empty
>
> xri-path-absolute = "/" [ xri-segment-nz *( "/" xri-segment ) ]
>
> xri-path-noscheme = xri-subseg-od-nx *xri-subseg-nc
> *( "/" xri-segment )
>
> xri-segment = xri-subseg-od *xri-subseg
>
> xri-segment-nz = xri-subseg-od-nz *xri-subseg
>
> xri-subseg = ( "*" / "!" ) (xref / *xri-pchar)
>
> xri-subseg-nc = ( "*" / "!" ) (xref / *xri-pchar-nc)
>
> xri-subseg-od = [ "*" / "!" ] (xref / *xri-pchar)
>
> xri-subseg-od-nz = [ "*" / "!" ] (xref / 1*xri-pchar)
>
> xri-subseg-od-nx = [ "*" / "!" ] 1*xri-pchar-nc
>
> xri-gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "("
> / ")" / "*" / gcs-char
>
> ***END***
>
> Second, following are the six changed lines of ABNF that
> would be needed.
> Two of them are revisions to current productions (gcs-authority and
> xri-gen-delims), two are renamed productions (gcs-char
> renamed rgcs-char,
> and gsc-authority renamed rgcs-authority), and two are new productions
> (pgcs-authority and xri-subseg-pt-nz). Note also that
> "xri-subseg-pt-nz" is
> named to match the rest of the "xri-subseg" productions, with the "pt"
> standing for "persistent".
>
> Also note that the only changes in the rgcs-char and xri-gen-delims
> productions are to move "!" out of the rgcs-char because of
> its special
> treatment vs. the other reassignable GCS characters.
>
> ****LINES THAT WOULD CHANGE IN THE REVISED ABNF****
>
> gcs-authority = pgcs-authority / rgcs-authority
>
> pgcs-authority = "!" xri-subseg-pt-nz *xri-subseg
>
> xri-subseg-pt-nz = "!" (xref / 1*xri-pchar)
>
> rgcs-authority = rgcs-char xri-segment
>
> rgcs-char = "=" / "@" / "+" / "$"
>
> xri-gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "("
> / ")" / "*" / "!" / rgcs-char
>
> ****END****
>
> The net effect of this change is simply to separate out the special
> restriction on using "!" as a GCS character. Because of the new
> pgcs-authority production, it is no longer ambiguous that a
> single bang
> represents a relative persistent subsegment, i.e., it is fine to write
> "!123".
>
> To see it all in context, following is the fully revised ABNF
> for this set
> of productions (again, only the six lines above have changed):
>
> ****FULL REVISED ABNF****
>
> xri-authority = gcs-authority / xref-authority
>
> gcs-authority = pgcs-authority / rgcs-authority
>
> pgcs-authority = "!" xri-subseg-pt-nz *xri-subseg
>
> rgcs-authority = rgcs-char xri-segment
>
> xref-authority = xref *xri-subseg
>
> rgcs-char = "=" / "@" / "+" / "$"
>
> xri-path = xri-path-absolute
> / xri-path-noscheme
> / ipath-empty
>
> xri-path-absolute = "/" [ xri-segment-nz *( "/" xri-segment ) ]
>
> xri-path-noscheme = xri-subseg-od-nx *xri-subseg-nc
> *( "/" xri-segment )
>
> xri-segment = xri-subseg-od *xri-subseg
>
> xri-segment-nz = xri-subseg-od-nz *xri-subseg
>
> xri-subseg = ( "*" / "!" ) (xref / *xri-pchar)
>
> xri-subseg-nc = ( "*" / "!" ) (xref / *xri-pchar-nc)
>
> xri-subseg-od = [ "*" / "!" ] (xref / *xri-pchar)
>
> xri-subseg-od-nz = [ "*" / "!" ] (xref / 1*xri-pchar)
>
> xri-subseg-od-nx = [ "*" / "!" ] 1*xri-pchar-nc
>
> xri-subseg-pt-nz = "!" (xref / 1*xri-pchar)
>
> xri-gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "("
> / ")" / "*" / "!" / rgcs-char
>
> ***END***
>
> =Drummond
>
>
>
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