[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message]

RE: RE : Comparison of Xml documents

• To: <bob@w...>, "'Bob Foster'" <bob@o...>
• Subject: RE: RE : Comparison of Xml documents
• From: "Alessandro Triglia" <sandro@m...>
• Date: Mon, 24 Nov 2003 00:10:04 +0900
• Cc: "'Alaric B Snell'" <alaric@a...>, <xml-dev@l...>
• Importance: Normal
• In-reply-to: <200311230627.AJN04525@m...>

Bob Wyman wrote:
> 
> Bob Foster wrote:
> > In ASN.1, does SET mean that order is both lexically
> > and semantically insignificant (as it does in
> > mathematics)?    
> For more information on ASN.1, I suggest you take a look at: 
> X.680. You can find it at: 
> http://www.itu.int/ITU-T/studygroups/com17/languages/
> 
> The definition of Sequence and Set types from that document 
> are below:
>     "3.6.60 sequence types: Types defined by referencing a 
> fixed, ordered list of types (some of which may be declared
> to be optional); each value of the sequence type is an 
> ordered list of values, one from each component type.
> NOTE â?? Where a component type is declared to be optional, a 
> value of the sequence type need not contain a value of that 
> component type."
>     "3.6.64 set types: Types defined by referencing a fixed, 
> unordered, list of types (some of which may be declared to be 
> optional); each value in the set type is an unordered list of 
> values, one from each component type. NOTE â?? Where a 
> component type is declared to be optional, a value of the set 
> type need not contain a value of that component type."
> 
> SET and SEQUENCE in ASN.1 are statements about the permited 
> lexical order of elements. In such cases that I've seen 
> significance given to order, it has been done in 
> specifications that accompany or incorporate the ASN.1 
> definitions. However, the rules for Canonical and 
> Distinguished encoding both require that the elements of SETs 
> are, in fact, ordered according to their tag numbers. Thus, 
> any semantic significance of order would be discarded if SETs 
> were encoded with CER or DER. 


I disagree on this particular view.

There cannot be any significance in the order of a SET (OF).  The fact that DER/CER "remove" the freedom of order of BER indicates precisely that a SET (OF) is inherently unordered at the abstract level.  Encoder's options (such as the SET component order in BER or the attribute order in EXTENDED-XER) cannot be used to convey any semantic significance.  They are there just for convenience.  They carry zero bits of information, they are pure entropy.

Alessandro


> For instance, X.690 says, in 
> defining CER:
> 
> "9.3 Set components: The encodings of the component values of 
> a set value shall appear in an order determined by their tags 
> as specified in 8.6 of ITU-T Rec. X.680 | ISO/IEC 8824-1. 
> Additionally, for the purposes of determining the order in 
> which components are encoded when one or more component is an 
> untagged choice type, each untagged choice type is ordered
> as though it has a tag equal to that of the smallest tag in 
> that choice type or any untagged choice types nested within."
> 
> >Finally, if SET does mean that order is lexically and  semantically 
> >insignificant, how does one specify in ASN.1  that order is 
> lexically 
> >insignificant but semantically  significant?
>    I'm not aware of any schema languages that allow the 
> specification semantic signifance to order independent of 
> lexical significance. Can you provide an example of one that 
> does? I am curious to see how this concept is used and 
> expressed.
> 
>     bob wyman
> 

• References:
  • Re: RE : Comparison of Xml documents
    • From: Bob Wyman <bob@w...>

• Prev by Date: RE: RE : Comparison of Xml documents
• Next by Date: crimson package in j2sdk1.4.1
• Previous by thread: Re: RE : Comparison of Xml documents
• Next by thread: Re: RE : Comparison of Xml documents
• Index(es):
  • Date
  • Thread