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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 – 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 – 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. 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
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