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RE: Substitution groups and xsi:type

From: Jeff Lowery <jlowery@scenicsoft.com>
Date: Fri, 18 May 2001 10:01:28 -0700
Message-ID: <3549BAFD79A7D411A1CF00508B62B5BC766C06@exchange-us.scenicsoft.com>
To: "'Mike_Leditschke@nemmco.com.au'" <Mike_Leditschke@nemmco.com.au>, xmlschema-dev@w3.org
Don't confuse element substitution with type substitution. The reason to use
xsi:type in a document is indicate that the default element type (the one
defined for the element in the schema) has be 'substituted' with a derived

A substitution group allows one element to be substituted for another, so
long at the types are the same or the substituted element's is a derived
type. The xsi:type isn't necessary, becuase the defualt type for the
substituted element is in use.

It is possible to combine element substitution with type substitution:
substitute an element, then substitute its type with a derived type.

I hope that's not too confusing of an explanation (and it may even be
substantially correct).


> -----Original Message-----
> From: Mike_Leditschke@nemmco.com.au
> [mailto:Mike_Leditschke@nemmco.com.au]
> Sent: Friday, May 18, 2001 9:36 AM
> To: xmlschema-dev@w3.org
> Subject: Substitution groups and xsi:type
> In section 4.6 of the primer, two statements are made...
> 1. "Elements in a substitution group must have the same
> type as the head element, or they can have a type that has been
> derived from the head element's type"
> then later
> 2. "..when an instance document contains element substitutions
> whose types are derived from those of their head elements, it is
> not necessary to identify the derived types using the xsi:type
> construction..."
> I think the wording of the second is tripping me up a bit given
> that, assuming the first statement, I can't see how you'd
> ever get a case where the second didn't apply, that is,
> an instance document containing element substitutions, whose
> types WEREN'T derived from those of the head etc and hence
> who WOULD have to use the xsi:type attribute.
> Is the intent to say, if you use an element substitution, the 
> rules about
> providing xsi:type attributes in instance documents don't apply?
> On a similar vein, it appears that for substitution groups to work,
> validators are capable of working
> out "for themselves" which particular substitution group element has
> been provided in an instance document without the instance author
> explicitly telling the validator.
> It seems a small step to ask why then, I need to provide xsi:type
> attributes at all, given that the validator knows the set of 
> types that
> could possbily be used i.e. those derived from the base type used
> in the schema defining the instance document.
> What additional "magic" is enabled by explictly tying a series of
> related elements (and by implication types) together via
> substitutionGroup attribute
> I've probably gone off the road completely somewhere. A pointer to
> the skid marks would be appreciated.
> Regards
> Michael
Received on Friday, 18 May 2001 13:02:17 UTC

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