- From: Kevin Braun <kbraun@obj-sys.com>
- Date: Tue, 08 Mar 2011 09:46:29 -0500
- To: "Costello, Roger L." <costello@mitre.org>
- CC: "xmlschema-dev@w3.org" <xmlschema-dev@w3.org>
As I recall, in order to substitute one element for another (as in substitution groups), you have to be using global elements and their types must be compatible. The choice allows for local elements (they can be in no namespace) and they can have types that are not at all related. In your example, train, plane, and automobile happen to have the same content models, but that may not always be the case. Kevin -- Objective Systems, Inc. REAL WORLD ASN.1 AND XML SOLUTIONS http://www.obj-sys.com On 3/8/2011 9:36 AM, Costello, Roger L. wrote: > Hi Folks, > > Here is an example of using xsd:choice. The content of<transportation> is a choice of either<train>,<plane>, or<automobile>. > > <xsd:element name="transportation"> > <xsd:complexType> > <xsd:choice> > <xsd:element name="train" type="xsd:string"/> > <xsd:element name="plane" type="xsd:string"/> > <xsd:element name="automobile" type="xsd:string"/> > </xsd:choice> > </xsd:complexType> > </xsd:element> > > Rather than using xsd:choice, the schema could be designed using xsd:sequence. The sequence consists of an element that ref's to an abstract mode-of-transportation element. The train, plane, and automobile elements are globally declared and are substitutable with mode-of-transportation. > > <xsd:element name="transportation"> > <xsd:complexType> > <xsd:sequence> > <xsd:element ref="mode-of-transportation" /> > </xsd:sequence> > </xsd:complexType> > </xsd:element> > > <xsd:element name="mode-of-transportation" abstract="true" type="xsd:string" /> > > <xsd:element name="train" substitutionGroup="mode-of-transportation"/> > <xsd:element name="plane" substitutionGroup="mode-of-transportation"/> > <xsd:element name="automobile" substitutionGroup="mode-of-transportation"/> > > I believe that the two designs produce identical results. > > Suppose the xsd:choice element were jettisoned from the XML Schema specification. Would there be any loss of functionality? Put another way, can you provide a real-world, compelling example of a schema that uses xsd:choice which cannot be expressed using abstract element plus substitution groups? > > /Roger > >
Received on Tuesday, 8 March 2011 14:47:02 UTC