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Subject: RE: [xml-dev] Is there a way to reuse and extend an enumeration in XML schema
Henry, Thanks for your interesting post. I am the editor of the UBL code list specification and I think your solution may help us trade off some difficulties we have with our present solution. We have considered substitution groups because: 1) You could define a substitution once and have it reflect everywhere without modifying the base schema or, also importantly, a third party code list schema that UBL has incorporated by reference. 2) Your modified schema's namespace would prefix all uses so that you explicitly know which fields are modified in the instance file. Problems with the substitution group method are: 1) Many UBL elements are derived from the code list types. Therefore additional definitions would be needed -- eg <xsd:element name='myCode' type='stdCodeListType'/> instead of using the global code by reference. 2) Problem number 1 is compounded by the desire to use code lists as attributes as well as elements. Thus you would have to have all elements containing code attributes be global and substitute them. 3) Because substitution groups are unconstrained, discipline would need to be used to ensure that it was only used for code list extension / restriction. This gives the UBL naming and design rules team great concern. I guess the only minor downsides I see to your redefine method is: 1) you would have to look to the schemaLocation attribute in the instance file to recognize that the redefinitions occurred. 2) you would not be able to pick and choose which instances of the code list is overridden -- e.g. one instance uses <cl:code>foo</> and another uses <customCl:code>foo</>. 3) This is more of a question -- is it possible to redefine several types from different namespaces in a single schema file so there would be a single incorporation of all changes referenced in the schemaLocation attribute of the instance file. Thanks again for highlighting a promising solution that might be able to meet all the UBL requirements. Marty Burns, Hypertek for NIST 1(301)315-9101 burnsmarty@aol.com -----Original Message----- From: Henry S. Thompson [mailto:ht@inf.ed.ac.uk] Sent: Thursday, March 10, 2005 6:35 AM To: William J. Kammerer Cc: XML Developers List Subject: Re: [xml-dev] Is there a way to reuse and extend an enumeration in XML schema I've worked a trivial example to fill out and illustrate the redefine approach I sent earlier. Here's the base schema, published by the namespace owner: curEnumBase.xsd: <xs:schema targetNamespace="http://www.example.com/fakeUBL" xmlns="http://www.example.com/fakeUBL" xmlns:xs="http://www.w3.org/2001/XMLSchema"> <!-- Example base schema for extensible enumerations --> <xs:element name="currency" type="currencyCodeType"/> <xs:simpleType name="currencyCodeType"> <xs:restriction base="xs:NMTOKEN"/> </xs:simpleType> </xs:schema> Here's the vanilla driver schema, likewise published by the namespace owner: curEnumDriver.xsd: <xs:schema targetNamespace="http://www.example.com/fakeUBL" xmlns="http://www.example.com/fakeUBL" xmlns:xs="http://www.w3.org/2001/XMLSchema"> <!-- Example driver schema for extensible enumerations --> <xs:redefine schemaLocation="curEnumBase.xsd"> <xs:simpleType name="currencyCodeType"> <xs:restriction base="currencyCodeType"> <xs:enumeration value="UKL"/> <xs:enumeration value="USD"/> </xs:restriction> </xs:simpleType> </xs:redefine> </xs:schema> And here's a valid instance: <currency xmlns="http://www.example.com/fakeUBL" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.example.com/fakeUBL curEnumDriver.xsd"> UKL </currency> And an invalid one: <currency xmlns="http://www.example.com/fakeUBL" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.example.com/fakeUBL curEnumDriver.xsd"> CAD </currency> To make it valid, we make our own extended driver: curEnumExt.xsd: <xs:schema targetNamespace="http://www.example.com/fakeUBL" xmlns="http://www.example.com/fakeUBL" xmlns:xs="http://www.w3.org/2001/XMLSchema"> <!-- Example extended schema for extensible enumerations --> <xs:redefine schemaLocation="curEnumBase.xsd"> <xs:simpleType name="currencyCodeType"> <xs:restriction base="currencyCodeType"> <xs:enumeration value="UKL"/> <xs:enumeration value="USD"/> <xs:enumeration value="CAD"/> </xs:restriction> </xs:simpleType> </xs:redefine> </xs:schema> and if we change the xsi:schemaLocation of our example to point to this driver, it's valid. As it happens, Dan Vint and colleagues had arrived at this solution long before I proposed it. He described it in much more detail in a recent post to xmlschema-dev [1], including the following, which says it all, IMO: 1) Produce a base schema that references a type for each list with no enumerations [e.g. curEnumBase]. 2) Produce a second schema that redefines those list types to the enumerated values [e.g. curEnumDriver]. Anyone needing to modify those lists modifies the second redefining schema [e.g. curEnumExt]. Now when I release the next version, all that has to happen is the modifier reviews the new redefining schema for new lists. These changes have to be copied into the file they originally modified. They also have to find any changes to the existing lists as well. They then use the newly produced base schema and point their modified redefine schema to point at the this file instead of the original base schema. This has the advantage of creating one single file with all the modifications. It still is not a perfect solution, but it is the best compromise that we could come up with. We also made the type of the lists to be QNAME and we require that anyone adding a value to a list use an appropriate namespace prefix to identify their additions. ['e.g.'s added] Some further observations: 1) The publisher could make this approach easier if they used an external general entity to include the enumerations in the redefining schema document: curEnumDriver.xsd: <!DOCTYPE xs:schema [ <!ENTITY currencyEnumeration SYSTEM "http://www.example.com/fakeUBL/currencies.xnt"> ]> <xs:schema targetNamespace="http://www.example.com/fakeUBL" xmlns="http://www.example.com/fakeUBL" xmlns:xs="http://www.w3.org/2001/XMLSchema"> <!-- Example driver schema for extensible enumerations --> <xs:redefine schemaLocation="curEnumBase.xsd"> <xs:simpleType name="currencyCodeType"> <xs:restriction base="currencyCodeType"> ¤cyEnumeration; </xs:restriction> </xs:simpleType> </xs:redefine> </xs:schema> Then the extension can track the official list and changes thereto more easily: curEnumExt.xsd: <!DOCTYPE xs:schema [ <!ENTITY currencyEnumeration SYSTEM "http://www.example.com/fakeUBL/currencies.xnt"> ]> <xs:schema targetNamespace="http://www.example.com/fakeUBL" xmlns="http://www.example.com/fakeUBL" xmlns:xs="http://www.w3.org/2001/XMLSchema"> <!-- Example driver schema for extensible enumerations --> <xs:redefine schemaLocation="curEnumBase.xsd"> <xs:simpleType name="currencyCodeType"> <xs:restriction base="currencyCodeType"> ¤cyEnumeration; <xs:enumeration value="CAD"/> </xs:restriction> </xs:simpleType> </xs:redefine> </xs:schema> 2) This approach works directly for types used for attributes _or_ elements, whereas any approach using substitution groups (which are very useful for many things, but not this problem, in my opinion) only works directly for elements. Hope this helps -- if you think it does, please pass it on to the UBL list. . . ht [1] http://lists.w3.org/Archives/Public/xmlschema-dev/2005Mar/0008.html -- Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh Half-time member of W3C Team 2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440 Fax: (44) 131 650-4587, e-mail: ht@inf.ed.ac.uk URL: http://www.ltg.ed.ac.uk/~ht/ [mail really from me _always_ has this .sig -- mail without it is forged spam] ----------------------------------------------------------------- The xml-dev list is sponsored by XML.org <http://www.xml.org>, an initiative of OASIS <http://www.oasis-open.org> The list archives are at http://lists.xml.org/archives/xml-dev/ To subscribe or unsubscribe from this list use the subscription manager: <http://www.oasis-open.org/mlmanage/index.php>
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