2222 2005-09-14 19:22:42 +0000 R-230: Question on maxOccurs=0, particles and restriction 2009-04-21 19:21:37 +0000 1 1 1 Unclassified XML Schema Structures: XSD Part 1 1.1 only All All CLOSED FIXED medium, easy, restriction cluster resolved P4 normal --- 1 sandygao cmsmcq www-xml-schema-comments oldest_to_newest 6245 0 sandygao 2005-09-14 19:22:42 +0000 Please clarify an issue regarding the use of maxOccurs="0", particularly within a type derived by <restriction> From various places in the Structures recommendation I have gathered the following information: 1. Where minOccurs=maxOccurs=0, the item "corresponds to no component at all." [repeated] 2. The Particle Schema Component: {max occurs} - "Either a non-negative integer or unbounded." http://www.w3.org/TR/xmlschema-1/#p-max_occurs 3. Particle Correct: for all particles "{max occurs} must be greater than or equal to 1." http://www.w3.org/TR/xmlschema-1/#p-props-correct Also, from the Primer: 4. An element with minOccurs="0" and maxOccurs="0" means the element "must not appear." http://www.w3.org/TR/xmlschema-0/#cardinalityTable 5. In restriction, minOccurs="0" and maxOcurs="0" means "exclusion of an optional component." http://www.w3.org/TR/xmlschema-0/#restrictsTable It seems to me that (2) and (3) conflict. (2) effectively says that a particle may have maxOccurs="0" while (3) says this would not be a correct particle. Is an incorrect particle still a particle?? I'm not sure that (1) excuses this conflict. Looking at the Errata I cannot see any substantive changes to these definitions. The problem I have specifically relates to restriction of a complexType containing a sequence. As I understand it, the derived restricted sequence can either simply not mention the element I wish to 'prohibit' or can explicitly prohibit it by specifying maxOccurs="0" and this should mean the same thing. However, (3) implies that the latter would not validate. E.g. <complexType name="base"> <sequence> <element name="a" minOccurs="0" maxOccurs="1"/> <element name="b" minOccurs="0" maxOccurs="1"/> <element name="c" minOccurs="0" maxOccurs="1"/> </sequence> </complexType> <complexType name="deriveByOmission"> <complexContent> <restriction base="base"> <sequence> <element name="a" minOccurs="0" maxOccurs="1"/> < prohibit element "b" by omission --> <element name="c" minOccurs="0" maxOccurs="1"/> </sequence> </restriction> </complexContent> </complexType> <complexType name="deriveByZeroOccurrence"> <complexContent> <restriction base="base"> <sequence> <element name="a" minOccurs="0" maxOccurs="1"/> <element name="b" minOccurs="0" maxOccurs="0"/> <!-- prohibit element "b" --> <element name="c" minOccurs="0" maxOccurs="1"/> </sequence> </restriction> </complexContent> </complexType> (Helpfully, of the seven schema tools I've tried, 5 accept both derivations (but given other experiences are maybe just too lax) and 2 refuse both...) Which of these 2 derivations are correct? See: http://lists.w3.org/Archives/Public/www-xml-schema-comments/2003JulSep/0042.html 12982 1 sandygao 2006-11-22 04:08:46 +0000 Both derivation should be valid. "deriveByOmission" and "deriveByZeroOccurrence" will have the same content model, because minOccurs=maxOccurs=0 "corresponds to no component at all." #2 and #3 are confusing. Proposal: to change the component description and disallow "0" as a valid value for maxOccurs. And to remove "{max occurs} must be greater than or equal to 1" from "Particle Correct". Resolution: adopted at 2006-11-17 telecon.