Cart
|
[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: The Rule of Least Power
 > Original Message From: "Costello, Roger L." > > Given the task of declaring an element to have a value that is one of a > > finite list of strings (or some other simple data type), you should > > declare the element using the least powerful methodâenumerations. > > > > Example: Create an XML Schema that declares a Color element to have one of > > these strings: red, white, or blue. One way to implement Color is with a > > simpleType that lists the values using enumeration facets: > > > > <xs:element name="Color"> > > <xs:simpleType> > > <xs:restriction base="xs:string"> > > <xs:enumeration value="red" /> > > <xs:enumeration value="white" /> > > <xs:enumeration value="blue" /> > > </xs:restriction> > > </xs:simpleType> > > </xs:element> > > > > A second way to implement Color is with a simpleType that lists the values > > using a regular expression in a pattern facet: > > > > <xs:element name="Color"> > > <xs:simpleType> > > <xs:restriction base="xs:string"> > > <xs:pattern value="red|white|blue" /> > > </xs:restriction> > > </xs:simpleType> > > </xs:element> > > > FWIW, using enumerations where possible instead of patterns and asserts has > benefits with data binding because the code generator can generate mirroring > programming language enumerations (Java, C++ etc.) to represent the XSD > enumerations, which is not so easy with the other forms of expressing the > constraint. And also allows XML-aware editors to provide better content assist (e.g. providing the user with a drop-down list of the enumeration values). > Pete Cordell > Codalogic Ltd > Interface XML to C++ the easy way using C++ XML > data binding to convert XSD schemas to C++ classes. > Visit http://codalogic.com/lmx/ or http://www.xml2cpp.com/ > for more info > ----- Original Message ----- > From: "Costello, Roger L." <costello@mitre.org> > To: <xml-dev@lists.xml.org> > Sent: Thursday, June 21, 2012 5:44 PM > Subject: The Rule of Least Power > > > > Hi Folks, > > > > Below is a discussion of the rule of least power and how it applies to XML > > Schema design. The rule of least power is very cool. Comments welcome. > > /Roger > > > > > > The rule of least power says that given a choice of suitable ways to > > implement something, choose the least powerful way. > > > > The following example illustrates the rule of least power. > > > > The XML Schema enumeration facets are less powerful than regular > > expressions (used by the XML Schema pattern facet), which is less powerful > > than XPath (used by the XML Schema 1.1 assert element): > > > > enumerations < regular expressions < XPath expressions > > > > Given the task of declaring an element to have a value that is one of a > > finite list of strings (or some other simple data type), you should > > declare the element using the least powerful methodâenumerations. > > > > Example: Create an XML Schema that declares a Color element to have one of > > these strings: red, white, or blue. One way to implement Color is with a > > simpleType that lists the values using enumeration facets: > > > > <xs:element name="Color"> > > <xs:simpleType> > > <xs:restriction base="xs:string"> > > <xs:enumeration value="red" /> > > <xs:enumeration value="white" /> > > <xs:enumeration value="blue" /> > > </xs:restriction> > > </xs:simpleType> > > </xs:element> > > > > A second way to implement Color is with a simpleType that lists the values > > using a regular expression in a pattern facet: > > > > <xs:element name="Color"> > > <xs:simpleType> > > <xs:restriction base="xs:string"> > > <xs:pattern value="red|white|blue" /> > > </xs:restriction> > > </xs:simpleType> > > </xs:element> > > > > You should use the first way, not the second. > > > > Why? > > > > Answer: With the first way it is easier for applications to analyze the > > XML Schema Color element for its list of valid values. In the second way > > applications must understand the regular expression (regex) language. > > Although this particular regex is simple, the regex language is complex > > and creating applications to determine what set of strings an arbitrary > > regex accepts is non-trivial. > > > > The regular expression language has more power than enumeration facets. If > > your task is to constrain the value of an element to a specified list of > > values, then use enumeration facets, not the pattern facet. > > > > There is a third way to implement this example, using the XML Schema 1.1 > > assert element: > > > > <xs:element name="Color"> > > <xs:complexType> > > <xs:simpleContent> > > <xs:extension base="xs:string"> > > <xs:assert test=". = ('red', 'white', 'blue')" /> > > </xs:extension> > > </xs:simpleContent> > > </xs:complexType> > > </xs:element> > > > > In the assert element the value of the test attribute is an XPath > > expression: > > > > <xs:assert test="XPath" /> > > > > XPath is a powerful language, even more powerful than regular expressions. > > For applications to analyze the XML Schema Color element for its list of > > valid values will require applications to understand the XPath language--a > > daunting task indeed. > > > > Lesson Learned: When creating an XML Schema, determine the suitable ways > > of implementing each feature and choose the one with the least power. > > > > W3C Paper: Tim Berners-Lee and Noah Mendelsohn wrote a wonderful paper on > > the rule of least power: > > > > http://www.w3.org/2001/tag/doc/leastPower.html > > > > Here are a few fascinating snippets from the paper: > > > > Powerful languages inhibit information reuse. > > > > Expressing constraints, relationships and processing instructions in less > > powerful languages increases the flexibility with which information can be > > reused: the less powerful the language, the more you can do with the data > > stored in that language. > > > > Less powerful languages are usually easier to secure ⦠Because programs in > > simpler languages are easier to analyze, it's also easier to identify the > > security problems that they do have. > > > > ⦠characteristics that make languages powerful can complicate or prevent > > analysis of programs or information conveyed in those languages ⦠Indeed, > > on the Web, the least powerful language that's suitable should usually be > > chosen. This is The Rule of Least Power > > > > ⦠the suggestion to use less powerful languages must in practice be > > weighed against other factors. Perhaps the more powerful language is a > > standard and the less powerful language not, or perhaps the use of simple > > idioms in a powerful language makes it practical to use the powerful > > languages without unduly obscuring the information conveyed. Overall, the > > Web benefits when less powerful languages can be successfully applied. > > Michael Glavassevich XML Technologies and WAS Development IBM Toronto Lab E-mail: mrglavas@ca.ibm.com E-mail: mrglavas@apache.org 
[Date Prev] | [Thread Prev] | [Thread Next] | [Date Next] -- [Date Index] | [Thread Index] |
PURCHASE STYLUS STUDIO ONLINE TODAY!Purchasing Stylus Studio from our online shop is Easy, Secure and Value Priced! 
Download The World's Best XML IDE!Accelerate XML development with our award-winning XML IDE - Download a free trial today! Subscribe in XML format
|
|||||||||
