Scala, XML and GAE

scala, xml and gae mark@ryall.name

xml literals

val document = <root> <child> <grandchild an_attribute=\"value1\" /> <grandchild an_attribute=\"value2\" /> </child> </root>

val document = scala.xml.Elem(null, \"root\", scala.xml.Null, scala.xml.TopScope, scala.xml.Elem(null, \"child\", scala.xml.Null, scala.xml.TopScope, scala.xml.Elem(null, \"grandchild\", new scala.xml.UnprefixedAttribute(\"an_attribute\",\"value1\", scala.xml.Null), scala.xml.TopScope, scala.xml.Text(\"content1\") ), scala.xml.Elem(null, \"grandchild\", new scala.xml.UnprefixedAttribute(\"an_attribute\",\"value2\", scala.xml.Null), scala.xml.TopScope, scala.xml.Text(\"content2\") ) ) )

extraction operators

// immediate descendant of document called 'child' document \\ \"child\" // any descendant of document called 'another_grandchild' document \\\\ \"another_grandchild\" // any descendant attribute called 'an_attribute' document \\\\ \"@an_attribute\"

pattern matching

• match single node with {variable} • match node sequence with {variable @ _*} • cannot match attribute directly

val a = document match { case Elem(_, \"root\", _, _, _*) => true case _ => null }

val a = document match { case <root>{ _* }</root> => true case _ => false }

def process(node: scala.xml.Node): Unit = { println(node) node match { case <root>{contents @ _* }</root> => for (a <- contents) process(a) case <child>{contents @ _*}</child> => for (a <- contents) process(a) case <grandchild>{contents @ _*}</grandchild> => for (a <- contents) process(a) case _ => println(\"leaf\") } }

Concatenation def add(p: Node, newEntry: Node ): Node = p match { case <div>{ ch @ _* }</div> => <div>{ ch }{ newEntry }</div> }

Extracting attributes document match { case n @ <grandchild>{ _* }</grandchild> => println(n.attribute(\"an_attribute\")) }

persisting and restoring

scala.xml.XML.saveFull(\"filename.xml\", document, \"UTF-8\", true, null) val document2 = scala.xml.XML.loadFile(\"filename.xml\")

stream parsing

val src = scala.io.Source.fromString(\"<root><child></child></root>\") val er = new scala.xml.pull.XMLEventReader().initialize(src) er.next ...

building xml

var variable1 = \"value1\" val variable2 = <another_grandchild an_attribute=\"value2\" /> var document = <root> <child> <grandchild an_attribute={variable1} /> {variable2} </child> </root>

google app engine

• ‘cloud’ web hosting environment on google’s infrastructure • uses non relational ‘bigtable’ for data persistance • user authentication uses google accounts or openid • also has email (sending), memcache and image manipulation services

• working example: • code: http://bitbucket.org/markryall/scalagae • live: http://pairs.appspot.com • use latest datanucleus enhancer from http:// www.datanucleus.org/downloads/maven2- nightly/org/datanucleus/datanucleus- enhancer/1.1.2/

basics

class Servlet extends HttpServlet { override def doGet(request: HttpServletRequest, response: HttpServletResponse): Unit = { val out = response.getWriter() ... out.println(<html>{head}<body>{body}</body></html>) } }

authentication

var userService = UserServiceFactory.getUserService(); var user = userService.getCurrentUser(); if (user == null) { response.sendRedirect(userService.createLoginURL(request.getRequestURI())) return }

persistence

@PersistenceCapable {val identityType = IdentityType.APPLICATION} class Person(@Persistent var firstName: String,@Persistent var lastName: String) { @PrimaryKey @Persistent {val valueStrategy = IdGeneratorStrategy.IDENTITY} var id: java.lang.Long = null }

Scala, XML and GAE

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