Thursday, March 24, 2016

Playing with xml

XML is a convenient way to define external DSL. On one of my previous posts (Filtering using Scala Parser Combinators) I used parser combinators JavaTokenParsers with PackratParsers This time I will use different approach using XML.
XML is easy to handle by non-developers and easy to read. We can query XML using XPATH like expressions and using pattern matching. In this post we will define some business rules using XML and we will treat our XML as first class citizen. Source code can be found here.
let's define our DSL :
Our rules will use AND/OR operators but to make it more interesting, our operators will not use the following pattern:
<expression><operator><expression> (where expression evaluates to boolean).
because XML have opening and closing tags we can define something like :
<operator><expression1><expression2>...<expression N></operator>
now we can define our AND/OR operators :
  • AND: a set of expressions that all must be true
  • OR : a set of expressions that at least one must be true
  • SINGLE: contain our value and the operator used to evaluate the expression.
for example we want our condition to look something like this :
 val simpleRuleXml =
      <RULE id="1" description="5 top" status="true">
            <SINGLE operator="eq"> 2 </SINGLE>
            <SINGLE operator="eq"> 4 </SINGLE>
            <SINGLE operator="eq"> 8 </SINGLE>
            <SINGLE operator="gt"> 25 </SINGLE>
            <SINGLE operator="st"> 100 </SINGLE>
that means that our value must be one of the following values: 2 or 4 or 8 or between 25 to 100
let's start by defining our value objects
sealed trait Expression

case class Or(ps: Seq[Expression]) extends Expression

case class And(ps: Seq[Expression]) extends Expression

case class Single(operand:String, operator:String) extends Expression
working with xml allows to navigate between the nodes and parse it very easily by using pattern matching and parse it recursively :
for example this expression will match all nodes between the TAG and bind it to the variable xs using a wild card _ and a Kleen star * which actually means "match any sequence " <TAG>{ xs @ _* }</TAG>
object Expression{
  def apply(ns: NodeSeq):Expression ={
    ns.head match  {
      case <CONDITIONS>{ xs @ _* }</CONDITIONS> => Expression(xs) 
      case <AND>{ xs @ _* }</AND> => And(xs map(Expression(_)))
      case <OR>{ xs @ _* }</OR> => Or(xs map(Expression(_)))
      case <SINGLE>{ s @ _* }</SINGLE> =>Single(s.text,ns\@"condition")
That's it ! well , almost... we still need to evaluate our rule, and check if our conditions are met .
object Rule{
  def apply(node: Seq[xml.Node]): Rule = {
    val id =node\s"@id"
    val desc =node\s"@description"
    val conditions = (node\\"CONDITIONS").map(Expression(_))
    Rule.apply(id.text,desc.text,conditions )
A rule is mainly a set of conditions that needs to be met. As described above - all expressions between the AND tags must be true,
at least on expression between OR tags must be true.

case class Rule(id:String,desc:String,conditions:Seq[Expression]){
  def isValid[T:Ordering](value:T)(implicit f:String => T):Boolean = {
    def run (pr:Expression):Boolean = {
      pr match {
        case Single(name,operator) =>
          import scala.math.Ordering.Implicits._
            val n = f(name)
            operator match {
              case "eq" => value == n
              case "gt" => value > n
              case "st" => value < n
        case And(ps) => ps forall run
        case Or(ps) => ps exists run
    conditions forall run
and we are done !

for completion let's define some explicits so we can convert our value in the xml which is a String to T .
object Implicits {
//Since Java does not define toInt on String but on Int, so we need to be define which "toInt" implicit to use  
  implicit def string2Int(s: String): Int = augmentString(s).toInt
  implicit def string2Double(s: String): Double = augmentString(s).toDouble
  implicit def string2Float(s: String): Float = augmentString(s).toFloat

usage (using the above xml) :
val trimmedNode = scala.xml.Utility.trim(simpleRuleXml)

    val v = Rule(trimmedNode)
hope you enjoyed it . The Complete Source Code can be found here ,Feedback and remarks are always welcome