Scala tip: Making it easier to add attributes to an XML element

In most respects Scala makes it very easy to handle XML, but there are a few things that are hard to do. One is changing the value of an existing attribute on an element, or adding a new attribute to an element.

The simplest way I found to add new attributes c="CCC" d="DDD" is to do:

    val modified = elem %
                    new UnprefixedAttribute("c","CCC",Null) %
                    new UnprefixedAttribute("d","DDD",Null)

This is a bit verbose. What if instead you could do

    val modified = elem % Map( "c"->"CCC", "d"->"DDD" )

Well you can, if you add the following implicit somewhere in scope:

    implicit def pimp(elem:Elem) = new {
      def %(attrs:Map[String,String]) = {
        val seq = for( (n,v) <- attrs ) yield new UnprefixedAttribute(n,v,Null)
        (elem /: seq) ( _ % _ )
      }
    }

This uses the library pimping pattern discussed elsewhere to effectively add a new version of the % operator to Elem that takes a Map of attributes.

The new % operator uses a for-yield construct to convert the attrs Map into a sequence of UnprefixedAttribute objects. It then uses the /: fold operator to repeatedly apply the built-in % operator, reducing down to an element with all the attributes added.