Is it possible to chain methods from different traits?

Question

I have the following code:

class Parameterizable{
  var map: Map[String, String] = new scala.collection.immutable.HashMap() 
  def put(entry: Tuple2[String, String]) = {
    map = map + entry; this
  }
}

class Query() extends Parameterizable{
  override def toString = {
    map.isEmpty match{
      case true => ""
      case false => "?" + map.map{case (key, value) => key + "=" + value}.mkString("&")
    }
  }
}

trait PageParameter extends Parameterizable{
  def page(page: Int) = put(("page" -> page.toString))
  def pageSize(pageSize: Int) = put(("pagesize" -> pageSize.toString))
}

trait DateParameter extends Parameterizable{
  def fromDate(date: java.util.Date) = put(("fromdate" -> (date.getTime()/1000L).toString()))
  def toDate(date: java.util.Date) = put(("todate" -> (date.getTime()/1000L).toString()))
}
//and other similar traits

I would like to do something like:

class ExtendedQuery extends Query with PageParameter with DateParameter
val query = new ExtendedQuery
query.page(4).pageSize(5).fromDate(new java.util.Date)

or:

query.and().page(4).and().pageSize(5).and().fromDate(new java.util.Date)

Is it possible in Scala?

Answer 1

You can declare the methods as returning this.type and then return this from them:

trait PageParameter extends Parameterizable{
  def page(page: Int) : this.type = { put(("page" -> page.toString)); this }
  def pageSize(pageSize: Int): this.type = { put(("pagesize" -> pageSize.toString)); this }
}

At the use-site, you can then chain the calls as you wanted. See this example:

scala> trait Wibble {
 | def foo : this.type = { println("foo"); this }
 | }
defined trait Wibble

scala> trait Wobble extends Wibble {
 | def bar: this.type = { println("bar"); this }
 | }
defined trait Wobble

scala> trait Wubble extends Wibble {
 | def baz: this.type = { println("baz"); this }
 | }
defined trait Wubble

Now I can test it

scala> new Wibble with Wobble with Wubble
res0: java.lang.Object with Wibble with Wobble with Wubble = $anon$1@937e20

scala> res0.bar.baz.foo
bar
baz
foo
res1: res0.type = $anon$1@937e20