Scala asInstanceOf with parameterized types

Question

I would like to write a function that casts to type A, where A can be e.g. List[Int], or a more complicated parameterized type like Map[Int, List[Int]].

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Answer 1

Consider this solution:

trait -->[A, B] {
  def ->(a: A): B
}

implicit val StringToInt = new -->[String, Int] {
  def ->(a: String): Int = a.toInt
}

implicit val DateToLong = new -->[java.util.Date, Long] {
  def ->(a: java.util.Date): Long = a.getTime
}

def cast[A,B](t:A)(implicit ev: A --> B):B= ev.->(t)

The advantage is that:

1. It is type safe - the compiler will tell you if the type cannot be casted
2. You can define casting rules by providing proper implicits

Now you can use it so:

scala>  cast(new java.util.Date())
res9: Long = 1361195427192

scala>  cast("123")
res10: Int = 123

EDIT

I've spent some time and wrote this advanced code. First let me show how to use it:

scala>    "2012-01-24".as[java.util.Date]
res8: java.util.Date = Tue Jan 24 00:00:00 CET 2012

scala>    "2012".as[Int]
res9: Int = 2012

scala>    "2012.123".as[Double]
res12: Double = 2012.123

scala>    "2012".as[Object]   // this is not working, becouse I did not provide caster to Object
:17: error: could not find implicit value for parameter $greater: -->[String,Object]
"2012".as[Object]
^

Pretty nice? See the scala magic:

trait -->[A, B] {
  def ->(a: A): B
}

implicit val StringToInt = new -->[String, Int] {
  def ->(a: String): Int = a.toInt
}

implicit val StringToDate = new -->[String, java.util.Date] {
  def ->(a: String): java.util.Date = (new java.text.SimpleDateFormat("yyyy-MM-dd")).parse(a)
}

implicit val StringToDouble = new -->[String, Double] {
  def ->(a: String): Double = a.toDouble
}

trait AsOps[A] {
  def as[B](implicit > : A --> B): B
}

implicit def asOps[A](a: A) = new AsOps[A] {
  def as[B](implicit > : A --> B) = > ->(a)
}