asInstanceOf with parameterized type


I have a rule trait that will be mixed-in to another trait called A for applying rule.

trait A[T] {
    def map(data: T)

trait Rule[E] {
    def rule: E => Boolean

class AImpl extends A[String] with Rule[String] ... { override def rule ... }

// desired logic 
val a =  new AImpl()  //  // get a instance of A

When Rule parameterized with String I know I can directly cast using a.asInstanceOf[Rule[String]]. However I will have other parameterized type later on. But asInstanceOf[Rule[…]] has limited (stated something like

Is there any recommended way to circumvent this problem? Mainly to achieve the effect that I can dynamically cast Rule with different parameterized type like

if(a.isInstanceOf[Rule[SomeType]]) a.asInstanceOf[Rule[SomeType]] ... // SomeType is the dest type to which I would like to cast.




Two solutions come to my mind:

(1) If the there are only a small fixed set of types that can be parameters, you could create a subtype for each, e.g.

class StringRule extends Rule[String]

(2) Obtain a TypeTag for the parameter, get the Type from it and store it.

Best, Oliver


You may want to consider designing your API such that the compiler does this check for you. For example:

def desiredLogic(a: Rule[String]) = Seq(/*...*/).filter(a.rule)
// or
def desiredLogic[T](a: Rule[T], xs: Seq[T]) = xs.filter(a.rule)


I decided to go with subclass e.g. StringRule extends Rule[String] tentatively. Then observe (and perhaps update this thread) how it works because at this stage not many variants needed to be dealt with.

Thanks again for all your help!