Pattern matching over typed parameters in Scala

Asked Apr 23 '19 at 21:07

Active Apr 23 '19 at 22:07

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Hey so I have the following ADT

 sealed trait MessageContext
 case class ThisMessageContext(...) extends MessageContext
 case class ThatMessageContext(...) extends MessageContext

and another class which is

 case class Message[+T <: MessageContext](...)

now I have this method

def doThisOrThat[T <: MessageContext](message: Message[T]) = message match {
    case mThis: Message[ThisMessageContext] = goThisWay(mThis)
    case mThat: Message[ThatMessageContext] = goThatWay(mThat)
}

private def goThisWay(msg: Message[ThisMessageContext]) = { // Some logic here}

private def goThatWay(msg: Message[ThatMessageContext]) = { // Some logic here}

Now I expect that for Message[ThatMessageContext] I am supposed to goThatWay but its evaluating to the first one. In fact due to type erasure its always hitting the first case no matter what I throw at it.

Is there a way to circumvent the type erasure here. AFAIK ClassTag is something I can use, but I do not how that works exactly. Also I do not like the idea of boxing the input.

edited Apr 23 '19 at 22:07

asked Apr 23 '19 at 21:07

Arunav Sanyal

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Instead of pattern matching on the type parameter, you could match on a value of your message which is the context. – Luis Miguel Mejía Suárez Apr 23 '19 at 22:01
@Lus Miguel, yes I can but I cannot pass that forward to gotThis or thatWay. Their arugments are specific impls of Message[T]. – Arunav Sanyal Apr 23 '19 at 22:06
@DmytroMitin Yep that works. Thanks. – Arunav Sanyal Apr 23 '19 at 22:07

Pattern matching over typed parameters in Scala

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