Yoav Yoav - 3 months ago 11
Scala Question

Scala trait as a method input - type mismatch error

I wrote a method that gets a trait type as an input.
This is the trait

Localizable
:

import com.vividsolutions.jts.geom.Coordinate
trait Localizable {
val location : Coordinate
}


This is the method:

def localizeWithId(rdd : RDD[Localizable]) : RDD[(BigInt,Localizable)] = {
return rdd.map { case place =>
(getIdFromLocation(place.location.x, place.location.y), place)
}
}


The issue is that when I try to call this method, and send a case class that extends this trait as a parameter, I get an error of type mismatch.

This is the case class:

case class At (
eventDate : DateTime,
location : Coordinate
) extends Localizable


and this is the call:

val ats : RDD[At] = ...
val atsLocalized : RDD[(BigInt, At)] = localizeWithId(ats)


How can I solve it? Thanks.

Answer

Your problem is that here:

val atsLocalized : RDD[(BigInt, At)] = localizeWithId(ats)

you said you expect RDD[(BigInt, At)] to be returned, while actual return type is declared as RDD[(BigInt,Localizable)].

RDD is invariant so you can't put RDD[B] where RDD[A] is expected even if B would be subtype of A. But that is not the case here anyway.

You can make your method generic like this:

def localizeWithId[A <: Localizable](rdd: RDD[A]): RDD[(BigInt, A)]

requiring A to be subtype of your trait.