Scala Typeclass for composites - scala

Question

I have a sum type, Mapping:
sealed trait Mapping
final case class XMapping(a:String)
final case class FallbackMapping(mappings: List[Mapping])
I have a typeclass defined as follows:
final case class Param(x:String)
trait ParameterLoader[T] {
def load(mapping:T) : List[Param]
}
With some instances:
object DefaultParameterLoaders {
implicit val xParameterLoader= new QueryParameterLoader[XMapping] {
override def load(mapping: XMapping): List[Param] = List(Param(mapping.a))
}
implicit val fallbackParameterLoader = new ParameterLoader[FallbackMapping] {
override def load(mapping: FallbackMapping): List[Param] =
mapping.mappings.flatMap(x => ???)
}
}
I can't find a way to have the implicit instances passed to the flatMap above. The error I get is that I'm missing an instance of ParameterLoader[Mapping]. Is there some way of telling the compiler that it should use whatever typeclass instances are in scope?

Answer 1

The type system is looking for a ParameterLoader[Mapping] specifically, meaning that a ParameterLoader[XMapping]/ParameterLoader[FallbackMapping] is not specific enough. You need to provide a ParameterLoader[Mapping]. You can do this using your existing definitions.
implicit def mappingLoader(implicit xpLoader: ParameterLoader[XMapping], fmLoader: ParameterLoader[FallbackMapping]) = new ParameterLoader[Mapping] {
def load(mapping: Mapping): List[QueryParam] =
mapping match {
case xm: XMapping = xpLoader.load(xm)
case fm: FallbackMapping => fmLoader.load(fm)
}
}
Alternatively, have your flatmap perform the matching logic:
implicit def fallbackParameterLoader(implicit xpLoader: ParameterLoader[XMapping]) = new ParameterLoader[FallbackMapping] {
override def load(mapping: FallbackMapping): List[Param] =
mapping.mappings.flatMap {
case xm: XMapping = xpLoader.load(xm)
case fm: FallbackMapping => this.load(fm)
}
}