CODEWITHSUNDEEP
scalacovariancetype-parameterimplicits
Deil
Deil

Reputation: 512

Scala covariance losing type constraints

I wonder how can I preserve my type constraints while trying to workaround over 'Covariant type parameter in a wrong position problem' situation. Here is the code:

trait Converter[SourceType, +JobType <: ConverterJobType[SourceType]] {
   def convert[JT >: JobType](job: JT):String = job.someMethodFromJobType()// method is not accessible here. I would like to use JobType but actually job is treated like type Any.
}

object Converter{
   implicit object CSVConverter extends Converter[CSV, CSVConverterJobType]{
     def converter....
   }
 }

I need covariance so that my implicit object could be looked up.

 case class CSVConverterJobType(...) extends ConverterJobType[SourceType]

 object Application {

    def process[T](job: List[T])(implicit 
       converter:Converter[T,ConverterJobType[T]]) = {...}

    val list:List[CSV] = ...
    process(list)

 }

In order to process method to be able to find implicit.... I need to make second type parameter covariant. But then I'm not able to use actual type information in convert method.

Any idea how to overcome this?

Upvotes: 0

Views: 109

Answers (1)

Ziyang Liu
Ziyang Liu

Reputation: 810

JT needs to be <: JobType in order for job.someMethodFromJobType() to work. If JT is an arbitrary supertype of JobType then it doesn't have a someMethodFromJobType() method.

You should convert the second type argument to a type member, especially if there is only one JobType per SourceType (if not, the implicit search won't work anyway as it won't know which JobType to choose):

trait Converter[SourceType] {
  type JobType <: ConverterJobType[SourceType]
  def convert...
}

object Converter{
  implicit object CSVConverter extends Converter[CSV]{
    type JobType = CSVConverterJobType
    def convert....
  }
}

Upvotes: 2

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