Calling type classes by using the type from a variable

Question

I have type classes depending on 2 types and one of the type contains the other

sealed trait Version
case class Version1() extends Version
case class Version2() extends Version

case class MyModel1(msg: String, version: Version)
case class MyModel2(msg: String, version: Version)

trait Validate[M, V] {
  def run : Boolean
}

object Validators {
  implicit object ValidateModel1 extends Validate[MyModel1, Version1] {
    override def run: Boolean = true
  }
  implicit object ValidateModel2 extends Validate[MyModel1, Version2] {
    override def run: Boolean = true
  }
}

And I would like to call my type classes this way:

object App {

  def main(args: Array[String]): Unit = {
    val model = MyModel1("Test", Version1())
    validate(model)
  }

  def validate(model: MyModel1) {
    import Validators._    

    val v = implicitly[Validate[MyModel1, model.version.type]]
    v.run
  }
}

But this doesn't work.

Error:(34, 23) could not find implicit value for parameter e: com.playground.Validate[com.playground.MyModel1,model.version.type] val v = implicitly[Validate[MyModel1, model.version.type]] Error:(34, 23) not enough arguments for method implicitly: (implicit e: com.playground.Validate[com.playground.MyModel1,model.version.type])com.playground.Validate[com.playground.MyModel1,model.version.type]. Unspecified value parameter e. val v = implicitly[Validate[MyModel1, model.version.type]]

Replacing the model.version.type with Version1 works

Any idea how to call my type class by pointing to the Version type from the MyModel1 ?

Feel free to suggest a more explicit title :)

Thank you

Edmondo · Accepted Answer · 2018-07-21 10:41:21Z

1

Remember that implicits are resolved at compile time so that the compiler cannot know whether inside

case class MyModel1(msg: String, version: Version)

version is of type Version1 or Version2. It is just not known. If you need to use that information you need to keep it in the signature of your class, either through a generic parameter or an abstract type (which you will use through an helper type that will turn into a generic, so you can use it in signature and express type-relationships)

So the solution is to generify the Model like so

case class MyModel1[V<:Version](msg:String, version:V)

edited Jul 21, 2018 at 10:41

answered Jul 21, 2018 at 10:07

Edmondo

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3 Comments

ccheneson Over a year ago

I have included the import in my test but not in the snippet. Thanks

Edmondo Over a year ago

Oh I totally misunderstood the question. Why are you using model.version.type ? It is a singleton type associated to the field version, are you sure that's what you meant?

ccheneson Over a year ago

It' s something I tried. I thought it returns the type of the version attribute from MyModel . Do you know how I can get the correct type?

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