Scala case class: generating Instances for type constructor cases?

1
votes

I want to use a generic function that returns the "default value" for a given type. I have implemented this with the help of the following case class construct:

case class DefaultOp[T](op: () => T)
implicit val defaultString = DefaultOp[String](() => "")
implicit val defaultInt = DefaultOp[Int](() => 0)
implicit val defaultFloat = DefaultOp[Float](() => 0.0f)
implicit val defaultDouble = DefaultOp[Double](() => 0.0d)
// ...

def default[T]()(implicit ev: DefaultOp[T]): T = ev.op()

That works fine, but I would like to extend it to also work on Types constructed from a type constructor. For instance, I would like to be the default value for any option type to be "None" and for any list type to be Nil. How can I implementd this? My first attemt:

implicit val defaultOption = DefaultOp[Option[_]](() => None)

compiles, but it does not compile when I use it in an application:

error: could not find implicit value for parameter ev: DefaultOp[Option[Float]]
var test: Option[Float] = default[Option[Float]]
                                  ^
one error found

Any suggestion how this can be made working?

Thanks!

1
scalacase-classtype-constructor

1 Answers

2
votes

The problem is that Option[Float] is not a Option[_], because we don't know what _ is supposed to be.

You can solve this by making defaultOption a def, and using a type parameter:

implicit def defaultOption[A] = DefaultOp[Option[A]](() => None)

scala> default[Option[Float]]
res3: Option[Float] = None