Compare types in Scala

Question

I have two objects, each with locally defined types, and I want to determine if the types are the same. For example, I'd like this code to compile:

trait Bar {
  type MyType
}

object Bar {
  def compareTypes(left: Bar, right: Bar): Boolean = (left.MyType == right.MyType)
}

However, compilation fails with "value MyType is not a member of Bar".

What's going on? Is there a way to do this?

Answer 1

You can do this, but it takes a little extra machinery:

trait Bar {
  type MyType
}

object Bar {
  def compareTypes[L <: Bar, R <: Bar](left: L, right: R)(
    implicit ev: L#MyType =:= R#MyType = null
  ) = ev != null
}

Now if we have the following:

val intBar1 = new Bar { type MyType = Int }
val intBar2 = new Bar { type MyType = Int }
val strBar1 = new Bar { type MyType = String }

It works as expected:

scala> Bar.compareTypes(intBar1, strBar1)
res0: Boolean = false

scala> Bar.compareTypes(intBar1, intBar2)
res1: Boolean = true

The trick is to ask for implicit evidence that L#MyType and R#MyType are the same, and to provide a default value (null) if they aren't. Then you can just check whether you get the default value or not.