How to get the classtag of a path-dependent type

Question

I have an abstract path-dependent type that I need to ClassTag of. Is there a better way than manually pulling the implicit for each concrete derived class?

trait Foo {
  type A // : ClassTag // Need the ClassTag of A later
  val ctA: ClassTag[A] // But can't put a context-bound on the type
}

class IntFoo extends Foo {
  type A = Int
  val ctA = implicitly[ClassTag[Int]]
}

class StringFoo extends Foo {
  type A = String
  val ctA = implicitly[ClassTag[String]]
}

Answer 1

You have to conjure a class tag where you know the type.

scala> :pa
// Entering paste mode (ctrl-D to finish)

trait Foo {
type A
def ct[B: ClassTag] = implicitly[ClassTag[B]]
}

// Exiting paste mode, now interpreting.

defined trait Foo

scala> :pa
// Entering paste mode (ctrl-D to finish)

class IntFoo extends Foo {
type A = Int
def myct = ct[A]
}

// Exiting paste mode, now interpreting.

defined class IntFoo

scala> new IntFoo().myct
res2: scala.reflect.ClassTag[Int] = Int

But macros are pretty easy to write these days.

scala> :pa
// Entering paste mode (ctrl-D to finish)

object M {
def ct[A: c.WeakTypeTag](c: Context): c.Expr[ClassTag[A]] = {
import c.universe._
val a = c.prefix.tree.tpe.member(TypeName("A")).typeSignature
c.Expr(q"implicitly[ClassTag[$a]]").asInstanceOf[c.Expr[ClassTag[A]]]
}}

// Exiting paste mode, now interpreting.

scala> class Foo { type A = Int ; def f: ClassTag[A] = macro M.ct[A] }
defined class Foo

scala> new Foo().f
res15: scala.reflect.ClassTag[Int] = Int

scala> class Bar { type A = Char ; def f: ClassTag[A] = macro M.ct[A] }
defined class Bar

scala> new Bar().f
res16: scala.reflect.ClassTag[Char] = Char

so

scala> trait Foo { type A ; def ct = macro M.ct[A] }
defined trait Foo

scala> class Bar extends Foo { type A = Int ; def p = println(ct) }
defined class Bar

scala> new Bar().p
Int