DbRep2.scala

#!/usr/bin/env scala
trait DbRep[A] {
  type Rep
}

class Email

object EmailRep extends DbRep[Email] {
  override type Rep = String
}

class Foo

object FooRep extends DbRep[Foo] {
  override type Rep = Int
}

object Main extends App {
  // error: an implicit parameter section must be last
  // def lower[A](a: A)(implicit dbRep: DbRep[A])(implicit eq: dbRep.Rep =:= String): A = ???

  def map[A](dbRep: DbRep[A])(f: dbRep.Rep => dbRep.Rep): A => A = { a => a }

  def lower[A](dbRep: DbRep[A])(a: A)(implicit eq: dbRep.Rep =:= String): A = map[A](dbRep)(x => x)(a)

  println(lower[Email](EmailRep)(new Email))

  // error: Cannot prove that FooRep.Rep =:= String.
  println(lower[Foo](FooRep)(new Foo))

}