IsNotFuture - Scala

IsNotFuture

trait IsNotFuture[F] {
  type T
  def apply(f: F): T
}

object IsNotFuture {
  def apply[F](implicit isf: IsNotFuture[F]) = isf

  implicit def mkFuture[A] = new IsNotFuture[Future[A]] {
    type T = Int // Doesn't matter
    def apply(f: Future[A]): T = 0
  }
  implicit def mk[A] = new IsNotFuture[A] {
    type T = A
    def apply(f: A): A = f
  }
}
// Alternatively, you could synchronize all futures...
//  implicit def mkFuture[A] = new IsNotFuture[Future[A]] {
//    type T = A
//    def apply(f: Future[A]): A = Await.result(f, 1 seconds)
//  }

// Here's my use case - don't want fn to leak a Future that keeps using the session after cleanup
def inSession[T](fn: (Session) => T)(implicit isNotFuture: IsNotFuture[T]): isNotFuture.T = {
  isNotFuture(db withSession fn)
}

// Compiler fails as desired 
inSession { session => Future(1) }

You don't need the dependent type. See here: https://gist.github.com/wheaties/a49e2f5c2e1e3c3286f3

Otherwise, good use of implicit divergence.