Monad Transformers for the working programmer

cats.scala

import cats.data.OptionT, cats.std.future._

def findAddressByUserId(id: Long): Future[Option[Address]] =
  (for {
    user    <- OptionT(findUserById(id))
    address <- OptionT(findAddressByUser(user))
  } yield address).value

final-solution.scala

def findUserById(id: Long): OptionT[Future, User] =
  OptionT { ??? }

def findAddressByUser(user: User): OptionT[Future, Address] =
  OptionT { ??? }

def findAddressByUserId(id: Long): OptionT[Future, Address]] =
  for {
    user    <- findUserById(id))
    address <- findAddressByUser(user))
  } yield address