Scala: ReaderT composition with different contexts and dependencies - scala

Question

Example of s3f1 and s3f2 functions that return different ReaderT:
type FailFast[A] = Either[List[String], A]
trait Service1 { def s1f:Option[Int] = Some(10) }
trait Service2 { def s2f:FailFast[Int] = Right(20) }
import cats.instances.option._
def s3f1: ReaderT[Option, Service1, Int] =
for {
r1 <- ReaderT((_: Service1).s1f)
} yield r1 + 1
import cats.syntax.applicative._
import cats.instances.either._
type ReaderService2FF[A] = ReaderT[FailFast, Service2, A]
def s3f2: ReaderService2FF[Int] =
for {
r1 <- ReaderT((_: Service2).s2f)
r2 <- 2.pure[ReaderService2FF]
} yield r1 + r2
I try to compose these two functions that return readers with different F[_] context and dependencies: ReaderT[Option, Service1, Int] and ReaderT[FailFast, Service2, Int]
I have to combine somehow the F[_] context, which means combine FailFast with Option. I assume, it makes sense to combine it to FailFast[Option]:
type Env = (Service1, Service2)
type FFOption[A] = FailFast[Option[A]]
type ReaderEnvFF[A] = ReaderT[FFOption, Env, A]
How to compose s3f1 and s3f2:
def c: ReaderEnvFF[Int] =
for {
r1 <- //s3f1
r2 <- //s3f2
} yield r1 + r2

Answer 1

Since you try to compose monads FailFast and Option in FFOption, you should use one more monad transformer, so FFOption[A] should be OptionT[FailFast, A] rather than just FailFast[Option[A]].
import cats.instances.option._
import cats.instances.either._
import cats.syntax.applicative._
import cats.syntax.either._
import cats.syntax.option._
type Env = (Service1, Service2)
type FFOption[A] = OptionT[FailFast, A]
type ReaderEnvFF[A] = ReaderT[FFOption, Env, A]
def c: ReaderEnvFF[Int] =
for {
r1 <- ReaderT[FFOption, Env, Int](p => OptionT(Either.right(s3f1.run(p._1))))
r2 <- ReaderT[FFOption, Env, Int](p => OptionT(s3f2.run(p._2).map(_.some)))
} yield r1 + r2
This can be rewritten with with local and mapF:
def c: ReaderEnvFF[Int] =
for {
r1 <- s3f1.local[Env](_._1).mapF[FFOption, Int](opt => OptionT(opt.asRight))
r2 <- s3f2.local[Env](_._2).mapF[FFOption, Int](ff => OptionT(ff.map(_.some)))
} yield r1 + r2