I have a monad transformer stack on top of a Kleisli defined as:
type Env = Map[String,Int]
type MyState = List[Int]
type S[A] = EitherT[StateT[WriterT[Kleisli[List,Env,?],String,?],MyState,?], String, A]
and I want to define a local method with the following signature:
def localE[A](f: Env => Env)(sa: S[A]): S[A] = ???
Is it possible?
I know that there is a local method in MonadReader with the signature:
def local[A](f: R => R)(fa: F[A]): F[A]
So the simplest solution would be to obtain the MonadReader implicit from S, however, I could not find how to do it.
A simple snippet of my code would be the following:
package examples
import cats._, data._
import cats.implicits._
object local {
type Env = Map[String,Int]
type MyState = List[Int]
type S[A] = EitherT[StateT[WriterT[Kleisli[List,Env,?],String,?],MyState,?], String, A]
// The following definition doesn't compile
// implicit lazy val mr = MonadReader[S,Env]
// Modify the environment
def localE[A](f: Env => Env)(sa: S[A]): S[A] = ???
}
A possible solution is to unlift the monad stack manually. In this case, I defined the following two auxiliary functions:
def localW[A](f: Env => Env)
(c: WriterT[Kleisli[List,Env,?],String,A]):
WriterT[Kleisli[List,Env,?],String,A] = {
type WriterTF[F[_],A] = WriterT[F, String, A]
val r: WriterT[Kleisli[List,Env,?],String,(String,A)] =
c.run.local(f).liftT[WriterTF]
r.mapBoth { case (_, (w, x)) => (w, x) }
}
def localS[A](f: Env => Env)
(c: StateT[WriterT[Kleisli[List,Env,?],String,?],MyState,A]):
StateT[WriterT[Kleisli[List,Env,?],String,?],MyState,A] = {
StateT(s => localW(f)(c.run(s)))
}
And the local function can be defined as:
def localE[A](f: Env => Env)(sa: S[A]): S[A] = {
EitherT(localS(f)(sa.value))
}
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