sample code for Cats MTL ApplicativeLocal

applicativeLocal.sc

import cats.data.ReaderT
import cats.effect.IO
import cats.syntax.flatMap._
import cats.syntax.functor._
import cats.{Applicative, Monad}
import cats.mtl.{ApplicativeAsk, ApplicativeLocal}

type Config = Map[String, String]

def get42[F[_]: Applicative](implicit F: ApplicativeAsk[F, Config]): F[String] =
  F.ask.map(_.getOrElse("42", "none"))

def both[F[_]: Monad](implicit F: ApplicativeLocal[F, Config]): F[(String, String)] =
  for {
    m <- F.local(_.updated("42", "modified"))(get42[F])
    o <- get42[F]
  } yield (m, o)

import cats.mtl.instances.local._
both[ReaderT[IO, Config, ?]].run(Map("42" -> "original")).unsafeRunSync()
//  (modified,original)