Scala 3 context functions

context-functions.scala

import cats.Monad
import cats.syntax.functor.*
import cats.effect.IO
import cats.effect.unsafe.implicits.global

case class Frunfles(id: Long, name: String)

trait DatabaseConnection:
  def exec[F[_]: Monad](op: String): F[Unit]

object DatabaseConnection:
  given DatabaseConnection with
    def exec[F[_]: Monad](op: String): F[Unit] = Monad[F].pure(println(s">>> DB op = $op"))

// single-dependency "primitive" environments
type MonadEnv[F[_], A] = Monad[F] ?=> F[A]
type DatabaseEnv[A] = DatabaseConnection ?=> A

// composing envs by nesting
// both are equivalent!
type Env[F[_], A] = MonadEnv[F, DatabaseEnv[A]]
type Env2[F[_], A] = DatabaseEnv[MonadEnv[F, A]]

// multi-dependency environments
// both are equivalent!
type Env3[F[_], A] = (DatabaseConnection, Monad[F]) ?=> F[A]
type Env4[F[_], A] = (Monad[F], DatabaseConnection) ?=> F[A]

// need "Env"
def findAllFrunfles[F[_]]: Env[F, List[Frunfles]] =
  summon[DatabaseConnection].exec("findAll").map(_ => List.empty)
def findFrunflesById[F[_]](id: Long): Env[F, Option[Frunfles]] =
  summon[DatabaseConnection].exec("findById").map(_ => None)

// need "Env2"
def insertFrunfles[F[_]](f: Frunfles): Env2[F, Unit] =
  summon[DatabaseConnection].exec("insert")

// need "Env3"
def updateFrunfles[F[_]](f: Frunfles): Env3[F, Unit] =
  summon[DatabaseConnection].exec("update")

// need "Env4"
def deleteFrunfles[F[_]](id: Long): Env4[F, Unit] =
  summon[DatabaseConnection].exec("delete")

// nesting order does not matter!
// parameter order does not matter!
// mixing different envs does not matter!
// the functions are called as if the envs don't exist at all!
val io: IO[Unit] =
for
  _ <- insertFrunfles[IO](Frunfles(1L, "frunfles"))
  _ <- findFrunflesById[IO](1L)
  _ <- updateFrunfles[IO](Frunfles(1L, "sbrubbles"))
  _ <- deleteFrunfles[IO](1L)
yield ()

val res1 = io.unsafeRunSync()
val res2 = findAllFrunfles[IO].unsafeRunSync()