Default implicit instance for Console[IO]

[EnverOsmanov]

Maybe make Console.io object implicit?

implicit object io extends SyncConsole[IO]

[gvolpe]

Not sure what's the benefit other than saving two characters ._?

Haven't seen many people using implicit objects for similar use cases, for example http4s also uses a similar approach to import a particular instance: import org.http4s.client.dsl.io._ See source

[EnverOsmanov]

I think it's needed for final tagless.
Let's take the example from the guide. It doesn't work like that:

import cats.effect.IO
import cats.effect.Console

myProgram[IO]

and works that way:

import cats.effect.IO
import cats.effect.Console

implicit val ConsoleIO = Console.io
myProgram[IO]

[gvolpe]

I don't think that's related to final tagless @EnverOsmanov

You can get an implicit instance of Console[IO] by just importing import cats.effect.Console.io._ as shown in the guide.

[EnverOsmanov]

Have you tried running (or compile) myProgram[IO] with import cats.effect.Console.io._? It doesn't work for me.

[gvolpe]

I don't really know what myProgram[IO] is, can you share the code?

[EnverOsmanov]

It is from the guide.
Ok, I'll share the code too:

object App {

  import Guide.myProgram
  import cats.effect.IO
  import cats.effect.Console.io._

  def main(args: Array[String]): Unit = {
    myProgram[IO].unsafeRunSync()
  }

}

object Guide {
  import cats.Monad
  import cats.effect.Console
  import cats.syntax.flatMap._
  import cats.syntax.functor._

  def myProgram[F[_]: Monad](implicit C: Console[F]): F[Unit] =
    for {
      _ <- C.putStrLn("Please enter your name: ")
      n <- C.readLn
      _ <- if (n.nonEmpty) C.putStrLn(s"Hello $n!")
      else C.putError("Name is empty!")
    } yield ()
}

[gvolpe]

That's right. In the guide the program that uses IO doesn't abstract over the effect.

In order to get your program working you need an implicit instance of Console[IO], for example:

object App {

  import Guide.myProgram
  import cats.effect.IO

  implicit val consoleIO: Console[IO] = new SyncConsole[IO] // or Console.io'

  def main(args: Array[String]): Unit = {
    myProgram[IO].unsafeRunSync()
  }

}

[EnverOsmanov]

Ok. I just thought if there is a default implicit Monad[IO], would be nice to have default implicit Console[IO]. Thanks for the library :) .

[gvolpe]

Not really. you can look here there is a default instance for any F[_]: Sync] but it is not available implicitly to avoid conflicts.

You can also look more at the history and arguments against providing a default implicit instance here since it was an original PR of cats-effect:

Thanks 😃