Providing a SyncConsole[F] instance, opt-in. Removing console from IO…

…App.

core/js/src/main/scala/cats/effect/internals/IOAppPlatform.scala

@@ -18,7 +18,6 @@ package cats
 package effect
 package internals
 
-import cats.Show
 import cats.implicits._
 import scala.concurrent.duration._
 
@@ -57,12 +56,4 @@ private[effect] object IOAppPlatform {
   }
 
   val defaultTimer: Timer[IO] = IOTimer.global
-
-  def defaultConsole: Console[IO] =
-    new Console[IO] {
-      def putStrLn[A: Show](a: A): IO[Unit] = IO(println(a.show))
-      def error[A: Show](a: A): IO[Unit] = IO(System.err.println(a.show))
-      def readLine: IO[String] = IO(scala.io.StdIn.readLine)
-    }
-
 }

core/jvm/src/main/scala/cats/effect/internals/IOAppPlatform.scala

@@ -18,7 +18,6 @@ package cats
 package effect
 package internals
 
-import cats.Show
 import cats.implicits._
 import java.util.concurrent.CountDownLatch
 
@@ -64,12 +63,4 @@ private[effect] object IOAppPlatform {
   }
 
   def defaultTimer: Timer[IO] = IOTimerRef.defaultIOTimer
-
-  def defaultConsole: Console[IO] =
-    new Console[IO] {
-      def putStrLn[A: Show](a: A): IO[Unit] = IO(println(a.show))
-      def error[A: Show](a: A): IO[Unit] = IO(System.err.println(a.show))
-      def readLine: IO[String] = IO(scala.io.StdIn.readLine)
-    }
-
 }

core/shared/src/main/scala/cats/effect/Console.scala

@@ -19,6 +19,7 @@ package effect
 
 import cats.Show
 import cats.instances.string._
+import cats.syntax.show._
 
 import scala.annotation.implicitNotFound
 
@@ -52,7 +53,7 @@ trait Console[F[_]] {
   /**
    * Reads line from the standard console.
    *
-   * @return a [[String]] value representing the user's input or raise an error in the F context.
+   * @return a value representing the user's input or raise an error in the F context.
    */
   def readLine: F[String]
 }
@@ -78,3 +79,48 @@ object Console {
         F.liftIO(console.readLine)
     }
 }
+
+/**
+ * A default instance for any `F[_]` with a `Sync` instance.
+ *
+ * Use it either in a DSL style:
+ *
+ * {{{
+ *   import cats.effect._
+ *
+ *   object io extends SyncConsole[IO]
+ *
+ *   import io._
+ *
+ *   val program: IO[Unit] =
+ *     for {
+ *       _ <- putStrLn("Please enter your name: ")
+ *       n <- readLine
+ *       _ <- if (n.nonEmpty) putStrLn(s"Hello $n!")
+ *            else error("Name is empty!")
+ *     }
+ * }}}
+ *
+ * Or in tagless final style:
+ *
+ * {{{
+ *   import cats.effect._
+ *
+ *   def myProgram[F[_]](implicit C: Console[F]): F[Unit] =
+ *     for {
+ *       _ <- C.putStrLn("Please enter your name: ")
+ *       n <- C.readLine
+ *       _ <- if (n.nonEmpty) C.putStrLn(s"Hello $n!")
+ *            else C.error("Name is empty!")
+ *     }
+ * }}}
+ *
+ */
+class SyncConsole[F[_]](implicit F: Sync[F]) extends Console[F] {
+  def putStrLn[A: Show](a: A): F[Unit] =
+    F.delay(println(a.show))
+  def error[A: Show](a: A): F[Unit] =
+    F.delay(System.err.println(a))
+  def readLine: F[String] =
+    F.delay(scala.io.StdIn.readLine)
+}

core/shared/src/main/scala/cats/effect/IOApp.scala

@@ -77,9 +77,4 @@ trait IOApp {
    * On scala.js, the default is `Timer.global`.
    */
   protected implicit def timer: Timer[IO] = IOAppPlatform.defaultTimer
-
-  /**
-   * Provides a default instance for Console[IO].
-   */
-  def console: Console[IO] = IOAppPlatform.defaultConsole
 }