Type Members

1.  trait Async [F[_]] extends Sync[F] with LiftIO[F] with Serializable

   A monad that can describe asynchronous or synchronous computations that produce exactly one result.

   A monad that can describe asynchronous or synchronous computations that produce exactly one result.

   Annotations

   @implicitNotFound( ... )

2.  trait Effect [F[_]] extends Async[F] with Serializable

   A monad that can suspend side effects into the F context and that supports lazy and potentially asynchronous evaluation.

   A monad that can suspend side effects into the F context and that supports lazy and potentially asynchronous evaluation.

   Annotations

   @implicitNotFound( ... )

3.  sealed abstract class IO [+A] extends AnyRef

   A pure abstraction representing the intention to perform a side effect, where the result of that side effect may be obtained synchronously (via return) or asynchronously (via callback).

   A pure abstraction representing the intention to perform a side effect, where the result of that side effect may be obtained synchronously (via return) or asynchronously (via callback).

   Effects contained within this abstraction are not evaluated until the "end of the world", which is to say, when one of the "unsafe" methods are used. Effectful results are not memoized, meaning that memory overhead is minimal (and no leaks), and also that a single effect may be run multiple times in a referentially-transparent manner. For example:

   val ioa = IO { println("hey!") }
   
   val program = for {
     _ <- ioa
     _ <- ioa
   } yield ()
   
   program.unsafeRunSync()

   The above will print "hey!" twice, as the effect will be re-run each time it is sequenced in the monadic chain.

   IO is trampolined for all synchronous joins. This means that you can safely call flatMap in a recursive function of arbitrary depth, without fear of blowing the stack. However, IO cannot guarantee stack-safety in the presence of arbitrarily nested asynchronous suspensions. This is quite simply because it is impossible (on the JVM) to guarantee stack-safety in that case. For example:

   def lie[A]: IO[A] = IO.async(cb => cb(Right(lie))).flatMap(a => a)

   This should blow the stack when evaluated. Also note that there is no way to encode this using tailRecM in such a way that it does not blow the stack. Thus, the tailRecM on Monad[IO] is not guaranteed to produce an IO which is stack-safe when run, but will rather make every attempt to do so barring pathological structure.

   IO makes no attempt to control finalization or guaranteed resource-safety in the presence of concurrent preemption, simply because IO does not care about concurrent preemption at all! IO actions are not interruptible and should be considered broadly-speaking atomic, at least when used purely.

4.  trait LiftIO [F[_]] extends Serializable

   Annotations

   @implicitNotFound( ... )

5.  trait Sync [F[_]] extends MonadError[F, Throwable] with Serializable

   A monad that can suspend the execution of side effects in the F[_] context.