parapet | purely functional library to build | Functional Programming library

The first thing you need to do is to add two dependencies into your project: parapet-core and interop-{effect_library} for a specific effect library. You can find the latest version in maven central. For Cats Effect add libraryDependencies += "io.parapet" %% "interop-cats" % "0.0.1-RC1". For Monix add libraryDependencies += "io.parapet" %% "interop-monix" % "0.0.1-RC1". For Scalaz ZIO add libraryDependencies += "io.parapet" %% "interop-scalaz-zio" % "0.0.1-RC1". Once you added the library, you can start writing your first program. However, it’s worth taking a few minutes and getting familiar with two main approaches to write processes: generic and effect specific. I’ll describe both in a minute. For those who aren’t familiar with effect systems like Cats Effect, I’d strongly recommend you to read some articles about IO monad. Fortunately, you don’t need to be an expert in Cats Effect to use Parapet. The first approach we’ll consider is Generic. It’s recommended to stick to this style when writing processes. Let’s develop a simple printer process that will print users requests to the system output. Let’s walk through this code. You start writing your processes by extending Process trait and parameterizing it with an effect type. In this example, we left so-called hole F[] in our Printer type which can be any type constructor with a single argument, e.g. F[] is a generic type constructor, cats effect IO is a specific type constructor and IO[Unit] is a concrete type. Starting from this moment, it should become clear what it means for a process to be generic. Simply speaking, it means that a process doesn’t depend on any specific effect type e.g. IO. Thus we can claim that our Printer process is surely generic. The next step is to define a process API or contract that defines a set of events that it can send and receive. Process contract is an important part of any process specification that should be taken seriously. API defines a protocol that other processes will use to communicate with your process. Please remember that it’s a very important aspect of any process definition and take it seriously. The next step would be importing DSL, Parapet DSL is a small set of operations that we will consider in detail in the next chapters. In this example, we need only eval operator that suspends a side effect in F, in our Printer process we suspend println effectful computation. Finally, every process should override handle function defined in Process trait. handle function is a partial function that matches input events and produces an executable flows. If you ever tried Akka framework you may find this approach familiar (for the curious, Receive is simply a type alias for PartialFunction[Event, DslF[F, Unit]]). In our Printer process, we match on Print event using a well known pattern-matching feature in Scala language. If you are new in functional programming, I’d strongly recommend to read about pattern-matching - it’s a very powerful instrument. That’s it. We have considered every important aspect of our Printer process. Let’s move forward and write a simple client process that will talk to our Printer. As you already might have noticed, we are repeating the same steps we made when were writing our Printer process: * Create a new Process with a hole F[_] in its type definition * Extend io.parapet.core.Process trait and parametrizing it with generic effect type F * Implement handle partial function.
io.parapet.core.Event.Start is sent to a process once it’s created in Parapet system
io.parapet.core.Event.Stop is sent to a process when an application is interrupted with Ctrl-C or when some other process sent Stop or Kill event to that process. The main difference between Stop and Kill is that in the former case a process can finish processing all pending events before it will receive Stop event, whereas Kill will interrupt a process and then deliver Stop event, all pending events will be discarded. If you familiar with Java ExecutorService then you can think of Stop as shutdown and Kill as shutdownNow.