add to the managed process tutorial

Author: Tim Watson

tutorials/3.managedprocess.md

@@ -10,6 +10,12 @@ In order to explore the `ManagedProcess` API, we will present a simple
 example taken from the test suite, which exercises some of the more
 interesting features.
 
+The main idea behind `ManagedProcess` is to separate the functional
+and non-functional aspects of a process. By functional, we mean whatever
+application specific task the process performs, and by non-functional
+we mean the *concurrency* or, more precisely, handling of the process'
+mailbox.
+
 Let's imagine we want to execute tasks on an arbitrary node, using a
 mechanism much as we would with the `call` API from distributed-process.
 As with `call`, we want the caller to block whilst the remote task is
@@ -29,4 +35,21 @@ from the backlog and executed.
 such as this, based on the synchronous `call` and asynchronous `cast`
 functions. Although `call` is synchronous, communication with the
 *server process* is out of band, both from the client and the server's
-point of view.
+point of view. The server implementation chooses whether to reply to
+a call request immediately, or defer its reply until a later stage
+and go back to receiving messages in the meanwhile.
+
+### Implementation Sketch
+
+We start out with some types: the tasks we perform and the maximum
+pool size:
+
+{% highlight haskell %}
+type PoolSize = Int
+type SimpleTask a = Closure (Process a)
+{% endhighlight %}
+
+To submit a task, our clients will submit an action in the process
+monad, wrapped in a `Closure` environment. We will use the `Addressable`
+typeclass to allow clients to specify the server's location in whatever
+manner suits them.