A Practical Intro to Scala

1. Try this in Scala REPL

   List( "one", "two", "three", "four" ) .
     foreach {
       name =>
         new Thread {
             override // needed
             def run() = {
               println(
                "Thread " + name +
                " [" + this + "] " +
                "says Hello, World!"
               )
             }
         }
         .  start
     }
   

1. Read: Andrews, Chapter 6: Monitors.
2. def synchronized[A] (e: => A): A Executes e in mutex mode
3. def wait() Running thread suspends itself by waiting on a signal. (Unrelated to Linux signals.)
4. def wait(msec: Long) As above, but waits at most msec milli-seconds.
5. def notify() Cooperate with a wait -ing process, and check its condition C and inform. Even so, do while (!C) wait()
6. def notifyAll() How many being waken up: one in notify() and all in notifyAll() Many subtleties. Look up the docs.

1. We had a pristine description of what semaphores are. Scala semaphores are close, but no cigar.

   package scala.concurrent
   class Lock {
     var available = true
   
     def acquire = synchronized {
       while (!available) wait()
       available = false
     }
   
     def release = synchronized {
       available = true
       notify()
     }
   }
   

2. Similar to our DC Official Binary Semaphores (but not exactly). The above is implemented using a monitor.

class BoundedBuffer(N: Int) {
  val buf = new Array[Int](N)
  var in, out = 0	// indices of buf
  var n = 0		// #items in buf

  def put(x: Int) = synchronized {
    while (n >= N) { println("buf full") ; wait() }
    buf(in) = x ; in = (in + 1) % N ; n += 1
    if (n == 1) notifyAll() }

  def get: Int = synchronized {
    while (n == 0) wait()
    val x = buf(out) ; out = (out + 1) % N ; n -= 1
    if (n == N - 1) notifyAll()
    x }
}

complete source: ./scalaExamples/bounded-buffers.scala

class MailBox {
  def send(msg: Any) synchronized { ... }
  def receive[A](f: PartialFunction[Any, A]): A // uses synchronized { ... }
  def receiveWithin[A](msec: Long)(f: PartialFunction[Any, A]): A
}

1. type Any is the super-type of all (other) types.
2. PartialFunction

class Channel[A] {
  private var written = new LinkedList[A]
  private var lastWritten = written
  private var nreaders = 0
  def write(x: A) = synchronized {
    lastWritten.elem = x
    lastWritten.next = new LinkedList[A]
    lastWritten = lastWritten.next
    if (nreaders > 0) notify()   }

  def read: A = synchronized {
    if (written.next == null) {
      nreaders += 1; wait(); nreaders -= 1
    }
    val x = written.elem
    written = written.next
    x  }
}

API scala.concurrent.package; cf. CEG 7370 Semi AMP

class SyncChannel[A] {
  private var data: A = _
  private var reading = false
  private var writing = false
  def write(x: A) = synchronized {
    while (writing) wait()
    data = x
    writing = true
    if (reading) notifyAll()
    else while (!reading) wait()
  }
  def read: A = synchronized {
    while (reading) wait()
    reading = true
    while (!writing) wait()
    val x = data
    writing = false
    reading = false
    notifyAll()
    x
  }
}

API scala.concurrent.package; cf. CEG 7370 SMP

package scala.concurrent
class SyncVar[A] {
  private var isDefined: Boolean = false
  private var v: A = _
  def get = synchronized
      {while (!isDefined) wait(); v}
  def set(x: A) = synchronized
      {v = x; isDefined=true; notifyAll()}
  def isSet: Boolean = synchronized {isDefined}
  def unset = synchronized {isDefined = false} // API
}

1. API scala.concurrent.SyncVar

1. An expression yet to be computed. In the future. When possible. When needed. In parallel with something else.

2. Use

   import scala.concurrent.ops._
   
   val fut = future(someLengthyComputation)
   anotherLengthyComputation
   val y = f(fut()) + g(fut())
   

3. API Futures

   def future[A](p: => A): Unit => A = {
     val result = new SyncVar[A]
     fork { result.set(p) }
     (() => result.get)
   }
   

4. More on futures in akka

import scala.actors.Actor

List("one", "two", "three", "four").foreach {
 name =>
   new Actor {
     override
     def act() = {
       println("Thread " + name + " says Hello World!")
     }
   }
  .start
}

1. Use akka.actors
2. read actors and akka

1. Identifiers, operators, … as in most languages.
2. ! ? with Hoare's CSP semantics.
3. recvActor ! msg send a message msg to recvActor
4. Unusual symbolic tokens: ::= :, :+ and :\ TBD
5. new special tokens: _, <-, :: (+ several more)
6. object, Unit, Any, Nil, None, Some(x)
7. case is used in different ways
8. Constructors: Array, List
9. var x, y, z: Int = 0
10. val x, y, z: Int = 0 assigned one-time
11. no ++, or --; use instead +=1 or -=1

1. Example expression: (x + y*2) / (1 + 3*z)

2. Develop a tree representation for use in a Calc program.

   abstract class Tree
   case class Sum(l: Tree, r: Tree) extends Tree
   case class Var(n: String) extends Tree
   case class Const(v: Int) extends Tree
   

3. ./scalaExamples/calc.scala source code for a complete program but without * and /

1. Genericity is the ability to write code where types can be substituted.

2. class Reference[T] {
     private var contents: T = _
     def set(value: T) { contents = value }
     def get: T = contents
   }
   

3. Here, the underscore stands for the default value (of type T). This default is 0 for numerics, false for Boolean, () for the Unit type and null for all object types.

4. object IntegerReference {
     def main(args: Array[String]) {
       val cell = new Reference[Int]
       cell.set(13)
       println("Reference contains the half of " + (cell.get * 2))
     }
   }
   

5. source code: ./scalaExamples/generics.scala

1. toString
2. Cf Scala traits v Java interfaces
3. Java static v Scala object

1. Implicit arguments

1. % scalac -help Print a synopsis of options.
2. % scalac -X Print a synopsis of advanced options.
3. % scalac -print pingObj-actor.scala Print program with Scala-specific features removed.
4. % scalac -Xprint:typer fileName.scala Syntax trees at end of typer.
5. % scalac -Xprint:all fileName.scala Check it out!

1. scala invokes java with Scala's libraries.
2. scala and scalac are shell scripts on Linux. 200+ lines. Differ in just one line. On windows, scala.bat and scalac.bat

3. Scala version I am using [date: Mar 30, 2014] is

   Welcome to Scala version 2.11.0-RC3
   (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0).
   Type in expressions to have them evaluated.
   Type :help for more information.
   

1. bash:
   1. export SCALA_HOME=/usr/local/scala-2.11.0-RC3
   2. alias scala=/usr/local/scala-2.11.0-RC3/bin/scala
   3. alias scalac=/usr/local/scala-2.11.0-RC3/bin/scalac
   4. PATH=$PATH:$SCALA_HOME/bin
2. scala classpath example
   1. % scalac -classpath /usr/share/java/scala-swing.jar HelloGui.scala
   2. % scala -classpath .:/usr/share/java/scala-swing.jar HelloWorld

1. % javap HelloWorld.class HelloWorld\$.class

   Compiled from "hello-world-println.scala"
   public final class HelloWorld {
     public static void main(java.lang.String[]);
   }
   Compiled from "hello-world-println.scala"
   public final class HelloWorld$ {
     public static final HelloWorld$ MODULE$;
     public static {};
     public void main(java.lang.String[]);
   }
   

1. http://www.flotsam.nl/dispatch-periodic-table.html "All operators of Scala's marvelous Dispatch library on a single page."
2. https://airtable.com/shrDCOpnMP8iA6kOn 2019 Scala Developer Survey
3. https://people.cs.ksu.edu/~schmidt/705a/Scala/
4. https://scastie.scala-lang.org/ Scastie is Scala + sbt in your browser!
5. https://developer.lightbend.com/start/
6. Intellij IDEA for Scala