Functional Programming

• Functional programming: Functions are values
• In Java, values are
  • Primitive types int, double, etc.
  • Objects (Note: Arrays are objects)
  • null
• A method is not a value in Java
  • Cannot store it in a variable

Functional Programming in Scala

• Scala: Multi-paradigm language (functional, OO, host for domain-specific languages)
• Functions can be values

  val num = 3.14
  val fun = Math.ceil _
  fun(num) // prints 4

• Functions can be anonymous...

  (x : Double) => x * x

• ...just like numbers

  3.14

Immutability

• Immutable: Cannot change
• In Java, strings are immutable
  • "Hello".toUpper() doesn't change "Hello" but returns a new string "HELLO"
• In Scala, val are immutable

  val num = 3.14
  num = 1.42 // Error

• Pure functional programming: No mutations
• Don't mutate—return the result as a new value

Why Functional Programming?

• Simpler and clearer programming style
• Often useful to pass functions as parameters

  (1 to 10).map((x : Double) => x * x)

• In Java, this requires cumbersome objects

  strings.sort(new Comparator<String>() {
    public void compare(String a, String b) {
      return a.length() - b.length();
    }
  });

• Functions that don't mutate state are inherently parallelizable
• Potential solution to the end of Moore's Law

Lab

• Format of classes: approx. 20 minutes lecture, 45 minutes lab, 10 minute wrap-up
• Use your laptop (with JDK 1.6 installed)
• Work with a buddy (Homework: Watch the pair programming video)
• One of you fusses with the code, the other types up answers
• Switch roles each time
• Submit lab work to eCampus.

Step 1: Installing Scala

• Download the Scala software from http://www.scala-lang.org/downloads and uncompress to a suitable place. Add the bin subdirectory of that directory to your PATH. Here are the directions for Linux, Mac OS X, or Cygwin under Windows.
  1. cd ; tar xvfz downloaddir/scala-version.tar.gz
  2. Add these two lines to the end of your ~/.bashrc file:

     export SCALA_HOME=/home/username/scala-version
     export PATH=$SCALA_HOME/bin:$PATH

  If you choose to run Windows without Cygwin, you will have to go through the hell of the Windows environment editor. Don't blame Scala for this. Keep these points in mind:

  • DO NOT install into a directory that contains spaces.
  • If you use Vista and run the installer, choose a directory inside your home directory. The security settings won't allow you to install outside. Or install as administrator.
  • You need to add installdir\scala-version\bin and a semicolon before the existing PATH.
  • Don't know how to edit the PATH? Look here. Under Vista, you need to be administrator to do this.
  • If all this is too much trouble, install VMWare and grab a Ubuntu VM.
• Open a shell window and type scala. What do you get?

Step 2: The Scala Interpreter

1. After you typed scala, you should have received a prompt of the Scala interpreter. Type 3 * 2. (Don't type the period, and hit ENTER after each input line.) What do you get?
2. Type res0. What do you get?
3. Type val a = 3 * 4. What do you get?
4. Type a. What do you get?
5. Type a = 9. What do you get?
6. Type res0 = 9. What do you get?

Step 3: Functions are Values

1. Type val triple = (x : Int) => 3 * x. What do you get?
2. Type triple(5). What do you get?

   HINT: These “What do you get” exercises are a lot more effective when you and your buddy first discuss whar you think you'll get. “Form a hypothesis” is an essential part of the learning path.

3. Type triple. What do you get?
4. What is the type of triple in Scala?
5. What is the type of 5 in Scala?

Step 4: Functions as Parameters

1. Type 1 to 10. What do you get?
2. Type (1 to 10).map(triple). What do you get? Why?
3. How do you get a list of the squares of the numbers from 1 to 10?
4. How do you get a list of the squares of the numbers from 1 to 10 without using val?