Writing Scala Programs

Command Line There are three common operating systems: Windows (various flavors; I recommend Windows 7) UNIX or Linux (basically the same thing) Mac OS X, built on top of UNIX All have a “command line” or “shell” interface Mac: Use the Terminal application Windows: Two choices. From the Start menu, type: cmd, to get the DOS interface (not recommended) powershell, to get a better interface PowerShell has many conveniences, including some UNIX-like commands You can copy the PowerShell icon to your Start menu for easier access Once everything has been installed properly, you can type scala at the command line to enter the Scala REPL 2

Minor points Little things add up! “Watch the pennies and the dollars will take care of themselves.”—Benjamin Franklin Syntax coloring is when your editor uses different colors for keywords, strings, numbers, etc. You can ignore the colors; you’ll soon start to notice when they indicate errors Other editor features you should learn to use: Parenthesis matching Automatic indentation Indent/dedent large blocks of code Use a good font, so that you don’t even have to think about distinguishing a 1 (one) from an l (lowercase L) or a | (vertical bar) My favorite is Consolas (compare 1 and l, for example) Not so minor: RSI, Repetitive Strain Injury, is an occupational hazard for programmers Learn at least the very basics of ergonomics 3

Editors You should get a good editor and get familiar with its features The following work on both Mac and Windows, and have syntax coloring for Scala Sublime Text 2 or newer—recommended by Atomic Scala Excellent, but it will nag you occasionally to pay for it Oddly, it has no way to access a printer (to make paper copies) jEdit is a longtime favorite of mine For the Mac, TextMate is a free alternative to Sublime Text 2 For Windows, Notepad++ is a fine editor but does not come with Scala syntax coloring If you are already familiar with Eclipse, there is a Scala plugin for it 4

The REPL REPL stands for “Read-Eval-Print-Loop” You can enter expressions directly in the REPL, and the result will be printed (That’s the “P” in REPL) scala> "Hello from the REPL!" res1: String = Hello from the REPL! This tells you that the REPL has put your result in a val named res1 that you can use later, and that res1 is a String You can also use print and println in the REPL, but usually you don’t need to scala> println("Hello from the REPL!") Hello from the REPL! The result of calling println is “unit” Unit is, and the REPL usually doesn’t print it You can give commands to the REPL, such as :help, :load, and :quit To load a program from a file into the REPL, use :load For example, to load the program Hello.scala, enter :load Hello.scala 5

More about the REPL Because the REPL is designed for trying things out, it lets you redeclare a val : scala> val x = 1 x: Int = 1 scala> x = 2 :8: error: reassignment to val x = 2 ^ scala> val x = 2 x: Int = 2 You can’t do this in a “real” program In the REPL, if an expression seems to be complete, it will be executed if (x < y) min = x // will execute as soon as you hit Enter else min = y // then this will be an error You can avoid this problem by using :paste (see :help ) 6

Programs A program must contain a main method inside an object: object HelloWorld { def main(args: Array[String]) { println("Hello, World!") } } Running the program from the (PowerShell) command line: PS C:\Users\dave\Scala programs> scala HelloWorld.scala Hello, World! Loading and running the program from the Scala REPL: scala> :load HelloWorld.scala Loading HelloWorld.scala... defined module HelloWorld scala> HelloWorld.main(Array()) Hello, World! Note: Array() is a required argument to the method 7

Scripts A script is a file containing any number of Scala expressions to be executed On the file: val hello = "Hello, Scala!" println(hello + " (scripted)") Running the script from the (PowerShell) command line: PS C:\Users\dave\Scala programs> scala ScriptDemo.scala Hello, Scala! (scripted) Running the script from the REPL: scala> :load ScriptDemo.scala Loading ScriptDemo.scala... Hello, Scala! (scripted) How this works: Scala turns your script into a program by creating an object containing a main method, and putting your script inside that main method Scripts are useful for small, simple programs that you might want to run from the command line 8

9 The End