Scala:How Many Steps Beyond Java?

R. Clayton

Monmouth County ACM • 7 June 2008

Outline

• History and background.

• Wikis

• Java vs. Scala.

  • Basics

  • Classes, interfaces, and inheritance.

What is Scala?

• Scala is an exercise in modern programming language development.

• The intention is to develop a scalable language.

  • “Scalable” means extensible in a consistent syntax and semantics.

    a = b + c*e
    
    a = b.add(c.multiply(e))
    

  • Yes, like C++ in more ways than one.

Scala vs. Java

• In many senses, Scala can be thought of as Java++.

  • Scala's design takes off from Java's design.

  • Scala's implementation is based on the Java Virtual Machine (JVM).

  • The Scala run-time integrates the Java Class libraries.

• Scala's additions to Java are extensive, and not consistent with Java.

History

• Development started in 2001, released in 2004 (current version 2.7.1).

• Martin Odersky is the designer and principle investigator.

  • Worked on Java generics definition and implementation.

  • Wrote the javac knock-off compiler espresso.

• Work based at The Programming Methods Laboratory at École Polytechnique Fédérale de Lausanne.

Scala High Points

• Integrates object-oriented and functional programming.

• Static typing (mostly) with inferencing.

  var l = new List[Color]()
  

• Configurable syntax.

  ("(" ~> expr <~ ")" <~ ";") ^^ DoLoop
  

• Explicit control over evalution time (lazy vs. eager).

• An interpreter for scripting and a compiler.

Talk Objectives

• Should I be paying attention to Scala? Why?

  • Should I ditch Java for Scala?

  • Should I pay attention to Scala or Haskell?

What’s a Wiki?

A wiki provides directly editable web pages. Invented by Ward Cunningham in 1995. “Wiki wiki” is Hawaiian for “quick”.

What’s a Wiki?

What’s a Wiki?

Wow, that's great! How can I get one? There are plenty of wikis available, but DIY is fun.

Wiki Use Case

• Assume nothing about the browser, not even JavaScript.

• Implement everything on the server side.

Analysis

• The server should do three things:

  1: Serve pages (as normal).
  2: Serve edit pages.
  3: Store edited pages and serve them.
  

Server-Side Implementations

• Available implementation choices include

  • A custom wiki server.

  • An existing server and CGI script.

  • An existing server and a servlet.

• A custom server is the most work.

• I don't remember how to do servlets.

• CGI wins by default.

Common Gateway Interface

• The common gateway interface (CGI) mechanism extends a web server's behavior.

• CGI relies on a any program that can

  • access environment variables and

  • access std-in and std-out (the console).

• CGI is a simple, effective, but expensive way to extend servers.

CGI Example

Wiki URL

• The wiki URL has the format

  url/wiki?action=action&page=page
  

  where

  action is either edit or submit.
  page is a page name.

• An example wiki URL:

  url/wiki?action=edit&page=poem.html
  

Java main()

class Wiki {
  public static void main(String args[]) {

    final Map<String, String> 
      input = getInput(args);
    final String page = input.get("page");

    if (input.get("action").equals("edit"))
      doEdit(page);
    else
      doSubmit(page, input.get("pagesource");
    }
  }
  // and so on
}

Scala main()

import java.util.Map
import java.util.HashMap

object Wiki {
  def main(args: Array[String]) {

    val input = getInput(args)
    val page = input.get("page")

    if (input.get("action").equals("edit"))
      doEdit(page)
    else
      doSubmit(page, input.get("pagesource")
    }
  // and so on
  }

Scala main() Observations

• Works with Java run-time.

  import java.util.HashMap
  

• Type inferencing.

  val input = getInput(args)
  

• Array's a proper class.

  def main(args: Array[String]) {
  

• An object is a singleton class (no static class features).

  object Wiki {
    def main(args: Array[String]) {
  

Java Action Methods

class Wiki {

  // blah blah blah 

  private static void
  doEdit(String pageName) {
    sendCGIResponse(makeEditPage(pageName));
    }

  private static void
  doSubmit(String pageName, String pageSource) {
    writeWikiPage(pageName, pageSource);
    sendCGIResponse(makeWikiPage(pageName));
    }

  // blah blah blah 

Scala Action Methods

object Wiki {

  // blah blah blah 

  private def 
  doEdit(pageName: String) {
    sendCGIResponse(makeEditPage(pageName))
    }

  private def 
  doSubmit(pageName: String, pageSource: String) {
    writeWikiPage(pageName, pageSource)
    sendCGIResponse(makeWikiPage(pageName))
    }

  // blah blah blah 

Scala Method Observations

• No static class features

  private def 
  doEdit(pageName: String) {
  

• Scala follows Java's packaging scope rules.

  • This is a disappointment, but probably required.

  • Compatibility with the Java run-time, particularly security.

Java Page Classes

class EditWikiPage
extends HTMLPage
implements SafeString {

  EditWikiPage(String fn) { super(fn); }

  EditWikiPage() { this(""); }

  public String
  decontaminate(String str, Sanitizer sfun) {
    // blah blah blah
    }

  // blah blah blah

  }

Scala Page Classes

class EditPageWiki(fn: String)
extends HTMLPage with SafeString {

  private val filename = fn

  def this() { this("") }

  // blah blah blah

  }

Scala Class Observations

• Scala classes have a primary constructor.

  class EditWikiPage(fn: String) 
  /* blah blah blah */ {
  
    private val filename = fn 
  

• Other constructors are secondary constructors.

  def this() { this("") }
  

• Where's decontaminate()?

String Scrubbing

• CGI programs accept arbitrary input from anywhere in the world.

  • Inspect and massage data carefully before use.

• Outgoing strings should also be scrubbed of danger.

  • Strings from other sources may not have been scrubbed.

    John <script> ... </script> Doe
    

Java Interfaces

interface SafeString {

  // Return the string scrubbed of the 
  // dangerous constructs defined in the 
  // given sanitizer.

  String decontaminate(
    String str, Sanitizer sfun);
  }

interface Sanitizer {
  String scrub(String str);
  }

Scala Traits

trait SafeString {
  def decontaminate(
    str: String, sfun: Sanitizer) {
    // blah blah blah
    }
  }


abstract class Sanitizer {
  def scrub(str: String): String
  }

Scala Traits Observations

• Traits replace interfaces.

• The SafeString trait defines, not just declares, decontaminate().

  • EditWikiPage inherits it.

• Traits lead to mix-in style programming.

  • Adding specific clots of function to classes.

• Otherwise it's still single inheritance and abstract classes.

Java Functional Parameters

abstract class HTMLPage {

  void process(TagTransformer f) {
    List<HTMLTag> newPage = 
      new ArrayList<HTMLTag>();
    for (HTMLTag tag: page) {
      final HTMLTag newTag = f.munge(tag);
      if (newTag != null)
        newPage.add(newTag);
      }
    page = newPage;
    }
  }

interface TagTransformer {
  HTMLTag munge(HTMLTag tag);
  }

Example Use

page = 
  page.transform(
    new TagTransformer() {
      public HTMLTag munge(HTMLTag tag) {
	return tag.isTag("script") 
               ? null 
               : tag;
	}
      }
    );

Scala Functional parameters

abstract class HTMLPage {

  def process(munge: HTMLTag => HTMLTag) {
    var newPage: List[HTMLTag] = List()
    for (tag <- page) {
      val newTag = munge(tag)
      if (newTag != null)
        newPage = newTag :: newPage
      }
    page = newPage
    }
  }

Functional Parameter Observations

• Direct syntax for functions.

  def process(munge: HTMLTag => HTMLTag) {
  
  val newTag = munge(tag)
  

• Type inferencing fails sometimes.

  var newPage: List[HTMLTag] = List()
  

• Immutable generic lists.

  newPage = newTag :: newPage
  

Example Use

page = page.transform(
         (tag: HTMLTag) => 
           if (tag.isTag("script")) null 
           else tag)

or

val scrub = 
  (tag: HTMLTag) => 
    if (tag isTag "script") null else tag

page = page.transform(scrub)

Observations

• Functions are class instances with special constructor syntax.

  val scrub = 
    (tag: HTMLTag) => 
      if (tag isTag "script") null else tag
  

• Unary methods can be written with infix notation.

  if (tag.isTag("script")) null else tag
  
  if (tag isTag "script") null else tag
  

Scala For Comprehensions

• Run through a sequence, doing something for each element and returning a list of results.

def process(munge: HTMLTag => HTMLTag) {
  page = for (t <- page) yield munge(t)
  page = for (t <- page if t != null) yield t
  }

def process(munge: HTMLTag => HTMLTag) {
  page =
    for (t <- (for (t <- page) yield munge(t)) 
         if t != null) yield t
  }

Other Scala Data Structures

• Streams are lists with lazy evaluation.

  • Create infinite lists and other data structures.

• N-ary tuple class with built-in literal syntax.

  • Support includes matching over tuple coordinates, similar to functional languages.

• Built-in XML data-types (!?) with built-in syntax.

  • Support includes an XPath-like match operator.

Other Scala Function Features

• Returned function instances are closures.

  • Provides Smalltalk- or JavaScript-like blocks.

• Functions can be curried.

  • Allows for selective specialization and (theoretically) faster code.

• Scala is mostly tail recursive.

  • This is a platform dependence.

Summary

• Scala is a multi-paradigm language integrated with Java.

  • Principally functional and object-oriented paradigms.

• A broad design philosophy and an eclectic (and perhaps eccentric) implementation.

• Scalability is oriented towards small- and medium-scale programming.

  • No advances in large-scale programming.

Personal Conclusions

• Should I pay attention to Scala or Haskell?

  • If push comes to shove, Haskell.

• Should I ditch Java for Scala?

  • Yes; it's a better (that is, more functional) Java.

• Should I be paying attention to Scala? Why?

  • Yes, but not full attention.

  • Greater complexity provides greater leverage.

Resources

• The Scala web site www.scala-lang.com .

• The book Programming in Scala by by Martin Odersky, Lex Spoon, and Bill Venners.

• The Scala aggregator www.drmaciver.com/planetscala/ .