WebObjects with Scala

=== What is Scala? ===

[[Scala>>http://en.wikipedia.org/wiki/Scala_(programming_language)]] is a JVM language that is a hybrid of Object-Oriented and Functional styles. It is useful as complete general purpose replacement for Java. But its built-in Actors library makes it especially attractive for concurrent computing. In this day and age of multi-core processors, concurrent computing can't be ignored.

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Many of Scala's features have been designed with concurrency in mind, primarily a preference for immutability and the use of other functional language paradigms.

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Some of these may not be unfamiliar to Objective-C or WebObjects developers.

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Here's a quick summary:

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|= |= Objective-C |= Java |= Scala

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|= Separation of Mutable & Immuable Datatypes | Collections //e.g: NSArray/NSMutableArray// | No | Yes

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|= Closures | Blocks (//Extension//) | No | Anonymous Functions

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|= Static variables | Yes | Yes | No

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|= Static methods or functions | Yes | Yes | No

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|= Concurrency | [[Grand Central Dispatch>>http://en.wikipedia.org/wiki/Grand_Central_Dispatch]] (//Extension//) | //Threads// | [[Actors>>http://en.wikipedia.org/wiki/Actor_model]]

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|= |= Weakly Typed |= --Strongly Typed-- |= Strongly Typed

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Other notable features include:

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|= |= Objective-C |= Java |= Scala

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|= Parametered methods | Yes //e.g: addObject: to~:// | No | Yes //e.g: add(object= ,to=)//

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|= Class composition | Categories | Interfaces | Traits

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=== Why Use Scala? ===

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Scala can help you to write thread-safe code.

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It has concurrency that is built-in to the standard library, primarily via Actors.

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So for WebObjects developers, Scala offers itself as a powerful, safe and easy-to-use solution for [[concurrent applications>>Building Concurrent Applications with WebObjects and Scala]]. (In other words, Scala Actors can be used for problems that would have normally required threads).

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=== Can WebObjects be Programmed In Scala? ===

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Yes. It is very simple.

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Scala compiles to java bytecode. Hence using it with WebObjects is fairly straightforward.

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However, tool support is a weak point. You should use Eclipse 3.6 with bundle-less WO builds.  If your Application class is in Scala then you will have to create an Eclipse launch configuration manually. One weird thing is that once you add the Scala nature to a WO project WOD completion will stop working - this is unfortunate. Also the [[Scala plugin>>http://www.scala-ide.org]] is rather slow and still very buggy.

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= WebObjects In Scala =

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The following highlights some of the differences between Java and Scala in WebObjects:

== EOs in Scala ==

=== Thread-Safe Shared Vars ===

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Scala doesn't have static variables or methods. Instead Scala employs the [[Singleton Pattern>>http://en.wikipedia.org/wiki/Singleton_pattern]] which is built into the language and is **thread-safe**: a class can have a //Companion Object// that will allow you to achieve something equivalent to static variables - but better. Is this true?  I don't think Scala "object" instances (with the object keyword) are guaranteed to be thread-safe; they are just singletons

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So you don't have to worry about synchronizing access to shared mutable fields in a concurrent application.

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The following is an example of the use of a //Companion Object// for Talent in Scala instead of Talent static fields in Java.

Java:

public class _Talent extends EOGenericRecord {

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public static final String ENTITY_NAME = "Talent";

Scala:

object Talent extends EOGenericRecord {

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val ENTITY_NAME = "Talent"

This value will be accessed exactly the same way in both languages:

Talent.ENTITY_NAME

==== Compacted imports ====

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Two lines in Java are compacted into one in Scala.

In Java:

import com.webobjects.eocontrol.EOGenericRecord;

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import com.webobjects.eocontrol.EORelationshipManipulation;

In Scala:

import com.webobjects.eocontrol.{EOGenericRecord, EORelationshipManipulation}

== WOComponents in Scala ==

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==== Compact Constructors ====

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Scala allows for simpler use of multi-valued constructors than Java.

In Java:

public class MenuHeader extends WOComponent {

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public MenuHeader(WOContext aContext) {

super(aContext);

}

In Scala:

class MenuHeader(context: WOContext) extends WOComponent(context: WOContext) {

==== Simplified Exception Handling ====

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Scala doesn't force you to catch exceptions unlike in Java.

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In addition, the syntax employs Scala's very powerful **pattern matching** to handle exceptions.

In Java:

try {

EditPageInterface epi = D2W.factory().editPageForNewObjectWithEntityNamed(_manipulatedEntityName, session());

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epi.setNextPage(context().page());

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nextPage = (WOComponent) epi;

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} catch (IllegalArgumentException e) {

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ErrorPageInterface epf = D2W.factory().errorPage(session());

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epf.setMessage(e.toString());

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epf.setNextPage(context().page());

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nextPage = (WOComponent) epf;

}

In Scala:

try {

var epi: EditPageInterface = D2W.factory.editPageForNewObjectWithEntityNamed(_manipulatedEntityName, session)

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epi.setNextPage(context.page)

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nextPage = epi.asInstanceOf[WOComponent]

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} catch {

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case e: IllegalArgumentException => {

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var epf: ErrorPageInterface = D2W.factory.errorPage(session)

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epf.setMessage(e.toString)

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epf.setNextPage(context.page)

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nextPage = epf.asInstanceOf[WOComponent]

}

}

==== Scala Annotations vs. Generated Accessors ====

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An example of accessing variables in WebObjects with the following languages:

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|= |= Objective-C |= Java |= Scala

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|= getter | ##object name## | ##object.name()## | ##object.name##

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|= setter | ##object setName:aName## | ##object.setName(aName)## | ##object.name = aName##

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Of course in Java, we may generate WebObjects classes with "get" methods as well in order to stick to convention.

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In scala there is an additional convenience we may use to produce "get" and "set" methods in addition to the default Scala accessors - Scala Annotations.

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E.g, in Main.scala we annotate our component keys with ##@BeanProperty## to automatically create public "set" and "get" methods.

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These variables can then be accessed via //KVC//.

import scala.reflect.BeanProperty

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@BeanProperty var username = new String()

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@BeanProperty var password = new String()

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@BeanProperty var isAssistantCheckboxVisible = false

== How to Use Scala Collections with EOF ==

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To use the Scala Collections API with an NSArray or NSDictionary you simply need to add an import:

import scala.collection.JavaConversions._

After that, you may access the typical Scala collection methods directly on NSArray.  This employs a feature of Scala known as implicit conversions to automagically cast a NSArray (a Java Iterable) into a Scala Iterable while leaving the actual object unchanged.  Alternatively, you could generate an actual new scala.List instance by calling myNSArray.toList.

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== How to Add Scala to a WO Project ==

This is for Eclipse/WOLips IDE

== WO Scala Example ==

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The following example is an almost 100% Scala WO app. In reality it is a mixed Java/Scala app:

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All the EO logic and WO components are in Scala.

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Only the Application class remains Java.

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It is based on the D2W Movies example.

=== Setup ===

1. [[Install the Scala eclipse IDE>>http://www.scala-ide.org/]]

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1. Right-click on Application.java and run as a WOApplication (as usual).

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Application can be made into a Scala class as well, but then you will have to create a launcher in Eclipse manually.

== EO Templates ==

When you create your ##.eogen## file, be sure to make the following changes in the EOGenerator Editor:

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1. Point to the local [[Scala versions>>http://wiki.objectstyle.org/confluence/display/WOL/EOGenerator+Templates+and+Additions]] of the .eotemplate files for ##Entity## and ##Entity##

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1. Change the File Names Extension to "scala"

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1. In Destination Paths set the Superclass Package (e.g: base)

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1. Uncheck Java under Options

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== How to Build & Deploy a WebObjects Scala Project with Ant ==

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1. [[Download>>http://www.scala-lang.org/downloads]] and install Scala