WebObjects with Scala

=== What is Scala? ===

[[Scala>>http://www.scala-lang.org/]] is a modern, multi-paradigm JVM language that is most often compared to [[Groovy>>http://en.wikipedia.org/wiki/Groovy_(programming_language)]], [[Clojure>>http://en.wikipedia.org/wiki/Clojure]] or [[Erlang>>http://en.wikipedia.org/wiki/Erlang_(programming_language)]]. Its [[functional language>>http://en.wikipedia.org/wiki/Functional_programming]] foundations and built-in Actors library make it especially attractive for concurrent computing. (Scala is an abbreviation for "scalable" hinting at its design goals). In this day and age of multi-core processors concurrent computing can not be ignored.

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Many of the design features of Scala have been chosen with concurrency in mind, some of which may not be unfamiliar to Objective-C or WebObjects developers. Here's a summary:

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

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|= Immutability | Partial - via collections //e.g: NSArray/NSMutableArray// | No | Yes

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|= Closures | Yes - via Blocks (//Extension//) | No | Yes - via 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 | Yes - via [[Grand Central Dispatch>>http://en.wikipedia.org/wiki/Grand_Central_Dispatch]] (//Extension//) | //Yes - via Threads// | Yes - via [[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 | Yes - via Categories | Yes - via Interfaces | Yes - via Traits

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=== Why Use Scala? ===

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For WebObjects developers, Scala offers itself as a powerful, safe and easy-to-use solution for [[concurrent computing>>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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By virtue of being a JVM-language, Scala compiles to java bytecode.

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Furthermore, being a multi-paradigm language grants Scala easy WebObjects-interoperability.

===== Caveats =====

Legacy tool support is often cited as a weak point. The [[Eclipse Scala plugin>>http://www.scala-ide.org]] has been found to be slow at times and sometimes 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.

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

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(This is not however true when for example you have a ##val## declared as a ##NSMutableArray##. You will still have to synchronize when adding to or removing from this mutable field).

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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 {

val ENTITY_NAME = "Talent"

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

Talent.ENTITY_NAME

==== Compacted imports ====

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.

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== How to Add Scala to a WO Project (in Eclipse) ==

== 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).

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