JRuby e DSLs
Luca Guidi - Sourcesense
Java Day Roma - 1 Dic 2007

DSLs cosa sono?

Domain Specific
Language

DSLs
• single purpose
• singolo dominio

Linguaggi di
programmazione
• general purpose
• multi dominio

YACC (1975)
Stephen C. Johnson

UNIX scripting

ls | grep dsl >> ~/list.txt

favoriscono le ontologie

Ontologia: rappresentazione strutturata
delle informazioni espressa tramite
gerarchie e relazioni tra classi.

Behavior Driven
Development

it \"tracks implicit observable models\" do
instance = FooObserver.new
instance.send(:observed_classes).should include(Foo)
instance.send(:observed_classes).should_not include(ObservedModel)
end

should

Secondo Martin Fowler
i DSL sono:
• Esterni (XML)
• Interni (Smalltalk, Ruby)

Java
• Non dinamico
• Strongly typed

<?xml version=\"1.0\" encoding=\"UTF-8\"?>
<book>
<attribute id=\"title\">
<required>true</required>
<length>
<minimum>1</minimum>
<maximum>23</maximum>
</length>
</attribute>
</book>

Annotations

import java.lang.reflect.*;
public class Book {
@Size(min=1, max=23)
protected String title;
public Book (String title) {
setTitle(title);
}
public String getTitle() {
return this.title;
}
public void setTitle(String title) {
this.title = title;
}
public boolean isValid()
throws NoSuchFieldException, SecurityException {
Size size = (Size) this.getClass().getDeclaredField(\"title\")
.getAnnotation(Size.class);
return ( title != null && ( title.length() >= size.min()
|| title.length() <= size.max() ) );
}
}

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface Size {
public int max() default 0;
public int min() default 0;
}

module Validations
def validates_presence_of(*attributes)
# ...
end
def validates_size_of(*attributes)
# ...
end
end
class Book
extend Validations
validates_presence_of :title
validates_size_of :title, 1..23
end

Nella mia esperienza,
la cosa più affascinante è che i
programmi Ruby ben scritti sono
già un DSL, a causa della sintassi
del linguaggio.
Jamis Buck - 37 signals

Ruby
• Symbol
• Block
• Module
• Splat
• eval, instance_eval, class_eval
• define_method, alias_method

Ruby DSLs

Method Class
class Carrot < ActiveRecord::Base
belongs_to :bunny
end
class Bunny < ActiveRecord::Base
has_many :carrots
end

Top Level
namespace :db do
desc \"Create a database connection.\"
task :connect do
# ...
end
desc \"Prepare the database for tests.\"
task :prepare => :connect do
# ...
end
end

Sandbox
ActiveRecord::Schema.define do
create_table :bunnies do |t|
t.column :name, :string, :null => false
end
end
ActiveRecord::Schema.define do
drop_table :bunnies
end

Piattaforma Java

Java => JRuby
require 'java'
include_class 'java.util.Random'
class JavaRandom
def self.next
@@r ||= Random.new
@@r.nextInt
end
end
JavaRandom.next # => 23

JRuby => Java
import org.jruby.*;
public class UsingRuby {
public static void main(String[] args) {
Ruby runtime = Ruby.getDefaultInstance();
runtime.evalScript(\"puts 1 + 2\");
}
}

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