internal dsls

internal DSLsZef Hemel

Tuesday, May 18, 2010

Internal DSLs are particular ways of using a host language to give the host language the feel of a particular language.

Martin Fowler


Internal DSLs are particular ways of using a host language to give the host language the feel of a particular language.

Martin Fowler

ab


fluent interfaces

flexible syntax

reflection

macros

method missing


fluent interfaces


Order o = new Order();Product p1 = new Product(1,Product.find(“Billy”));o.addProduct(p1);Product p2 = new Product(2,Product.find(”Janso"));o.addProduct(p2);Product p3 = new Product(4,Product.find(“Traby"));o.addProduct(p3);o.setPriorityRush(true);customer.addOrder(o);

http://www.st.ewi.tudelft.nl/~bouwers/main/slides/2008jspring.pdf




customer.newOrder() .with(1, "Billy") .with(2, "Janso") .with(4, "Traby") .priorityRush() .done();


public class Customer { ... public OrderBuilder newOrder() { return new OrderBuilder(this); }}


public class OrderBuilder { // ...

public OrderBuilder(Customer customer) { this.customer = customer; this.order = new Order(); }

public OrderBuilder with(int id, String name) { order.addProduct(new Product(id, name)); return this; }

public OrderBuilder priorityRush() { order.setPriorityRush(true); return this; }

public void done() { customer.addOrder(this.order); }}


flexible syntax


header("Add entry")form { table { row { col { text("Your name:") } col { newEntry.name = input(newEntry.name) } } row { col { text("Your message:") } col { newEntry.text = inputText(newEntry.text) } } } button("Post") { newEntry.save() goto(Home()) }}


in Scala


case class Home() extends Page { def ui { header("Welcome to my guestbook!") section { header("All entries") list { for (e <- cache("entries", Entry.all)) { listitem { form { text(e.name) text(": ") text(e.text) button("Delete") { e.delete() goto(Home()) } } } } } } }}


case class Home() extends Page { def ui { header("Welcome to my guestbook!") section { entries } }

def entries { header("All entries") list { for (e <- cache("entries", Entry.all)) { listitem { form { text(e.name) text(": ") text(e.text) button("Delete") { e.delete() goto(Home()) } } } } } }}


object DefaultStyle extends Style { block("headerblock") >> header { fontsize = 30 pt; width = 100 percent; bgcolor = "#eeeeee"; } section >> header { color = "#0c0ccc"; } body { fontfamily = "Helvetica, Arial, Verdana, sans-serif" }}


object DefaultStyle extends Style { block("headerblock").>>(header { fontsize = 30 pt; width = 100 percent; bgcolor = "#eeeeee"; }) section.>>(header { color = "#0c0ccc"; }) body { fontfamily = "Helvetica, Arial, Verdana, sans-serif" }}


object DefaultStyle extends Style { block("headerblock").>>(header(() => { fontsize = 30 pt; width = 100 percent; bgcolor = "#eeeeee"; })) section.>>(header(() => { color = "#0c0ccc"; })) body(() => { fontfamily = "Helvetica, Arial, Verdana, sans-serif" })}


a >> b == a.>>(b)


section { header("All entries") ...}


section(() => { header("All entries") ...})


section(() => { header("All entries") ...})

def section(content : => Unit) { write("<div class='section'>") content write("</div>")}


ruby


create_table :posts do |t| t.string :name t.string :title t.text :contentend


create_table(:posts,do |t| t.string(:name) t.string(:title) t.text (:content)end)


class Post < ActiveRecord::Base validates_presence_of :name, :title validates_length_of :title, :minimum => 5end


class Post < ActiveRecord::Base validates_presence_of(:name, :title) validates_length_of(:title, :minimum => 5)end


method missing


scala


width = 100 percent;


def width_=(w: UnitInt) { ...}

width = 100 percent;


width_=(100 percent);


width_=(100.percent);


width_=(100.percent());


implicit def int2UnitInt(i: Int) =new UnitIntWrapper(i)


width_=(100.percent());


width_=(int2UnitInt(100).percent());


class UnitIntWrapper(i: Int) { ...

def percent = new PercentUnitInt(i)

class PercentUnitInt(i: Int) extends UnitInt { override def toString = i + "%" }}


ruby


Person.find_by_name('Zef')


class Person def self.find(key, value) puts "You want results from #{key} with a value of #{value}" endend

Person.find('name', 'Zef')


class Person def self.find(key, value) puts "You want results from #{key} with a value of #{value}" end

def self.method_missing(id, *args) if id.id2name =~ /find_by_(.+)/ return self.find(Regexp.last_match(1), args[0]) else raise NoMethodError end endend

Person.find_by_name('Zef')


reflection


public class Person { @Persistent public String name; @Persistent public int age;}


void persist(Object obj) { Class cls = obj.getClass(); Field[] fields = cls.getFields(); for(Field f : fields) { Persistent anno = f.getAnnotation(Persistent.class); if(anno != null) { System.out.println(f.getName()); } }}


macros


int one() { printf("One!"); return 1;}

int two() { printf("Two!"); return 2;}

...

int c = choice(n == 1, one(), two());


int choice(BOOL c, int ifTrue, int ifFalse) { return c ? ifTrue : ifFalse;}


int one() { printf("One!"); return 1;}

int two() { printf("Two!"); return 2;}

...

int c = choice(n == 1, one(), two());


#define CHOICE(c,ifTrue,ifFalse) \ (c) ? (ifTrue) : (ifFalse)



int c = CHOICE(n == 1, one(), two());



int c = CHOICE(n == 1, one(), two());

int c = (n == 1) ? (one()) : (two());


clojure (a Lisp)


homoiconiccode is datadata is code


(+ 1 2 3)


(+ 1 2 3) = 6


'(+ 1 2 3)


(first '(+ 1 2 3))


(first '(+ 1 2 3)) = +


(cons '+ (reverse (rest '(+ 1 2 3))))



= (+ 3 2 1)


(cons '+ (reverse (rest '(+ 1 2 3))))(eval

)



= 6

(eval)


(if (= n 10) (print "It was ten!”) nil)


(when (= n 10) (print "It was ten!”))


(defn when [c ifTrue] (if c ifTrue nil))


(defmacro when [c ifTrue] (list 'if c ifTrue 'nil))


(defmacro when [c ifTrue] `(if ~c ~ifTrue nil))



(if (= n 10) (print "It was ten!”) nil)


(loop for i in *random* counting (evenp i) into evens counting (oddp i) into odds summing i into total maximizing i into max minimizing i into min finally (return (list min max total evens odds)))


(defent User [:username :string {:unique true}] [:openid :string] [:email :email] [:points :int])

http://github.com/zefhemel/adia




(defwebfn show [u User] [:h1 "Posted items"] [:ul (for [i (query Item :where {:author (:_id u)} :order-by {:date -1})] [:li (:title i)])])


advantages


easy to develop

builds on existing platform

existing community


disadvantages


<h1>Hello, Rails!</h1>

<%= link_to "My Blog", post_path %>

http://zef.me/2308/when-rails-fails




<h1>Hello, Rails!</h1>

<%= link_to "My Blog", post_path %>

post_url failed to generate from {:controller=>"posts", :action=>"show"} – you may have ambiguous routes, or you may need to supply additional parameters for this route. content_url has the following required parameters: ["posts", :id] – are they all satisfied?





<%= link_to 'Destroy', post, :confrm => 'Are you sure?', :method => :delete %>


class Post < ActiveRecord::Base validates_presence_of :namend


no static checking

late discovery of errors


non-domain specific error messages


errors hard to trace back to origin


Table t = table("table").as("t");Table t1 = table("table1").as("t1");Field tId = t.field("id");Field t1Id = t1.field("id");Field t1Time = t1.field("time");

Sql sql = select(tId).from(t).join(inner(t1, tId.eq(t1Id))) .where(and(tId.eq("'a'"), t1Time.between("'1900'", "'2000'"))) .groupBy(tId).having(tId.gt("1")) .orderBy(asc(tId));

http://www.jequel.de


http://www.jequel.de/

http://www.jequel.de/

limited freedom in syntax


+ -quick development lack of static checking

built on existing platform

errors hard to trace back to origin

existing community bad error messages

limited freedom in syntax


?Tuesday, May 18, 2010

internal dsls

Technology

public class customer

public class orderbuilder

public class person

order o

public void

public orderbuilder

new order product p1

setpriorityrushtrue