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Google Dart Eberhard Wolff Architecture & Technology Manager adesso AG
Dart: A Structure Web Programming Language ► New programming language
► New programming tools
► New open source project
► Currently in a preview
► …for feedback
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The Team Behind Dart ► Lars Bak
> Beta language > HotSpot Java VM > V8 JavaScript VM in Google Chrome > 18 software patents
► Gilad Bracha > Computational Theologist and later Distinguished Engineer at
Sun > Java Language Specification > Java Virtual Machine Specification
► Both worked on Strongtalk (Smalltalk + static typing)
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Why Dart? ► More and more web application with complex logic
► Will become a lot more: HTML5 on the rise
► So far: No really great languages designed to create large scale web applications
► Google: The competition is NOT JavaScript ... but fragmented mobile platforms
► GWT (Google Web Toolkit) already featured a Java to JavaScript compiler
► Dart is designed with simplicity and mass appeal in mind
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Runtime Environment ► Dart has its own VM
► Open Source project
► Needed for some advanced features
► Dart can compile into JavaScript
► Runs on any browser
► Currently not very efficient
► i.e. about the same performance as first V8 releases
► Size of JavaScript code currently considerable
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Hello World in Dart
► C like language
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main() { print('Hello, Dart!');}
int fib(int n) { if (n <= 1) return n; return fib(n - 1) + fib(n - 2);}main() { print('fib(20) = ${fib(20)}');}
► Seemingly static typing
Objects and Classes
► Easy to initialize fields
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class Person { String name; Person(this.name);}main() { Person p = new Person('Gilad'); print('Hi ${p.name} ');}
Objects and Classes
► No method or constructor overloading
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class Person { String name; String firstname; Person(this.name); Person.withFirstname(this.firstname,this.name);}main() { Person p = new Person.withFirstname('Gilad','Bracha'); print('Hi ${p.firstname} ${p.name}');}
Namespaces ► Much like Java packages
► Might contain classes, interfaces, variables and functions
► Definition:
#library("http");
► Using:
#import("http.dart");
► Optional prefix:
#import("http.dart”,”http”);
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Dart Library
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Object Construction With Factories
► Very elegant approach to allow for other object creation algorithms
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interface Person factory PersonFactory { Person(name); final name;}class PersonFactory { factory Person(name) { if (name == 'Wolff') { return new Adessi(name); } return new RealPerson(name); }}
class Adessi implements Person { Adessi(this.name); String name;}class RealPerson implements Person { RealPerson(this.name); String name;}main() { Person p = new Person('Wolff'); print(p is Adessi); p = new Person('Bracha'); print(p is Adessi);}
More Complex Example
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class Person {}class Customer extends Person { buy() {print("bought");}}main() { Person p = new Customer(); p.buy();}
More Complex Example
► Code actually compiles and runs
► There are no type errors – only Static Warnings
► Types are considered annotations
► Type annotations never change the semantics of a program 27.10.11 13
class Person {}class Customer extends Person { buy() {print("bought");}}main() { Person p = new Customer(); p.buy();}
On Types ► Type theory: A type defines a set of values and operations on them
> i.e.Java int: values: all integers from -2147483648 to 2147483647 > Operations on int: + - * etc
► Goal: Check types to find errors > i.e. multiply a String by an int > i.e. call a method that does not exist on an object
► Type checkers proof the partial correctness of programs
► i.e. at least types are correct
► Typing will only find basic errors
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Dynamic Typing ► At runtime
► Advantage > Simpler > More flexible > Can even implement methods on the fly – Meta programming
► Disadvantage > Perceived as less secure
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Static Typing ► Typing checked At compile time
► Advantage > IDE support > Documentation > Perceived as safer > …but problems should be found by Unit Tests, too!
► Disadvantage > Overhead if no type inference > Complex to get right in some case > Complex (Java Generic FAQ by Anglika Langer is 297 pages !)
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Generics in Java ► Does this code compile?
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public class Customer extends Person {…}public static void main(String[] args) { List<Person> listOfPerson = new ArrayList<Person>(); List<Customer> listOfCustomer = new ArrayList<Customer>(); listOfCustomer=listOfPerson; listOfPerson=listOfCustomer;}
Generics in Java ► Does this code compile?
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public class Customer extends Person {…}public static void main(String[] args) { List<Person> listOfPerson = new ArrayList<Person>(); List<Customer> listOfCustomer = new ArrayList<Customer>(); listOfCustomer=listOfPerson; listOfPerson=listOfCustomer;}
listOfPerson.add(new Person());listOfCustomer.add(new Person());Customer c = listOfCustomer.get(1);Customer c = listOfPerson.get(1);
Generics and Static Typing in Java ► You can still mess with it
► Basically each type cast disables static typing
► i.e. it can introduce problems that otherwise would have been discovered by the compiler
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listOfPerson.add(new Person());Object obj = listOfPerson;listOfCustomer = (List<Customer>) obj;Customer c = listOfCustomer.get(0);
Static vs. Dynamic Typing: My Take ► Errors found by a static type checker will also be found by unit tests
► The security of static typing is only perceived
► Real advantage: Documentation
► “I expect you to pass in a Customer!”
► “This gives you a Person!”
► Tool support
► Easier to come up with suggestions for content assist
► However, Dynamic languages tools are catching up
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Generics in Dart
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class Person {}class Customer extends Person { buy() {print("bought");}}main() { List<Customer> listOfCustomer = new List<Customer>(); List<Person> listOfPerson = listOfCustomer; listOfPerson.add(new Person()); Customer c = listOfCustomer[0]; c.buy();}
Generics in Dart
► Call to buy() won’t work
► Problem not found until the method is actually called
► Optional run time type checker would find the problem one line earlier
► Valid code: List<Customer> is a List<Person>
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class Person {}class Customer extends Person { buy() {print("bought");}}main() { List<Customer> listOfCustomer = new List<Customer>(); List<Person> listOfPerson = listOfCustomer; listOfPerson.add(new Person()); Customer c = listOfCustomer[0]; c.buy();}
Generics in Dart ► Generics in Dart are considered covariant
► i.e. List<Customer> is List<Person>
► This is in fact logical incorrect (see last slide)
► But: > Do you want to read 297 pages Generic FAQ? > And: It is just a help to spot basic errors
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Concurrency
► What is the problem?
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public class MyCache { Map<Integer, Integer> map = new HashMap<Integer, Integer>(); public void add(int i, int j) { map.put(i, j); }
public int read(int i) { return map.get(i); }}
Concurrency
► What is the problem?
► Code is not thread safe!
► Should use ConcurrentHashMap instead
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public class MyCache { Map<Integer, Integer> map = new HashMap<Integer, Integer>(); public void add(int i, int j) { map.put(i, j); }
public int read(int i) { return map.get(i); }}
Concurrency ► What is the real problem?
► Concurrency in object-oriented systems
► OO is based on concurrent access shared state > Hard to get correct > If you get it correct: performance bad > Locks > Synchronization > Etc
► Pretty low level concept
► This is why Carnegie Mellon dropped OO from the Computer Science curriculum!
► Can we do better?
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Object with State
Threads
Dart code is always single threaded!
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Isolates ► Inspired by Actor model
► As present in Erlang
► As done in Scala with Actors and Akka
► Idea: State is hidden in Isolates
► Isolates communicate with messages send to ports
► Each isolate only works on one message at a time
► No concurrency issues in an Isolate: Only one thread
► Still concurrent: Multiple isolates might be active at a time
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Isolate with State
Port
Isolates
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class Printer extends Isolate { main() { port.receive((message, replyTo) { if (message == null) port.close(); else print(message); }); }}main() { new Printer().spawn().then((port) { for (var message in ['Hello', 'from', 'other', 'isolate']) { port.send(message); } port.send(null); });}
More Fun With Isolates ► Isolates might be mapped to Web Worker in JavaScript
► Nothing is shared
► Isolates will have their own heap on the Dart VM > Messages must be serialized and deserialized > Garbage Collection can happen per Isolate > No more stop-the-world GC!
► Isolates might be used to introduce security concepts > Enforce security rules on ports > To replace Same Origin Policy
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More Fun With Isolates ► Isolates require a light weight concurrency model
> OS Thread switches take too much time > In Erlang 100.000 of Isolates equivalent are not uncommon > No way to run that many threads > Interesting issue for Dart VM
► Isolates for Remoting > Can send messages to a port on a different host > Semantics don’t change as serialization is done anyway
► Probably more high level concurrency models
► Specifications says that Isolates might run different versions of libraries
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Even More Fun With Isolates ► In Erlang the concept is extended for high availability
> Links allow to listen to events of Isolates > Supervisor can monitor other processes > If an error occurs the isolate crashes and is restarted > Hierarchies of supervisors can be created > State is a problem here > This makes high availability easy to develop > Dart might come up with a similar solution
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Snapshots ► VM can be snapshot
► Current heap etc
► Faster startup > No need to create initial state, config, … > Used in V8 already for standard JavaScript libraries > How long does a JVM need to start up?
► Possible answer to mobile applications killed due to memory constraints
► Could be used to migrate isolates between machines
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Dart Editor ► Editor for Dart applications
► Based on Eclipse
► Code in the Open Source Project
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More Complex Example ► Swarm: A newsreader
► Completely written in Dart > App code: 3210 LOC > UI library code: 13200 LOC > Animation yields 30 fps > Runs on iPad and Chrome
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Not Decided Yet… ► Currently a technology preview
► Some questions are open > Reflection? Probably using Mirrors > Changing classes on the fly? Probably not needed due to Isolates > What about Chrome?
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Links ► http://www.dartlang.org/
> Language Site > Tutorials > Language Spec > Library Spec > …
► http://dart.googlecode.com/ > Open Source project > Including compiler, VM etc.
► http://code.google.com/p/jdart/ > Dart on the JVM
► http://it-republik.de/jaxenter/artikel/Google-Dart-4121.html > German
► http://jonaswesterlund.se/dart.html > Pre compiled Dart for Mac OS X
► http://www.infoq.com/articles/google-dart 27.10.11 37
Possible Applications for Dart ► Google App Engine
> Isolates are a nice fit for GAE’s restriction > Snapshot will make it easy to move execution from
machine to machine
► Android > As a replacement for Java
► Web Browser > Compiling into JavaScript
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Dart: Conclusion – Huge Potential ► Best shot at next generation web language so far
► Language designed to appeal to the masses
► Solves a real problem: Complex web applications
► Already possible to create cross platform web applications
► Google has a lot of resources and this solves a real problem for them
► Isolates and future concurrency models very interesting > Client: Security > Server: Modern concurrency > …
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