Scala & symmetry
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Slides for the lunchtime talk on Scala & Symmetry on 11 June 2013 at ScalaDays 2013.
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Scala & symmetry
- 1. Scala Days 2013 Gilt Groupe ebowman@gilt.com http://www.flickr.com/photos/justusthane/1252907196/
- 2. Gilt provides insider access to today’s top designer labels, at up to 60% off retail. Through our daily flash sales, which begin at noon EST, we offer everything from high-end designer clothing and home furnishings to fine dining experiences. Our products are grouped into five stores: Women, Men, Baby & Kids, Home and City. Working from New York City, Dublin, or home/remotely, Gilt Tech’s 120 engineers develop all of our technology in-house. Since Gilt’s founding days, we have pursued a structure that enables us to address unique needs other e-commerce platforms simply cannot support. These needs include the ability to handle intense traffic spikes, offer prompt customer service, manage operations (inventory, payments and shipping), and execute many other tasks. As a rapidly growing company with an international customer base, Gilt’s priorities focus on scalability and personalization. Gilt.com was originally built on Ruby on Rails, but our rapid growth quickly led to scalability issues — prompting us to migrate many parts of our website to the Java Virtual Machine. In 2011 we began migrating our operations to Scala, which is currently our core stack.
- 8. EverythingisanObject Java Scala void dump(int x) { System.out.println( Integer.toString(x)); } def dump(x: Int): Unit = { println(x.toString) } void dump(Complex x) { System.out.println( x.toString()); } def dump(x: Complex): Unit = { println(x.toString) } :( def dump[T](x: T): Unit = { println(x.toString) }
- 9. UniformAccessPrinciple Java Scala public class Datum { public final int x; public Datum(int x) { this.x = x; } } class Datum(val x: Int) public class Datum { private final int y; public int x() { return f(g(y)); } public Datum(int y) { this.y = y; } } class Datum(y: Int) { def x = f(g(y)) } “All services offered by a module should be available through a uniform notation, which does not betray whether they are implemented through storage or through computation” - Bertrand Meyer http://en.wikipedia.org/wiki/Uniform_access_principle
- 10. Immutable|Mutable List<String> foo() { ArrayList<String> items = computeItems(); return items; }
- 11. Immutable|Mutable List<String> foo() { ArrayList<String> items = computeItems(); if (unlikelyThing()) { return items.subList( 0, items.size() / 2); } else { return items; } } List<String> foo() { ArrayList<String> items = computeItems(); return items; }
- 12. PatternMatching x match { case even if (x % 2) == 0 => even/2 case odd => odd + 1 }
- 13. PatternMatching x match { case even if (x % 2) == 0 => even/2 case odd => odd + 1 } catch { case io: IOException => ... case e: Exception => ... }
- 14. PatternMatching x match { case even if (x % 2) == 0 => even/2 case odd => odd + 1 } catch { case io: IOException => ... case e: Exception => ... } val split: String => String = { case null => None case str => Some(str) }
- 15. Monads for { x <- monad1 y <- monad2 } yield x op y
- 16. Monads for { x <- monad1 y <- monad2 } yield x op y Traversable Option Future IO Reader Writer Error Continuation
- 17. Asymmetry
- 18. PointlessSymmetry try { Class c = Class.forName("MyClass"); Object instance = c.newInstance(); c.getMethod("myMethod").invoke(instance); } catch (NullPointerException e) { } catch (IllegalAccessException e) { } catch (InvocationTargetException e) { } catch (ClassNotFoundException e) { } catch (NoSuchMethodException e) { } catch (InstantiationException e) { }
- 19. http://www.metalev.org/2011/04/source-code-symmetry-and-transcendent.html a[i].x += a[i - 1].x; a[i].y += a[i - 1].y; Ill-ConceivedSymmetry
- 20. http://www.metalev.org/2011/04/source-code-symmetry-and-transcendent.html a[i].x += a[i - 1].x; a[i].y += a[i - 1].y; a(i) += a(i - 1) Ill-ConceivedSymmetry
- 21. private def immediateAstNodes(n: Any): List[AstNode] = n match { case a: AstNode List(a) case t: Token Nil case Some(x) immediateAstNodes(x) case xs @ (_ :: _) xs flatMap { immediateAstNodes(_) } case Left(x) immediateAstNodes(x) case Right(x) immediateAstNodes(x) case (l, r) immediateAstNodes(l) ++ immediateAstNodes(r) case (x, y, z) immediateAstNodes(x) ++ immediateAstNodes(y) ++ immediateAstNodes(z) case true | false | Nil | None Nil } http://stackoverflow.com/questions/8618082/visitor-pattern-in-scala BeautifulAsymmetry