Talk given as part of the Refreshing Series, about how symmetry manifests itself in software, using Scala as an example where internal symmetry leads to much greater expressiveness.

1. Symmetry, Scala &
Software
Refresh Dublin
15 October 2013
Eric Bowman
@ebowman
ebowman@gilt.com
http://tech.gilt.com

4. “harmonious and
beautiful proportion
and balance”
http://en.wikipedia.org/wiki/Symmetry

5. “patterned
self-similarity”
http://en.wikipedia.org/wiki/Symmetry

7. Symmetry is changing
without changing.

8. Symmetry
in Scala

9. Scala
•Object-Oriented/Functional
Hybrid
•Statically Typed

10. public class HelloWorld {
public static void main(String[] args) {
System.out.println("hello, world");
}
}

11. public class HelloWorld {
public static void main(String[] args) {
System.out.println("hello, world");
}
}
object HelloWorld extends App {
println("hello, world")
}
!

12. public class HelloWorld {
public static void main(String[] args) {
System.out.println("hello, world");
}
}
object HelloWorld extends App {
println("hello, world")
}
!
println("hello, world")

13. for (int i = 1; i <= 10; i++) {
for (int j = 1; j <= 10; j++) {
System.out.println(i*j);
}
}
!
!

14. for (int i = 1; i <= 10; i++) {
for (int j = 1; j <= 10; j++) {
System.out.println(i*j);
}
}
!
!
for {
i <- 1 to 10
j <- 1 to 10
} println(i * j)
!

15. Future<Integer> f = callService1();
Future<Integer> f2 = callService2(f.get());
System.out.println(f2.get());

16. Future<Integer> f = callService1();
Future<Integer> f2 = callService2(f.get());
System.out.println(f2.get());
val f = for {
x <- callService1()
y <- callService2(x)
} yield y
println(Await.result(f, Duration.Inf))
!

17. String capitalize(String name) {
if (name == null) return null;
else return Character.toUpperCase(
name.charAt(0)) + name.substring(1);
}

18. String capitalize(String name) {
if (name == null) return null;
else return Character.toUpperCase(
name.charAt(0)) + name.substring(1);
}
def capitalize(n: Option[String]): Option[String] = {
for (name <- n) yield {
name(0).toUpper + name.substring(1)
}
}
!
!

19. for (int i = 1; i <= 10; i++) {
for (int j = 1; j <= 10; j++) {
System.out.println(i*j);
}
}
!
!
Future<Integer> f = callService1();
Future<Integer> f2 = callService2(f.get());
System.out.println(f2.get());
Ja
va
String capitalize(String name) {
if (name == null) return null;
else return Character.toUpperCase(
name.charAt(0)) + name.substring(1);
}
for {
i <- 1 to 10
j <- 1 to 10
} println(i * j)
!
val f = for {
x <- callService1()
y <- callService2(x)
} yield y
println(Await.result(f, Duration.Inf))
!
!
def capitalize(name: Option[String]): Option[String] =
for (n <- name) yield {
n(0).toUpper + n.substring(1)
}
!
!
Sc
al
a

20. Refactoring

21. “By continuously improving the design of code,
we make it easier and easier to work with. This
is in sharp contrast to what typically happens:
little refactoring and a great deal of attention
paid to expediently adding new features. If you
get into the hygienic habit of refactoring
continuously, you'll find that it is easier to
extend and maintain code.”
http://en.wikipedia.org/wiki/Code_refactoring

22. •Abstraction
•Composition/Decomposition
•Name & Location

23. 1. Finite, unchanging list of possible
refactorings.
2. Every refactoring is reversible.
3. Every refactoring is deterministic.
4. Refactorings can be combined in any
order.

24. Group Theory

25. “‘Numbers measure size;
groups measure symmetry.”
Groups and Symmetry, Armstrong, Spring-Verlag, 1988, p.1

26. “The most substantial piece of writing in the whole
literature of mankind”
http://upload.wikimedia.org/wikipedia/commons/5/53/Evariste_galois.jpg

27. http://opinionator.blogs.nytimes.com/2010/05/02/group-think/

28. http://opinionator.blogs.nytimes.com/2010/05/02/group-think/

29. “the symmetry group of an object is the
group of all isometries under which the
object is invariant with composition as the
operation”
http://en.wikipedia.org/wiki/Symmetry_group

30. “the symmetry group of a program ... is the
group of all refactorings under which the
program behaves identically after applying
refactorings in any order”

31. for (int i = 1; i <= 10; i++) {
for (int j = 1; j <= 10; j++) {
System.out.println(i*j);
}
}
!
!
Future<Integer> f = callService1();
Future<Integer> f2 = callService2(f.get());
System.out.println(f2.get());
Ja
va
String capitalize(String name) {
if (name == null) return null;
else return Character.toUpperCase(
name.charAt(0)) + name.substring(1);
}
for {
i <- 1 to 10
j <- 1 to 10
} println(i * j)
!
val f = for {
x <- callService1()
y <- callService2(x)
} yield y
println(Await.result(f, Duration.Inf))
!
!
def capitalize(name: Option[String]): Option[String] =
for (n <- name) yield {
n(0).toUpper + n.substring(1)
}
!
!
Sc
al
a

32. Breaking
Symmetry

33. a[i].x += a[i - 1].x;
a[i].y += a[i - 1].y;
http://www.metalev.org/2011/04/source-code-symmetry-and-transcendent.html

34. a[i].x += a[i - 1].x;
a[i].y += a[i - 1].y;
a(i) += a(i-1)
http://www.metalev.org/2011/04/source-code-symmetry-and-transcendent.html

35. http://pages.cs.wisc.edu/~dyer/vsam/images/mona.gif
http://www.leonardodavinci.net/images/gallery/monalisa_detail1.jpg
http://www.saylor.org/site/wp-content/uploads/2012/04/6-mona-lisa.gif

36. http://upload.wikimedia.org/wikipedia/commons/thumb/d/db/MaryRose-carpentry_tools1.jpg/250px-MaryRose-carpentry_tools1.jpg
http://test.classconnection.s3.amazonaws.com/324/flashcards/95324/png/altarpiece.png

37. http://upload.wikimedia.org/wikipedia/commons/thumb/d/db/MaryRose-carpentry_tools1.jpg/250px-MaryRose-carpentry_tools1.jpg
http://test.classconnection.s3.amazonaws.com/324/flashcards/95324/png/altarpiece.png

38. http://www.biographyonline.net/artists/images/Much-The_Scream.jpg
http://upload.wikimedia.org/wikipedia/commons/6/63/Michelangelos_David.jpg
http://amolife.com/image/images/stories/Art&Abstract/most_popular_sculptures%20(7).jpg
http://www.pics24h.com/img/artwork/5-most-famous-paintings/5-most-famous-paintings03.jpg

39. Fin
niF
join us.
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@gilttech
@ebowman
ebowman@gilt.com