An introductory presentation about Ruby metaprogramming presented at CodeMash '08

1. Rails
A Peek under the covers
Brian Sam-Bodden

2. Rails

3. A framework...
for building database-backed web applications
that at first wow us with speed

4. Rails can be seen as a collection of...
mini-languages
for implementing MVC
on the web

5. then came the headlines...
and we all wondered...
what am I doing wrong?

6. the answer
too much
and we reacted
the way we knew better
...with frameworks

7. Lessons of Rails

8. Integrated
Seamless front to back stack
Familiar idioms on every tier
Avoiding manually defining or gluing the tiers

9. CoC
Convention Over Configuration
“Flexibility is overrated, Constraints are liberating”

10. DRY
Do it once, do it right and do it in the right place

11. Instant Change
Tight feedback loop
We want the world and we want it NOW!

12. Terse Expressiveness
Getting advanced behavior with very little code
Without sacrificing readability

13. Did the clones catch up?
Yes , Sort of and Not quite
✓CoC ✓DRY
๏ Terse Expressiveness
✓ Instant Change ✓Integrated

14. What’s Missing?

16. ✓Object-Oriented
✓Elegant ✓General Purpose
✓ Multi-paradigm
Ruby ✓ Interpreted
is...
✓Reflective ✓Dynamic
✓ Garbage-collected

17. Without Ruby there would be no Rails
Ruby’s Dynamism is the key ingredient to
the Rails framework

18. Open Classes

19. Say we have an Array like:
[1,2,3,4,5]
and we want to sum of all of its elements

20. Let’s try something like:
[1,2,3,4,5].sum
Doh! Doesn’t work in Ruby … yet!

21. Ruby classes are open
We can teach the Array class a new behavior

22. Let’s try again!
Load the file containing the enhancements

23. Since Ruby classes are open...
You can statically enhance ANY class

24. Code as Data

25. When code can be manipulated as data
a whole world of possibilities opens
In Ruby you can evaluate the contents of a string

26. Code that spits out code

27. Ruby provides the eval family of methods for
runtime execution of code stored in strings
eval will evaluate any string

28. instance_eval can evaluate a string or a code block
in the context of the receiver

29. class_eval can evaluate a string or a code block
in the context of the class or module it is called on

30. Meta-programming

31. Meta-programming
...is about programs that write programs
...it’s a superb tool for building frameworks
...it’s the key ingredient for building domain-specific
languages

32. Ruby is a great language for meta-programming because
...is dynamic and reflexive
...open and malleable
...code is data, data is code
...clean unencumbered syntax
...programming event model

33. Rails
...uses the Ruby meta-programming
techniques to accomplish most of its “magic”
...uses meta-programming to bring the language
closer to the problem at hand
...is a domain-specific language for building
web applications

34. Singleton Class

35. All objects are open to modification
You can change a particular instance of a class
Ruby uses a proxy class known as the singleton class
Meta-class: The singleton for Class objects

36. With access to a class object’s meta-class we can
then use Ruby meta-programming techniques to
enhance the class

37. Rails relies heavily on Ruby’s ability to dynamically
enhance a class

38. The previous example could have also been
accomplished using the define_method

39. Accessing the singleton meta-class explicitly

40. Inheritance the
Ruby Way

41. Ruby Meta-programming techniques can be used to
write DSLs that use class methods to enhance
subclasses
Using the meta-class (singleton class) of the subclasses
Rails uses this technique in ActiveRecord

42. Let’s create a module with a “Base” class

43. Methods get added to our classes with simple declarations
Can you say DSL!

44. Rails uses this technique in ActiveRecord
Now our class has a new set of class methods

45. AOP the Ruby Way

46. With alias_method you can wrap an existing
method

48. Meta-Programming
Events

49. Included Hook

50. Ruby has a rich event model associated with
static and dynamic changes of the code

51. Method Missing

52. Ruby provides several hook methods that can
be used to created custom behaviors
method_missing
method_added
method_removed

53. Let’s implement the greeter example using
method_missing

55. There is more to Ruby than Rails!
Building a Framework? Give Ruby a try!
Build the language up towards the problem
Meta-programming is no longer just for the Lisp folks
The future? Powerful Java APIs exposed with Ruby DSLs

56. www.integrallis.com