The document discusses metaprogramming in Ruby. It provides an overview of Ruby's metaprogramming capabilities and how they enable dynamically modifying classes at runtime. This allows enhancing core classes like Array as well as building powerful frameworks like Ruby on Rails. The document also describes how Trellis, an experimental web framework created by the author, leverages metaprogramming techniques to provide component-based programming.

1. Metaprograming
in Ruby
Brian Sam-Bodden

2. What’s this all about?
‣ We’ll learn...
‣ mainly about Ruby’s Meta-programming power
‣ about a language doesn’t treat you like a moron
‣ what happens when dynamic meets object-oriented
‣ why Ruby is superb for building frameworks
‣ what happens when meta-programming
‣ ...meets someone with too much free-time

3. Ruby

4. Ruby Ideals:
Programming should be fun!
“I believe people want to express themselves when they
program.They don’t want to fight with the language.
Programming languages must feel natural to
programmers.”
Matz
The language should treat you like a responsible adult!

7. Ruby

8. Ruby
is...

9. Ruby
is...
✓Object-Oriented

10. Ruby
is...
✓Object-Oriented
✓General Purpose

11. Ruby
is...
✓Object-Oriented
✓ Interpreted
✓General Purpose

12. Ruby
is...
✓Object-Oriented
✓Dynamic
✓ Interpreted
✓General Purpose

13. Ruby
is...
✓Object-Oriented
✓Dynamic
✓ Interpreted
✓General Purpose
✓ Garbage-collected

14. Ruby
is...
✓Object-Oriented
✓Reflective ✓Dynamic
✓ Interpreted
✓General Purpose
✓ Garbage-collected

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

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

17. Without Ruby there would be
no Rails
Ruby’s meta-programing is the key
feature that makes Rails magical

18. Open
Classes

20. Say we have an Array like:

21. Say we have an Array like:
[1,2,3,4,5]

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

24. Let’s try something like:

25. Let’s try something like:
[1,2,3,4,5].sum

26. Let’s try something like:
[1,2,3,4,5].sum

27. Let’s try something like:
[1,2,3,4,5].sum
Rats! Doesn’t work in Ruby

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

30. Ruby classes are

31. Ruby classes are

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

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

35. Let’s try again!

36. Let’s try again!

37. Let’s try again!
All you need to do is load the file
containing the enhancements

38. Code as
Data
Bend at will your code should

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

40. Code that invokes code,
that invokes code

41. Code that invokes code,
that invokes code

42. Code that invokes code,
that invokes code
There you go, there you go

43. Ruby provides the eval family of
methods for runtime execution of
code stored in strings

44. Ruby provides the eval family of
methods for runtime execution of
code stored in strings

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

46. instance_eval can evaluate a
string or a code block

47. instance_eval can evaluate a
string or a code block

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

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

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

51. In Ruby we try to think about sending a message to
an object rather than calling a method on an object

52. In Ruby we try to think about sending a message to
an object rather than calling a method on an object
In simple cases (above) syntactic sugar hides it,
but we can use it in interesting ways...

53. Meta-
programming

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

55. Ruby’s is a great vehicle for
meta-programming because it is:
dynamic and reflexive
open and malleable
clean syntax
code is data, data is code
programming event model

56. ...uses meta-programming to bring
the language closer to the problem
...is a collection of domain-specific
languages (DSL) for building web
applications
Ruby on Rails

57. Singleton
Class

58. In Ruby you can enhance a
particular instance of a class
Ruby uses a proxy class known as
the singleton class
Meta-class:The singleton for
Class objects

59. class_eval indirectly uses the meta-class
adding a singleton method to the class

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

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

62. Shortcuts such as define_method
exist to make life easier

63. Accessing the singleton meta-class
explicitly

64. Child classes get enhanced with
class methods

65. Inheritance
the Ruby Way

66. Many Ruby DSLs create class
methods on subclasses using the
meta-class (singleton class)

67. Rails uses this technique in ActiveRecord

68. A module with a “Base” class

69. Methods get added to our classes with
simple declarations
Getting closer to a DSL!

70. Now our class has a new set of class
methods

71. Now our class has a new set of class
methods

72. AOP
the Ruby
Way

73. With alias_method you can wrap
an existing method...

74. ...effectively intercepting any calls
and injecting any desired behavior

75. ...effectively intercepting any calls
and injecting any desired behavior

76. Meta-Programming
Events

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

78. Method Missing

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

80. Ruby’s Markup Builder is an
example of what can be achieved
with method missing

81. Let’s implement the greeter example
using method_missing

82. greeter using method_missing

83. greeter using method_missing

84. Trellis...or what happens when you have too much time
in between consulting gigs
and your friends no longer call you

85. I was bored...
In the Ruby world I’ve worked with Rails and Merb
...thinking of web apps in terms of request & response
meanwhile... in the Java world I was working with
Tapestry and Wicket
...thinking about Pages, Components and Events

86. Started to hack to see what it would be to build
something like Tapestry/Wicket in Ruby
...actually it all started as an learning exercise
...but then I kept going
...somehow I ended building component-oriented
web framework in Ruby
...part of a conversation among friends about the need
for IoC and AOP frameworks for dynamic languages

87. You might be
asking yourself,
but why?

88. Why not?

89. My goals:
✓Magic like Rails; less code more action
✓ Small like Camping... a micro-framework
✓Components like Tapestry/Wicket
✓Clean HTML templates...
✓...but also programatic HTML generation
✓Non-managed components, no pooling

90. The big picture:
Trellis Application
Pages Components
has
has
{declares pages used
declares a home page
dispatches request to pages
} {
defines a template
provides event handlers
passes events to the
components
provides reusable logic
responds to events
can be stateless (tags)
or stateful

91. The simplest Trellis App, one Application and one Page
The Code
The Template

92. What did I use to put this thing together...
➡A boat load of meta-programming
➡Rack ...HTTP the Ruby way
➡Radius ...XML tags for the templates
➡Builder ...for building HTML/XML
➡Hpricot, REXML ...parsing searching HTML/XML
➡Paginator ...for duh, pagination
➡Parts of Rails ...so far ActiveSupport (Inflectors)

93. Let’s do some code spelunking on the Trellis codebase...
yes, there are some things that I’m not proud of but...
...this is pre-alpha, unreleased v0.000001;-)

94. Some of the example applications that I’ve put
together in order of complexity
➡hilo
➡guest_book
➡hangman
➡yui
➡stateful_counters
➡flickr
➡simple_blog
➡crud_components

95. What have
we learned?

96. There is more to Ruby than Rails!
Building a Framework?
Give Ruby a try!
Build the language up towards
the problem

97. Rack: http://rack.rubyforge.org
Radius: http://radius.rubyforge.org
Hpricot: http://code.whytheluckystiff.net/hpricot
REXML: http://www.germane-software.com/software/rexml
Builder: http://builder.rubyforge.org
Paginator: http://paginator.rubyforge.org
YUI (Yahoo! UI Library): http://open.yahoo.com/yui
Trellis (Not ReleasedYet)
Trellis @ RubyForge: http://rubyforge.org/projects/trellis
Trellis Website: http://www.trellisframework.org
Resources

98. www.integrallis.com