Metaprogramming in Ruby. A lecture for #pivorak summer Ruby course.

2. Volodymyr Byno
github.com/vbyno
volodymyr.byno@gmail.com
domain lead engineer at

3. Metaprogramming

4. What?
Writing code that writes code
Writing code that manipulates
language constructs at runtime

5. • write own DSL
• remove duplications in the code
• enhance basic Ruby
Why?

6. Object Model

7. Class is an object, Object is a class
class is a keyword, class is a method
method is a method, Method is a class
Object Model

8. Class is an Object

9. Module is an Object

10. Module Hooks

11. Class vs Module

12. Statement vs Method
method
statement

13. Singleton Method/Class
class A singleton class

14. Lookup

15. new?

16. Methods
unbound method

17. What does
«everything»
mean?
Everything
is an
object

18. Open class
NOT Objects
• comments (# comment, =begin =end)
• punctuation marks (, ; ( .)
• statements (if while for defined? yield super)
• operators (&. || && =)
• blocks (do..end, {..})
• methods (not method objects)
Methods are a fundamental part of Ruby's syntax, but they are not values that Ruby
programs can operate on. That is, Ruby's methods are not objects in the way that strings,
numbers, and arrays are. It is possible, however, to obtain a Method object that represents a
given method, and we can invoke methods indirectly through Method objects.
(c) The Ruby Programming Language. D. Flanagan; Y. Matsumoto
operators table

19. Open Class

20. Open Class

22. method already exists
Monkeypatching

23. Test: Ruby or Rails?
ActiveSupport
3 Ways to Monkey-Patch

24. why is nobody using refinements
Refinements
• Applicable only for classes
• Works in the current lexical scope
(module/class definition, EOF)

25. Dynamic
Method

26. Dynamic Dispatch

27. Dynamic Method
Comment it out!

28. Remove Method

29. Ghost
Method

30. Ghost Method

31. respond_to?
Rules:
• Redefine respond_to_missing?(name)
• Call super
respond_to? vs respond_to_missing?

32. Dynamic vs Ghost
use Dynamic Methods if you can
use Ghost Methods if you have to

33. Alias
Method

34. Alias Method
alias vs alias_method

35. Block
Proc
Lamda

36. Block
Reccomendation: 1 line - curly braces, multiple
lines - do..end

37. & operator
yield is not enough:
• pass block to another method
• convert block to Proc

38. Proc

39. LambdaProc
returns fom the scope
where it is defined returns from itself
checks the aritydoes not check the arity

40. Lambdas vs Methods
lambda is evaluated in the scope it is defined,
method is evaluated in the scope of object

41. Lexical
Scope

42. Lexical Scope
Flat
Scope
Scope
Gates
scope in ruby

43. *eval methods
*eval vs *exec methods

44. def current_user
def user_signed_in?
def authenticate_user!
def user_session

45. Variable Setter/Getter

46. Statements / Methods
• BasicObject#__send__
• BasicObject#method_missing
• Kernel#send
• Kernel#public_send
• Kernel#method
• Kernel#class
• Kernel#define_singleton_method
• Kernel#block_given?
• Kernel#respond_to_missing?
• Kernel#respond_to?
• Kernel#eval
• yield
• defined?
• alias
• super
• class
• module
• def
• Module#module_eval
• Module#define_method
• Module#alias_method
• Module#included
• Module#refine
• Module#using

47. Summary
• Avoid metaprogramming if it is possible!
• Be smart in monkeypatching
• Statements are not methods
• Prefer lambdas over procs
• Use super in method_missing
• Keep scope in mind
• Comment out!

48. 1. Metaprogramming Ruby 2.0
(book)
2. The Ruby Programming Language
(book, Reflection and Metaprogramming section)
3. Ruby Under a Microscope
(Objects and Classes, Constants Lookup)
4. Codebases lessons
Investigate Further