Metaprogramming code-that-writes-code

Code That Writes Code
metaprogramming

The Boss’ Challenge
1 class Person
2 include CheckedAttributes
3 attr_checked :age do |v|
4 v >= 18
5 end
6 end
7
8 me = Person.new
9 me.age = 39 # OK
10 me.age = 12 # Exception

Requirements
• Applies when class includes CheckedAttributs
Module
• Class Macro: attr_checked
• First Argument as attribute name
• Second Argument can accept a block as
validator of attribute

Bill steps
• Kernel method “add_checked_attribute” by
Kernel#eval
• Refactoring: remove Kernel#eval
• Block attribute validator
• Replace method to class macro for all classes
• A Module with attr_checked method, hooks to
class

Kernel#eval
• Spells: String of Code.
• The most Strings of Code feature some kind of
string substitution.
• Ex: ……
1 array = [10, 20]
2 element = 30
3 eval("array << element") # => [10, 20, 30]

eval(string [, binding [, ﬁlename [,lineno]]]) → obj
• Evaluates the Ruby expression(s) in String
• If binding is given, which must be a Binding
object, the evaluation is performed in its context.
• If the optional ﬁlename and lineno parameters are
present, they will be used when reporting syntax
errors

Ex 1. REST Client
• Its simple HTTP library for sending HTTP request.
1 #Originally, It needs to declare 4 methods
2 def get(path, *args, &b)
3 r[path].get(*args, &b)
4 end
5
6 def set(path, *args, &b)
7 r[path].set(*args, &b)
8 end
9
10 def put(path, *args, &b)
11 r[path].put(*args, &b)
12 end
13
14 def delete(path, *args, &b)
15 r[path].delete(*args, &b)
16 end

Ex 1. REST Client
• Its simple HTTP library for sending HTTP request.
1 # Using "String Of Code"
2 POSSIBLE_VERBS = ['get', 'put', 'post',
3 'delete']
4 POSSIBLE_VERBS.each do |m|
5 #syntax of heredoc
6 eval <<-end_eval
7 def #{m}(path, *args, &b)
8 r[path].#{m}(*args, &b)
9 end
10 end_eval
11 end

Here Document
1 hdoc = <<-hdoc_end
2 This is here document
3 hdoc_end
4
5 puts "Show:#{hdoc}"
6 #=> Show: This is here document
1 puts("Show:" + <<-hdoc_end)
3 hdoc_end
4 #=> Show: This is here document
1 puts("Show:" + <<-hdoc_end.gsub(/^s+/, ''))
3 hdoc_end
4 #=> Show:This is here document

Binding objects
• Objects of class Binding encapsulate the
execution context at some particular place in the
code and retain this context for future use.
• Kernel#binding returns the binding object.
1 class MyClass
2 def my_method
3 @x = 1
4 binding
5 end
6 end
7 b = MyClass.new.my_method
8 eval "p @x", b

Strings of Code vs. Blocks
• instance_eval and class_eval also accept String
as argument.
• Which one should you choose?
Avoid String of Code whenever you have an alternative.

The Trouble with eval()
• Difﬁcult to read and modify.
• Won’t report syntax error until the string is
evaluate.
• Security issue: Code injection!

Code Injection
1 def explore_array(method_name)
2 code = "[1,2,3,4].#{method_name}"
3 puts "Evaluating: #{code}"
4 eval code
5 end
6
7 orga_input = "find_index(2)"
8 p explore_array(orga_input) # => 1
9
10 otis_input = "map!(&:next)"
11 p explore_array(otis_input) # => [2, 3, 4, 5]
12
13 sunkai_input = "object_id; Dir.glob('*')"
14 p explore_array(sunkai_input) # => Some
15 sensitive information!!!

Defending
• Dynamic Method & Dynamic Dispatch
• Tainted Objects: object#tainted?
• Safe Levels: $SAFE = 0 ~ 3
• Clean Room: proc()

Step 1 - Checked Attributes
• Kernel method: add_checked_attribute.
• Raise exception if value of attribute is nil or false.
• Source Code

Step 2 - Remove eval()
• class name is variable: Open Class by class_eval
• method name is variable: Dynamic Methods
• instance variable name is variable:
instance_variable_get/instance_variable_set
• Source Code

Step 3 - Block Validator
• Attribute validation according to return value of
block.
• Source Code

Step 4 - Class Marco
• Class marco: attr_checked
• A class marco for all classes: Instance method of
Class/Module.
• Source Code

Hook Methods
• The object model is an eventful place.
• Class inherited
• Module included, prepended and extended
• Instance method added, removed and undeﬁned
• Singleton method added, removed and undeﬁned

Quiz
• Q: undef_method Vs remove_method?
• undef_method: added an exception to that
method.
• remove_method: remove the method at current
class, receiver will try to ﬁnd it from ancestor.

Quiz
• Q: which one
method is
useless?
1 #Which one method_hook is useless?
2 class Person
3 def self.singleton_method_added(m)
4 p "M1: #{m}"
5 end
6 def self.method_added(m)
7 p "M2: #{m}"
8 end
9 def singleton_method_added(m)
10 p "M3: #{m}"
11 end
12 def method_added(m)
13 p "M4: #{m}"
14 end
15 def hello
16 end
17 def self.hehe
18 end
19 end
20
21 person = Person.new
22 def person.yo
23 end
• method_added

Quiz
• Q: Other approaches for hook method?
• Override method and call super in the end of
method
• Around Alias

Step 5 - Final step
• Declare an CheckedAttributes Module
• Include module as class method: ClassMehtods-
plus-hook.
• Source Code

Epilogue
• Are you smart enough to forget what you have
learned?
• There’s no such thing as metaprogramming. It’s
just programming all the way through

Shuhari (Kanji: 守破離 Hiragana: ゅはり)
• Shu(protect, obey): learning fundamentals, techniques,
heuristics, proverbs.
• Ha(detach, digress): detachment from the illusions of
self.
• Ri(leave, separate): there are no techniques or proverbs,
all moves are natural, becoming one with spirit alone
without clinging to forms; transcending the physical

Not Only in “M” world
• Design Patterns
• TDD
• Scrum
• More…

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