rulebook

Allows you to define a set of 'rules' or dynamic methods to apply to a class.

Install

> gem update --system
> gem install rulebook

Simple Example

require 'rulebook'

class User
  follows_the_rules!

  def initialize(name)
    @name = name
  end
  
  def_rule(/say_(.+)/) do |what_to_say|
    puts "#{@name} says '#{what_to_say.gsub(/_/, ' ')}'"
  end
end

User.new('Ryan').say_hello_world # => Ryan says 'hello world'

Rulebook::Rule

An instance of Rulebook::Rule simply holds two attributes, @regexp and @block:

rule = Rulebook::Rule.new(/^regexp$/) { puts "Hello" } # => #<Rule:0x0000>
rule.regexp # => /^regexp$/
rule.block  # => #<Proc:0x0000>

It's a simple as that.
There are a couple of helper methods as well:

rule.match('regexp') # => #<MatchData "regexp">
rule['regexp'] # => #<MatchData "regexp">
rule.matches?('regexp') # => true

Rulebook

An instance of Rulebook holds a single attribute, @rules, which is a simple Array of Rulebook::Rule instances:

rulebook = Rulebook.new
rule = Rulebook::Rule.new(/^regexp$/) { "Hello" }

rulebook.rules << rule
rulebook.rules # => [#<Rule:0x0000>]

Rulebook also contains a few helper methods:

rulebook.add(/testing/) { "testing" }
rulebook.rules # => [#<Rule:0x0000>, #<Rule:0x0eb6>]

rulebook.rules_that_match_against("testing") # => [#<Rule:0x0eb6>]
rulebook.match("testing") # => [#<Rule:0x0eb6>]
rulebook["testing"] # => [#<Rule:0x0eb6>]

follows_the_rules!

When you require 'rulebook', we automatically define a method within Module named follows_the_rules!.
This defines two new class methods; def_rule and def_class_rule.

class User
end

p User.methods.grep(/follows|def_/) # => ["follows_the_rules!"]

class User
  follows_the_rules!
end

p User.methods.grep(/follows|def_/) # => ["follows_the_rules!", "def_rule", "def_class_rule"]

This ensures that only the Objects that you want to follow rules will have these methods available.
You may call follows_the_rules! Outside of the class as well:

class User
end

User.follows_the_rules!

p User.methods.grep(/follows|def_/) # => ["follows_the_rules!", "def_rule", "def_class_rule"]

def_rule

When you define a rule on a class using def_rule the first time, we will add an instance variable to the class named @rulebook that contains an instance of Rulebook. We then define the rule within that rulebook instance.

Now, when you call a method that does not yet exist in your class, we check the rulebook to see if the method called matches any of the defined rules.

If it doesn't we just go ahead and let method_missing do it's thing (A.K.A. throw a NoMethodError), otherwise we find the first rule that matches the method, take the block associated with that rule and define the method. The next step is to simply call the method.

Take the following for example (although a bit useless, it describes the usage pretty clearly):

class User
  follows_the_rules!
  
  def_rule(/foo(?:bar)?/) { "FOOBAR!" }
end

user = User.new
p user.methods - Object.new.methods # => []

As you can see, no instance methods are defined within the User object.
So when I call a method that matches the Regexp within the defined rule, it will actually define the instance method on the User object:

p user.foo # => "FOOBAR"
p user.methods - Object.new.methods # => [:foo]
p user.foobar # => "FOOBAR"
p user.methods - Object.new.methods # => [:foo, :foobar]

def_class_rule

def_class_rule works in much the same way as def_rule, except on the class.
Here's a small example if def_class_rule in action:

class User
  follows_the_rules!
  attr :role
  
  def_class_rule(/new_(admin|moderator)/) do |role|
    instance = new
    instance.role = role.to_sym
    instance
  end
  
  def initialize
    @role = :user
  end
  
  def role=(new_role)
    @role = new_role.to_sym
  end
end

User.new.role # => :user
User.new_moderator.role # => :moderator
User.new_admin.role # => :admin

Captures

As seen in the last example, when using def_rule or def_class_rule, you can pass the captures from the match to the block given.
Here is a nice example of that usage:

class User
  follows_the_rules!
  
  def_class_rule(/new_(admin|moderator|user)/) do |role|
    new.instance_eval { @role = role.to_sym; self }
  end
  
  def_rule(/is_(admin|moderator|user)\?/) do |role|
    @role == role.to_sym
  end
  
  def initialize
    @role = :user # default
  end
end

user = User.new_admin
user.is_user? # => false
user.is_admin? # => true

Arguments

If you would like to add arguments to the rule, simply tack them on the end of the block arguments. Just like you would define_method:

class Book
  def_rule(/^page_(\d+)$/) do |page_number, line_number=1|
    puts "This is the contents of page #{page_number}, line #{line_number}."
  end
end

book = Book.new
book.page_15 # => This is the contents of page 15, line 1.
book.page_15(3) # => This is the contents of page 15, line 3.

Method Missing

Don't use it.
Say you have the following class:

class Foo
  def bar
    "foobar"
  end
  def baz(str_to_append="!!!")
    "foobaz#{str_to_append}"
  end
end

And another class that wants to delegate missing methods to an instance of Foo:

class DelegateMeToFoo
  def initialize
    @foo = Foo.new
  end
  
  def method_missing(meth, *args, &blk)
    @foo.send(meth, *args, &blk)
  end
end

dmtf = DelegateMeToFoo.new
p dmtf.bar # => "foobar"
p dmtf.baz("??") # => "foobaz??"

This could easily be converted to use Rulebook:

class DelegateMeToFoo
  def initialize
    @foo = Foo.new
  end
  
  def_rule(/(.*)/) do |meth, *args, &blk|
    @foo.send(meth, *args, &blk)
  end
end

dmtf = DelegateMeToFoo.new
p dmtf.bar # => "foobar"
p dmtf.baz("??") # => "foobaz??"

The advantages of doing so aren't apparent at first, but you must realize that every single method (in this case, since no instance methods are defined within Foo) that is sent to your Foo instance must check all of it's defined methods for the method you are calling. When it isn't found, it goes to the parent class to see if the instance method is defined there. It continues up the chain in that fashion until it hits the top-level class. It then loops back around to your Foo instance to check for method_missing which is defined in the example above. Since it found method_missing, it calls it.

Ruby must do this every single time.

With Rulebook, when you call dmtf.bar the first time, we actually define the bar method on your DelegateMeToFoo instance, then calls it.
Now the next time dmtf.bar is called, it no longer has to do the method_missing dance and search the inheritance chain for the bar method. Ruby finds the newly defined method and runs it.

Note on Patches/Pull Requests

• Fork the project.
• Make your feature addition or bug fix.
• Add tests for it. This is important so I don't break it in a future version unintentionally.
• Commit, do not mess with rakefile, version, or history. (if you want to have your own version, that is fine but bump version in a commit by itself I can ignore when I pull)
• Send me a pull request. Bonus points for topic branches.

Copyright

Copyright (c) 2010-2011 Ryan Lewis. See LICENSE for details.