Refinements ↑

Due to Ruby's open classes you can redefine or add functionality to existing classes. This is called a “monkey patch”. Unfortunately the scope of such changes is global. All users of the monkey-patched class see the same changes. This can cause unintended side-effects or breakage of programs.

Refinements are designed to reduce the impact of monkey patching on other users of the monkey-patched class. Refinements provide a way to extend a class locally. Refinements can modify both classes and modules.

Here is a basic refinement:

class C
  def foo
    puts "C#foo"
  end
end

module M
  refine C do
    def foo
      puts "C#foo in M"
    end
  end
end

First, a class C is defined. Next a refinement for C is created using Module#refine.

Module#refine creates an anonymous module that contains the changes or refinements to the class (C in the example). self in the refine block is this anonymous module similar to Module#module_eval.

Activate the refinement with using:

using M

c = C.new

c.foo # prints "C#foo in M"

Scope ↑

You may activate refinements at top-level, and inside classes and modules. You may not activate refinements in method scope. Refinements are activated until the end of the current class or module definition, or until the end of the current file if used at the top-level.

You may activate refinements in a string passed to Kernel#eval. Refinements are active until the end of the eval string.

Refinements are lexical in scope. Refinements are only active within a scope after the call to using. Any code before the using statement will not have the refinement activated.

When control is transferred outside the scope, the refinement is deactivated. This means that if you require or load a file or call a method that is defined outside the current scope the refinement will be deactivated:

class C
end

module M
  refine C do
    def foo
      puts "C#foo in M"
    end
  end
end

def call_foo(x)
  x.foo
end

using M

x = C.new
x.foo       # prints "C#foo in M"
call_foo(x) #=> raises NoMethodError

If a method is defined in a scope where a refinement is active, the refinement will be active when the method is called. This example spans multiple files:

c.rb:

class C
end

m.rb:

require "c"

module M
  refine C do
    def foo
      puts "C#foo in M"
    end
  end
end

m_user.rb:

require "m"

using M

class MUser
  def call_foo(x)
    x.foo
  end
end

main.rb:

require "m_user"

x = C.new
m_user = MUser.new
m_user.call_foo(x) # prints "C#foo in M"
x.foo              #=> raises NoMethodError

Since the refinement M is active in m_user.rb where MUser#call_foo is defined it is also active when main.rb calls call_foo.

Since using is a method, refinements are only active when it is called. Here are examples of where a refinement M is and is not active.

In a file:

# not activated here
using M
# activated here
class Foo
  # activated here
  def foo
    # activated here
  end
  # activated here
end
# activated here

In a class:

# not activated here
class Foo
  # not activated here
  def foo
    # not activated here
  end
  using M
  # activated here
  def bar
    # activated here
  end
  # activated here
end
# not activated here

Note that the refinements in M are not activated automatically if the class Foo is reopened later.

In eval:

# not activated here
eval <<EOF
  # not activated here
  using M
  # activated here
EOF
# not activated here

When not evaluated:

# not activated here
if false
  using M
end
# not activated here

When defining multiple refinements in the same module inside multiple refine blocks, all refinements from the same module are active when a refined method (any of the to_json methods from the example below) is called:

module ToJSON
  refine Integer do
    def to_json
      to_s
    end
  end

  refine Array do
    def to_json
      "[" + map { |i| i.to_json }.join(",") + "]"
    end
  end

  refine Hash do
    def to_json
      "{" + map { |k, v| k.to_s.dump + ":" + v.to_json }.join(",") + "}"
    end
  end
end

using ToJSON

p [{1=>2}, {3=>4}].to_json # prints "[{\"1\":2},{\"3\":4}]"

Method Lookup ↑

When looking up a method for an instance of class C Ruby checks:

If no method was found at any point this repeats with the superclass of C.

Note that methods in a subclass have priority over refinements in a superclass. For example, if the method / is defined in a refinement for Numeric 1 / 2 invokes the original Integer#/ because Integer is a subclass of Numeric and is searched before the refinements for the superclass Numeric. Since the method / is also present in child Integer, the method lookup does not move up to the superclass.

However, if a method foo is defined on Numeric in a refinement, 1.foo invokes that method since foo does not exist on Integer.

super ↑

When super is invoked method lookup checks:

Note that super in a method of a refinement invokes the method in the refined class even if there is another refinement which has been activated in the same context. This is only true for super in a method of a refinement, it does not apply to super in a method in a module that is included in a refinement.

Methods Introspection ↑

When using introspection methods such as Kernel#method or Kernel#methods refinements are not honored.

This behavior may be changed in the future.

Refinement inheritance by Module#include ↑

When a module X is included into a module Y, Y inherits refinements from X.

For example, C inherits refinements from A and B in the following code:

module A
  refine X do ... end
  refine Y do ... end
end
module B
  refine Z do ... end
end
module C
  include A
  include B
end

using C
# Refinements in A and B are activated here.

Refinements in descendants have higher precedence than those of ancestors.

Further Reading ↑

See bugs.ruby-lang.org/projects/ruby-master/wiki/RefinementsSpec for the current specification for implementing refinements. The specification also contains more details.