= 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 <code>m_user.rb</code> where
<code>MUser#call_foo</code> is defined it is also active when
<code>main.rb</code> 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

Method lookup in Ruby is based on the ancestor chain. You can see the
ancestor chain for any object in Ruby by doing:

  object.singleton_class.ancestors
  # or, if the object does not support a singleton class:
  object.class.ancestors

The ancestor chain is constructed as follows:

* Subclasses are before superclasses in the ancestor chain
* Prepended modules are before the class they prepend in the ancestor
  chain, in reverse order in which they were prepended.
* Included modules are after the class they are included in in the
  ancestor chain, in reverse order in which they were included.

When looking up a method for an object, Ruby goes through each ancestor:

* If the class/module has been refined, Ruby will consider the refinements
  activated at the point the method was called, in reverse order of
  activation.
* Otherwise, Ruby will check the methods of the class/module itself.

If no method was found at either point this repeats with the next
ancestor.

Note that methods in a earlier ancestor have priority over refinements in a
later ancestor.  For example, if the method <code>/</code> is defined in a
refinement for Numeric <code>1 / 2</code> invokes the original Integer#/
because Integer is a comes before Numeric in the ancestor chain.  However,
if a method +foo+ is defined on Numeric in a refinement, <code>1.foo</code>
invokes that method since +foo+ does not exist on Integer.

== +super+

When +super+ is invoked, method lookup starts:

* If the method is in a refinement:
  * At the next active refinement in scope at the +super+ call site,
    excluding the current refinement, if such a refinement exists
  * Otherwise, at the refined class or module
* Otherwise, at the next ancestor

Method lookup then proceeds as described in the Method Lookup section
above.

Refinements activated at the call site of a refinement method do not
affect +super+ inside that method. Only refinements activated at the
point +super+ was called affect method lookup for that +super+ call.
You cannot use refinements to insert into the middle of a method
lookup chain, only to insert at the start of a method lookup chain,
unless you control the +super+ call sites.

Note that if you refine a module, the refinement method can call +super+
to call the method in the module, but the method in the module cannot
call +super+ to continue the method lookup process to further ancestors.

== 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 https://github.com/ruby/ruby/wiki/Refinements-Spec for the
current specification for implementing refinements.  The specification also
contains more details.
