ruby--ruby/doc/syntax/refinements.rdoc

= 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.

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.  Refinements only modify classes, not modules so the argument
must be a class.

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 only activate refinements at top-level, not inside any class, module
or method scope.  You may activate refinements in a string passed to
Kernel#eval that is evaluated at top-level.  Refinements are active until the
end of the file or the end of the eval string, respectively.

Refinements are lexical in scope.  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 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 a refine block
all refinements from the same module are active when a refined method 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}]"

You may also activate refinements in a class or module definition, in which
case the refinements are activated from the point where using is called to
the end of the class or module definition:

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

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

== Method Lookup

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

* If refinements are active for +C+, in the reverse order they were activated:
  * The prepended modules from the refinement for +C+
  * The refinement for +C+
  * The included modules from the refinement for +C+
* The prepended modules of +C+
* +C+
* The included modules of +C+

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 <code>/</code> is defined in a
refinement for Integer <code>1 / 2</code> invokes the original Fixnum#/
because Fixnum is a subclass of Integer and is searched before the refinements
for the superclass Integer.

If a method +foo+ is defined on Integer in a refinement, <code>1.foo</code>
invokes that method since +foo+ does not exist on Fixnum.

== +super+

When +super+ is invoked method lookup checks:

* The included modules of the current class.  Note that the current class may
  be a refinement.
* If the current class is a refinement, the method lookup proceeds as in the
  Method Lookup section above.
* If the current class has a direct superclass, the method proceeds as in the
  Method Lookup section above using the superclass.

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.

== Indirect Method Calls

When using indirect method access such as Kernel#send, Kernel#method or
Kernel#respond_to? refinements are not honored for the caller context during
method lookup.

This behavior may be changed in the future.

== Refinements and module inclusion

Refinements are inherited by module inclusion.  That is, using activates all
refinements in the ancestors of the specified module.  Refinements in a
descendant have priority over refinements in an ancestor.

== Further Reading

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