[ruby-core:33322] [Ruby 1.9-Feature#4085][Open] Refinements and nested methods

Feature #4085: Refinements and nested methods
http://redmine.ruby-lang.org/issues/show/4085

Author: Shugo Maeda
Status: Open, Priority: Normal
Category: core, Target version: 1.9.3

As I said at RubyConf 2010, I'd like to propose a new features called
"Refinements."

Refinements are similar to Classboxes.  However, Refinements doesn't
support local rebinding as mentioned later.  In this sense,
Refinements might be more similar to selector namespaces, but I'm not
sure because I have never seen any implementation of selector
namespaces.

In Refinements, a Ruby module is used as a namespace (or classbox) for
class extensions.  Such class extensions are called refinements.  For
example, the following module refines Fixnum.

  module MathN
    refine Fixnum do
      def /(other) quo(other) end
    end
  end

Module#refine(klass) takes one argument, which is a class to be
extended.  Module#refine also takes a block, where additional or
overriding methods of klass can be defined.  In this example, MathN
refines Fixnum so that 1 / 2 returns a rational number (1/2) instead
of an integer 0.

This refinement can be enabled by the method using.

  class Foo
    using MathN

    def foo
      p 1 / 2
    end
  end

  f =3D Foo.new
  f.foo #=3D> (1/2)
  p 1 / 2

In this example, the refinement in MathN is enabled in the definition
of Foo.  The effective scope of the refinement is the innermost class,
module, or method where using is called; however the refinement is not
enabled before the call of using.  If there is no such class, module,
or method, then the effective scope is the file where using is called.
Note that refinements are pseudo-lexically scoped.  For example,
foo.baz prints not "FooExt#bar" but "Foo#bar" in the following code:

  class Foo
    def bar
      puts "Foo#bar"
    end

    def baz
      bar
    end
  end

  module FooExt
    refine Foo do
      def bar
        puts "FooExt#bar"
      end
    end
  end

  module Quux
    using FooExt

    foo =3D Foo.new
    foo.bar  # =3D> FooExt#bar
    foo.baz  # =3D> Foo#bar
  end

Refinements are also enabled in reopened definitions of classes using
refinements and definitions of their subclasses, so they are
*pseudo*-lexically scoped.

  class Foo
    using MathN
  end

  class Foo
    # MathN is enabled in a reopened definition.
    p 1 / 2  #=3D> (1/2)
  end

  class Bar < Foo
    # MathN is enabled in a subclass definition.
    p 1 / 2  #=3D> (1/2)
  end

If a module or class is using refinements, they are enabled in
module_eval, class_eval, and instance_eval if the receiver is the
class or module, or an instance of the class.

  module A
    using MathN
  end
  class B
    using MathN
  end
  MathN.module_eval do
    p 1 / 2  #=3D> (1/2)
  end
  A.module_eval do
    p 1 / 2  #=3D> (1/2)
  end
  B.class_eval do
    p 1 / 2  #=3D> (1/2)
  end
  B.new.instance_eval do
    p 1 / 2  #=3D> (1/2)
  end

Besides refinements, I'd like to propose new behavior of nested methods.
Currently, the scope of a nested method is not closed in the outer method=
.

  def foo
    def bar
      puts "bar"
    end
    bar
  end
  foo  #=3D> bar
  bar  #=3D> bar

In Ruby, there are no functions, but only methods.  So there are no
right places where nested methods are defined.  However, if
refinements are introduced, a refinement enabled only in the outer
method would be the right place.  For example, the above code is
almost equivalent to the following code:

  def foo
    klass =3D self.class
    m =3D Module.new {
      refine klass do
        def bar
          puts "bar"
        end
      end
    }
    using m
    bar
  end
  foo  #=3D> bar
  bar  #=3D> NoMethodError

The attached patch is based on SVN trunk r29837.


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