From: Paul Brannan <pbrannan@...>
Date: 2005-10-20T02:16:31+09:00
Subject: namespaces implemented in ruby

Rich and Nathaniel and I were standing around at the conference this
weekend talking about Nathaniel's idea for implementing namespaces in
pure ruby, just so ruby users could play around with them until we get
them in ruby 2.0 (or maybe 1.9.1).  Here's my first pass at an
implementation of that idea.  It uses a slightly modified version of
Florian Gross's Binding.of_caller method.

The idea is pretty simple; each scope contains two local variables
called __ns (which namespace is currently being modified) and __use
(which namespaces are searched during method invocation).  When a method
is added, a proxy method is created that does the method searching.  No
method caching is done, but that could be implemented.  The global
namespace is known as :__global.

The syntax is something like this:

  namespace :namespace do
    # methods created in this scope will be added to :namespace
  end

  use :namespace do
    # methods called in this scope will be searched in :namespace first,
    # then :__global.
  end

Hopefully someone can take this and extend it to properly handle
singleton methods and inheritance.

There is also one bug that I don't know how to address:

  class Foo; def foo; puts "global namespace"; end; end
  namespace :n { class Foo; def foo; puts "namespace n"; end; end }
  p = proc { Foo.new.foo }
  use :n { p.call }

This should print "global namespace" but currently prints "namespace n".
The problem is that both the outer scope and the inner scope share the
same __use variable, and I don't know any good way to actually make the
variable truly block-local.  I'm all ears if anyone knows how to fix
this.

Paul


---

require 'thread'

def caller_binding(&block)
  old_critical = Thread.critical
  Thread.critical = true

  count = 0
  result, error = callcc do |cc|
    tracer = lambda do |*args|
      type, context = args[0], args[4]
      if type == "return"
        count += 1
        if count == 2
          set_trace_func(nil)
          cc.call(eval("binding", context), nil)
        end
      elsif type != "line"
        set_trace_func(nil)
        cc.call(nil, lambda { raise(ArgumentError, "unable to get binding" ) })
      end
    end
    set_trace_func(tracer)
    return nil
  end

  Thread.critical = false
  error.call if error
  yield result
end

def method_table
  h = {}
  methods.each do |name|
    h[name] = method(name)
  end
  return h
end

def namespace(ns)
  caller_binding do |b|
    old_ns = eval("defined?(__ns) ? __ns : nil", b)
    eval("__ns = #{ns.inspect}", b)
    eval("__mt ||= []; __mt << method_table", b)
    begin
      use(ns) do
        yield
      end
    ensure
      if old_ns then
        eval("__ns = #{old_ns.inspect}", b)
      else
        eval("__ns = nil", b)
      end

      eval("__mt.pop", b)
    end
  end
end

def use(ns)
  caller_binding do |b|
    eval("__use ||= []; __use << #{ns.inspect}", b)
    begin
      yield
    ensure
      eval("__use.pop", b)
    end
  end
end

class Object
  # TODO: need singleton_method_added too
  def self.method_added(name)
    caller_binding do |b|
      in_ma = eval("defined?(__in_ma) ? true : false", b)
      return if in_ma

      ns = eval("defined?(__ns) ? __ns : nil", b)
      ns ||= :__global
      mt = class << self; @mt ||= {}; end
      mt[ns] = instance_method(name)

      undef_method name
      self.class_eval <<-END
        __in_ma = true
        def #{name}(*args, &block)
          caller_binding do |b|
            use = eval("defined?(__use) ? __use : nil", b)
            use ||= []
            use = use.reverse + [:__global]
            use.each do |ns|
              # TODO: also search up the inheritance hierarchy
              mt = class << self.class; @mt ||= {}; end
              u = mt[ns]
              next if not u
              m = u.bind(self)
              return m.call(*args, &block)
            end
            raise NoMethodError, "undefined method `#{name}' for TODO"
          end
        end
      END
    end
  end
end

class Foo
  namespace :n do
    def foo; 42; end
  end

  namespace :m do
    def foo; 10; end
  end

  def foo; 1; end
end

def bar
  p Foo.new.foo
end

use :n do
  p Foo.new.foo #=> 42 (namespace n)

  use :m do
    p Foo.new.foo #=> 10 (namespace m)
  end

  use :z do
    p Foo.new.foo #=> 42 (namespace n)
  end

  bar #=> 1 (global namespace)
end

use :z do
  p Foo.new.foo #=> 1 (global namespace)
end

p Foo.new.foo #=> 1 (global namespace)