lib_rubygems_file_system.rb

module Gem
  class FileSystem

    attr_reader :path

    def initialize(*parts)
      @path = Path.new(*parts)
    end

    def bin
      path.add 'bin'
    end

    def cache
      path.add 'cache'
    end

    def specifications
      path.add 'specifications'
    end

    def gems
      path.add 'gems'
    end

    def doc
      path.add 'doc'
    end

    class Path
      def initialize(*parts)
        @path = File.join(*parts)
      end

      def readable?
        File.readable?(@path)
      end

      def writable?
        File.writable?(@path)
      end

      def path
        @path.dup
      end
      
      alias to_s path
      alias to_str path

      def add(filename)
        self.class.new(@path, filename)
      end
    end
  end
end

if __FILE__ == $0
  module Gem
    def self.os
      FileSystem.new('/tmp')
    end

    def self.user
      FileSystem.new(File.expand_path("~/.gems"))
    end
  end

  p Gem.os.path #==> /tmp
  p Gem.user.path #==> ~/.gems
  
  path = Gem.os.gems.add("some-gem-0.1.2.gem")

  p path #==> /tmp/gems/some-gem-0.1.2.gem
  require 'fileutils'
  FileUtils.mkdir_p Gem.os.gems

  p path.writable? #==> false (file doesn't exist)
  FileUtils.touch path
  p path.writable? #==> true
  p path.readable? #==> true
end