From: Chris Lowis <chris.lowis@...>
Date: 2007-08-30T00:07:06+09:00
Subject: Help design a plugin system for a DSL

Hello,

I've been working through the "Creating DSLs with Ruby" tutorial here :
http://www.artima.com/rubycs/articles/ruby_as_dsl4.html

In my DSL I'd like users to be able to write code such as this :

myDSL do
  plugin "extra_functions"

  a_special_function
  basic_function
end

that is, to extend the range of callable functions using a kind of
"plugin" system.

The code that I have to implement this so far looks like :

class myDSL
  def initialize(name)
    @name = name
  end

  def self.load(name)
    dsl = new(name)
    dsl = dsl.instance_eval(File.read(name))
    dsl
  end

  def myDSL
    yield
  end

  def plugin(file)
    puts "loading plugin: #{file}"
    require "./plugins/#{file}.rb"
  end

  def basic_function()
    # a "core" method of the DSL.
  end
end

which is called using a short loader file like this :

require 'myDSL'
name = ARGV.shift
name = myDSL.load(name)

In the extra_functions.rb "plugin" I have a method definition

 def a_special_function
   # does something special
 end

I'm concerned that this is poor design, as the plugin functions will not
be very well encapsulated. Requiring two plugins that define the same
method names will cause the wrong methods to be called from the DSL.

What is a sensible way to solve this problem ? My thought is to use
method_missing to traverse plugin Classes in the order they were loaded,
looking for the function called. Is this a good way to proceed ? Any
other suggestions ?

Chris
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