From: Gavin Kistner <gavin@...>
Date: 2005-05-26T15:36:20+09:00
Subject: Invoking a Class block on an Instance

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For "fun" I started writing my own finite state machine system  
tonight. (Or rather, what my idea of one is.)

I wanted to be able to subclass the FSM with specific classes that  
define the states; each state is a combination of a unique object,  
and a block that handles that state. Each time the state machine is  
fed a new piece of input, it yields that information to the block for  
the current state, and uses the return value as the next state.

I need a way to aggregate data as I crawl through the states, so I  
thought I'd use instance variables and call some convenience instance  
methods on my FSM subclass.

Unfortunately, I can't figure out how to get an instance of a class  
to invoke a block stored in its parent class, using the scope of the  
instance for the block. In JS this would be easy, but I don't yet  
grok how to have Ruby run a set of code on any arbitrary scope (while  
passing parameters to that code).

Code follows. I've pared down some of the (limited) bounds checks for  
cleanliness, and removed most of the (undone) states. The problem  
occurs when the :tag_start state tries to call the (instance)  
"start_element" method, and complains that no such method exists on  
the XMLParser class.

/Users/gavinkistner/Desktop/tmp.rb:61: undefined method  
`start_element' for XMLParser:Class (NoMethodError)

class FiniteStateMachine
   def self.states
   @states ||= {}
   end
   def self.add_state( start_state, &block )
   states[ start_state ] = block
   end

   def initialize( current_state=nil )
   @state = current_state
   end
   def transition( trigger )
   @state = self.class.states[ @state ].call( trigger )
   end
end

class XMLParser < FiniteStateMachine
   WHITESPACE = /\s/
   STARTCHAR = /[a-z_]/i
   NAMECHAR = /[a-z0-9_.-]/i

   def start_element( tag_name )
     if @doc
       new_element = @doc.create_element( tag_name )
       @current_parent.append_child( new_element )
       @current_parent = new_element
     else
       @current_parent = MyXML::Document.new( tag_name )
     end
   end

   def close_element
     @current_parent = @current_parent.parent_node
   end

   add_state( :pre ){ |char|
     @str = ''
     case char
       when WHITESPACE then :pre
       when '<' then :open
     end
   }

   add_state( :open ){ |char|
     case char
       when STARTCHAR
         @str << char
         :tag_start
     end
   }

   add_state( :tag_start ){ |char|
     case char
       when NAMECHAR
         @str << char
         :tag_start
       when WHITESPACE
         start_element( @str )
         :attrs
       when '>'
         start_element( @str )
         :chilluns
       when '/'
         start_element( @str )
         :empty_tag
     end
   }
end

parser = XMLParser.new( :pre )
'<root> <foo /> </root>'.each_byte{ |c|
   break unless parser.transition( c.chr )
}






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