From: James Edward Gray II <james@...>
Date: 2004-11-24T00:26:05+09:00
Subject: Re: Typical/idiomatic examples of dynamic code generation with Ruby?

On Nov 21, 2004, at 3:28 PM, Iwan van der Kleyn wrote:

> So my question: do you know of any examples on the subject of dynamic
> code generation which would be typical for Ruby and could not be
> easily implemented in languages like Python?

I don't know if this is the kind of thing you're after and I don't know 
Python, so I have no idea how it compares, but and interesting example 
did come up in my project today.

I'm building a server.  When some event happens in the server, it 
notifies monitoring code with an Event object.  My first crack at that 
class looked like:

	class Event
		CONNECT		= "Connect"
		LOGIN		= "Login"
		COMMAND		= "Command"
		DISCONNECT	= "Disconnect"
		
		def initialize( connection, identity,
						type, details = nil, time = Time.now )
			@connection	= connection
			@identity	= identity
			
			@type		= type
			@details	= details
			@time		= time
		end
		
		attr_reader :connection, :identity, :type, :details, :time
		
		def connect?()		@type == CONNECT	end
		def login?()		@type == LOGIN		end
		def command?()		@type == COMMAND	end
		def disconnect?()	@type == DISCONNECT	end
	end

That worked, of course.  However, one of my major goals with this 
server is to keep it very open to change.  I know for a fact it will be 
modified after deployment, so I'm planning ahead.

The problem with the above version comes when I add a new event type.  
I provide those little helper methods like command?(), to keep you from 
having to write:

	if event.type == Event::COMMAND
		# ...

	# can be written as...

	if event.command?
		# ...

However, when I decide I need a new event type, I have to add another 
constant and a new helper method.  That goes against DRY philosophy.  
So, I changed the class to:

	class Event
		CONNECT		= "Connect"
		LOGIN		= "Login"
		COMMAND		= "Command"
		DISCONNECT	= "Disconnect"
		
		def initialize( connection, identity,
						type, details = nil, time = Time.now )
			@connection	= connection
			@identity	= identity
			
			@type		= type
			@details	= details
			@time		= time
		end
		
		attr_reader :connection, :identity, :type, :details, :time
		
		constants.each do |e|
			value = const_get e
			define_method((e.downcase + "?").to_sym) do
				@type == value
			end
		end
	end

Problem solved.  Helper methods are now auto-generated from class 
constants.  Add a new constant, get a new helper method.

Hopefully, that's the kind of thing you are looking for.  Good luck 
with your paper.

James Edward Gray II