From: "Christoffer Lernö" <lerno@...>
Date: 2007-03-15T15:13:38+09:00
Subject: Re: Anyone playing with higher order messaging in ruby?

On Mar 14, 2007, at 23:48 , Benjohn Barnes wrote:

>
> On 14 Mar 2007, at 19:57, Jason Roelofs wrote:
>
>> I haven't personally messed with this kind of stuff but code like  
>> this just
>> makes me love Ruby even more. These types of, well DSL is really  
>> the best
>> way to describe it, in such little code, makes Ruby such a blast  
>> to program
>> in, and it's readable too!
>
> I think I must be getting too old, or I'm being a spring time  
> Scrouge...
>
> While it's neat, and it's cool Ruby can do that stuff, I've got to  
> ask "Why?".
>
> I've come across this used in DSLs, but I generally think, "I'm  
> sure you're going to get in to trouble down the line. Why don't you  
> just use the regular syntax?"
>
> I don't really see that it makes for an especially more concise  
> program, and really, it just seems to be introducing a different  
> syntax that's kind of crow bared in as an overloading of the  
> current syntax and semantics: as such, it doesn't seem to act in an  
> expected way. Frankly, it all seems reminiscent of some of the c++  
> template techniques which, while terribly clever, don't generally  
> seem to actually achieve a lot that can't be done more simply in  
> other ways; and introduce so much complexity that even a seriously  
> good coder's complexity budget is nearly all used up with a very  
> small problem.
>
> No, you're right, I am getting old ;-)

For code like this:

subscribers.send_message(:chat, @current_user, chat_text)

Instead of

subscribers.each { |s| s.add_to_queue(:send_message, [:chat,  
@current_user, chat_text]) }

It is also useful for transactions, where you can define a trampoline  
like this (writing this code in my mail program, it probably has bugs):

def transaction
	queue = []
	$exec = trampoline { |method, *args| @queue << [method, args] }
	yield
	queue.each do |method, args|
		__send__(method, *args)
	end
end

def exec
	$exec
end
	

Now you write code like this

transaction do
	do_something
	exec.start_system1
	do_something_else
	raise "Some error" if (something_failed)
	exec.start_system2
end

If the transaction works, both start_system1 and start_system2 is  
called. Otherwise neither happens.

It helps making the code straightforward, especially if the  
start_system calls are IO or similar that is difficult to reverse.


/Christoffer