From: "Слепнев Владимир" <slepnev_v@...>
Date: 2004-04-21T02:41:45+09:00
Subject: resumable exceptions

Hi all,

maybe I'm touching an old problem here...I wrote a "Hello World" 
program which uses a very simple "resumable exceptions" 
infrastructure. Here we go:


class ResourceRequest < Exception
  attr_accessor :key, :continuation

  def initialize(key,continuation)
   @key = key
   @continuation = continuation
   super("No resource found for key "+key.inspect)
  end
end

def get_resource(key)
  callcc {|cc| raise ResourceRequest.new(key,cc)}		
end

def with(hash)
  begin
   yield
  rescue ResourceRequest => e
   hash.has_key?(e.key) ? e.continuation.call(hash[e.key]) : raise(e)
  end
end


def print_string(str)
  puts(str || get_resource("string to print"))
end

with( "string to print" => "Hello World!" ) do print_string(nil) end


If a function can't find some resource, it throws a request for it, 
which is processed with scoping rules (#with calls can be nested), as 
opposed to calling a global/static resource getter somewhere. We can 
also play with the resource hash - for example, use class names as 
keys... or use allocator methods as values, de-facto giving the block 
a set of "lazily allocated" resources... or easily override the object 
a specific method call interacts with, for testing purposes...

Is this interesting? =) I've been thinking about how to make functions 
request missing parameters from callers; this is definitely not a 
complete solution...

Vladimir Slepnev