From: William Morgan <wmorgan-ruby-talk@...>
Date: 2005-03-09T10:38:25+09:00
Subject: Re: What is reason for callcc{|c|c}?

Excerpts from John Carter's mail of  6 Mar 2005 (EST):
> The excellent article in callcc at
> http://www.all-thing.net/Ruby/iterators_generators_and_continuations_in_ruby.html
> 
> makes the following statement...
> 
>   The way to create a continuation is with Kernel#callcc. For what I
>   imagine are historical reasons, the continuation is passed as an
>   argument to a block, rather than returned directly. So the idiom to
>   get a continuation at the current point in the code is this:
> 
>      c = callcc { |c| c }
> 
> Can anyone elaborate on this history?
> 
> Or is there a deeper reason than mere history?

The author of that article clearly didn't understand continuations as
well as he thought! The block syntax does indeed serve a purpose: it
becomes useful when you want to pass values around in continuation
calls, as the ol'

  if @cont = callcc { |c| c }
    ## before
  end
  ## before + after

trick doesn't work any more, if nil or false can be passed as values.
See e.g. http://www.all-thing.net/Ruby/coroutines.html for a usage case.

I've exchanged a few choice words with the author and he's updated the
article to reflect this.

-- 
William <wmorgan-ruby-talk@masanjin.net>