From: Joel VanderWerf <vjoel@...>
Date: 2005-03-07T07:58:48+09:00
Subject: Re: What is reason for callcc{|c|c}?

John Carter wrote:
> 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?

Two reasons, neither particularly historical,  come to mind:

1. So you can implement throw/catch-style control structures.

2. So a result can be passed to the continuation.

Both are illustrated by:

   p callcc { |c|
     c.call "result"
     puts "don't get here"
   }
   puts "got here"

Output:

"result"
got here