From: Jesse Yoon <jesse.yoon@...>
Date: 2006-02-14T17:28:25+09:00
Subject: Re: Continuation in ruby

Minkoo,

When you call cont.call, the control goes right back to the end of
callcc block with all the context in tact. So your program's control
flow looks like this:

1. cont = print_num # calls print_num method
2. callcc {|c| return c} # at i % 2 == 0, print_num returns with the
continuation as a return value, which is now assigned to cont
3. puts "Yae..."
4. cont.call # now the control goes back to "}" of callcc with the
variable i == 2
5. for loop continues
6. go to step 2 unless i == 10
7. at i == 10, callcc block returns one final time
8. puts ...
9. cont.call
10. now that i == 10 for loop finishes and the return value of
print_num at this time is (1..10), because "for loop" is essentially
(1..10).each {|i|} in disguise, and #each returns self
11. puts ...
12. cont.call # argh! this time you're calling (1..10).call
13. NoMethodError exception

This is what has happened.

Hope this helps.

Jesse

Minkoo Seo wrote:
> Hi all.
>
> I've written the following code to learn continuations in ruby only to
> fail. Please have a look at:
>
>
> def print_num
>     for i in (1..10)
>         puts i
>         if i % 2 == 0; callcc{|c| return c}; end
>     end
> end
>
> cont = print_num
> puts "Yae, I love even~"
> cont.call
> puts "Bye"
>
>
> This code produces:
>
> C:\WINDOWS\system32\cmd.exe /c ruby a.rb
> 1
> 2
> Yae, I love even~
> 3
> 4
> Yae, I love even~
> 5
> 6
> Yae, I love even~
> 7
> 8
> Yae, I love even~
> 9
> 10
> Yae, I love even~
> Yae, I love even~
> a.rb:10: undefined method `call' for 1..10:Range (NoMethodError)
> shell returned 1
> Hit any key to close this window...
>
> This is contrary to my intuition. I've called cont.call only once, and
> cont.call should put the following command, put "bye", into stack. I
> can't figure out the reason why cont.call is called indefinitely.
> 
> Best,
> Minkoo Seo