From: Jim Weirich <jim@...>
Date: 2005-09-21T23:36:30+09:00
Subject: Re: Continuation - where does it continue


michelemendel@gmail.com said:
> Why does the following code print the line "doing other stuff" twice?
>
>
01 def cc
02    puts "do A"
03    puts "----> 1. #{caller}"
04    callcc { |cont| return cont}
05    puts "----> 2. #{caller}"
06    puts "do B"
07 end
>
08 puts "calling cc"
09 cont = cc()
10 puts "doing other stuff"
11 cont.call() if cont
12 puts "end of work"
13 puts

This is the seqence of events (by line number).  Comments on what's
happening are in parenthesis.

08: calling cc
09     (calling cc)
02: do A
03: ----> 1. remove2.rb:59
04:    (returning the continuation)
10: doing other stuff
11:    (calling the continuation, so picking up in the middle of cc)
05: ----> 2. remove2.rb:59
06: do B
07:    (returning from cc to the original location cc call site.
    Now we return a nil value)
10: doing other stuff
11:    (since cc returned a nil the second time, we don't call the
       continuation this time)
12: end of work

-- 
-- Jim Weirich     jim@weirichhouse.org    http://onestepback.org
-----------------------------------------------------------------
"Beware of bugs in the above code; I have only proved it correct,
not tried it." -- Donald Knuth (in a memo to Peter van Emde Boas)