From: Sean O'Halpin <sean.ohalpin@...>
Date: 2008-09-11T03:27:07+09:00
Subject: Re: Local variable in loop affects callcc

On Tue, Sep 9, 2008 at 5:48 PM, Pinku Surana <suranap@gmail.com> wrote:
> I was trying to do some simple backtracking, but it kept failing for
> some reason. In the simplified version below, I use continuations to
> return either a 1 or 2 from interval. testcc assigns a value to the
> array, prints it out, then calls the next continuation on the stack
> (@next_cc) to jump back into interval and return the other number. The
> output should be:
> [1, 1]
> [1, 2]
> [2, 1]
> [2, 2]
>
> But if I introduce a temporary local variable, I get this instead:
> [1, 1]
> [1, 2]
> [1, 1]        # WRONG
> [1, 2]        # WRONG
>
> The local variable seems to cause the continuations to jump only to
> the point where i=1, not where i=0 where it should go. How does a
> local variable effect continuations like that?
>
> I'm using "ruby 1.8.6 (2007-09-24 patchlevel 111) [i486-linux]" built
> for Ubuntu. Thanks for your help.
>
>
>
> def interval
>  return callcc { |ret|
>    callcc { |k|
>      @next_cc.push(k)
>      ret.call(1)
>    }
>    ret.call(2)
>  }
> end
>
> def testcc
>  @next_cc = []
>  a = Array.new(2)
>  for i in 0...2
> # This produces the WRONG output
> #     x = interval
> #     a[i] = x
>
> # This produces the CORRECT output
>    a[i] = interval
>  end
>
>  puts a.inspect
>
>  while (not @next_cc.empty?) do
>    @next_cc.pop.call
>  end
> end
>
>

Like David, I haven't fully unravelled this myself, however it appears
to be a scoping issue. If you replace the

  for i in 0..2

with

  (0..2).each do |i|

you get the correct result in both cases.

Regards,
Sean