From: Pinku Surana <suranap@...>
Date: 2008-09-10T03:43:38+09:00
Subject: Re: Local variable in loop affects callcc

On Sep 9, 1:30 pm, "David A. Black" <dbl...@rubypal.com> wrote:
> Hi --
>
>
>
> On Wed, 10 Sep 2008, Pinku Surana 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
>
> I haven't unraveled it entirely but I believe it's not about the local
> variable itself; it's about the fact that the interval method gets
> called before the assignment to a[i]. If you do this:
>
>    a[i] = x = interval
>
> you'll get the result you want.
>
> David
>
> --
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"a[i] = x = interval" probably gets translated into "a[i] = interval"
anyway because x is not used. I want to do something with the value of
x before I assign it to the array. The sample code I posted is just a
simplified version of my code that still exhibits the bug.