From: Pinku Surana <suranap@...>
Date: 2008-09-10T01:48:41+09:00
Subject: Local variable in loop affects callcc

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