Question

I’ve start getting acquainted with Ruby and met a function never seen before — callcc.
I’ve understood some general idea of what’s it for but when i had tried to write an example and i’ve got unexpected result.

require 'continuation'
def callcc_func
  i = 0
  while true
    c = nil
    callcc {|x| c = x}
    i += 1
    puts i    
    return c if (i % 3) == 0
  end
end

c = callcc_func()
puts
callcc_func.call

The result is an endless loop. Why?
I’ve expected it to be:

# for `c = callcc_func()` line
1
2
3

# callcc_func.call
4
5
6
#end here because of `return c if (i % 3) == 0`

P.S.
Sorry for my English and thanks.

Answer 1

call-with-current-continuation (generally abbreviated as call/cc originated in Scheme, so if you want to know more about it, Scheme docs are a good starting point:

• call/cc in the Racket documentation
• call/cc on Wikipedia

As for your Ruby problem: look at this blog post title Continuations and ruby, it implements something very similar to what you are trying to do. You’ll find an explanation there:

As a script file run by the main ruby interpreter, this will loop
forerver as it captures the control state of the program when and
where it was called and this includes returning the continuation and
then calling it again.