This document discusses continuations in Ruby. It explains that continuations allow flow control by treating the rest of the program as an object that can be called later. Continuations act like goto statements but only jump backward in the code. They are similar to exceptions but can traverse deeper in the call stack. The document provides examples of using continuations to implement labels and goto statements, restartable exceptions, and generator objects.

1. Continuations
in Ruby
Antono Vasiljev
Ruby Open Air, 2012

2. Continuation is basically
Rest of the program

3. Continuations are
first class objects
in Ruby

4. irb(main)> c = callcc { |cont| cont }
=> #<Continuation:0x7f70ad16d030>

5. Continuations are about
Flow Control

6. 1: i = 0
2: callcc do |cont|
3: i = 1
4: cont.call
5: i = 2 # skipped
6: end
7:
8: # i => 1

7. 1: i = 0
2: i = callcc do |cont|
3: i = 1
4: cont.call(2)
5: i = 3 # skipped
6: end
7:
8: # i => 2 # See lines: 2, 4

8. 1| i = 0
2|
3| label :sum
4|
5| puts i += 1
6|
7| goto :sum, cond: (i < 10)

9. 1| LABELS = {}
2|
3| def label(name)
4| callcc { |cont| LABELS[name] = cont }
5| end
6|
7| def goto(label, args: {})
8| LABELS[label].call if args[:cond]
9| end

10. Continuation are like
goto
with parameters
(but jumps only backward)

11. continuations jumps far
def foo
bar
end
def bar
baz
end
def baz
$cont[0]
end
callcc { |c| $cont = c; foo }

12. Similar to exceptions.
But can go down through stack.
Time Machine!

13. Restartable Exceptions

14. 1| begin
2| hello
3| rescue Exception => e
4| e.restart
5| ensure
6| e.cleanup
7| end

15. 1| def hello
3| i = 0
4| restartable do
5| puts i += 1
6| raise Exception unless i == 5
7| end
8| end

16. 1| def restartable
2| cont = callcc { |c| c }
3| begin
4| yield
5| rescue Exception => e
6| e.continuation = cont
7| raise e
8| end
9| end

17. require 'continuation'
class Exception
class << self
attr_accessor :conts
end
def continuation=(cont)
self.class.conts ||= {}
self.class.conts[self.class] ||= cont
end

18. def restart
self.class.conts[self.class].call
end
def cleanup
self.class.conts.delete(self.class)
end
end

19. You can do with continuation:
Generator objects
Fibers/Coroutines
Exit from recursion
Other control structures

20. Thanks!

21. http://antono.info/