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/