Continuations

Continuations &
Web Applications
-Balaji Damodaran, ThoughtWorks

Contiuation

●
Continuation is Time Travel

●
Continuation is saving the control flow of
a program's execution

class ControlCentre

def exploreTerrain
marsPlateau = Plateau.new(5, 5)
alpha = Rover.new("alpha")
source = Position.new(1, 2, "N")
alpha.land(marsPlateau, source)
destination = alpha.explore("LMLMLMLMM")
print destination
end
end

class ControlCentre

def exploreTerrain
print destination
end
end

Control Scope
Execution Context
marsPlateau
alpha

class ControlCentre

def exploreTerrain
print destination
end
end

Control Scope Current Continuation
Execution Context source = Position.new(1, 2, "N")
marsPlateau alpha.land(marsPlateau, source)
alpha print destination

class ControlCentre

def exploreTerrain
callcc{|@cc|}
print destination
return @cc
end
end

class ControlCentre

def exploreTerrain
callcc{|@cc|}
print destination
return @cc
end
end

> c = exploreTerrain
“1 3 N”
=> #<Continuation:0xb744a8c4>

> c.call
“1 3 N”
=> #<Continuation:0xb744a8c4>

Normal Control Flow with
Continuation invoked
Control Flow Continuations

continuation continuation

Isn't that just a glorified GOTO?

class ControlCentre

def exploreTerrain
c = nil
source = callcc do |cc|
c = cc
Position.new(1, 2, "N")
end
print destination
return c
end
end

> c = exploreTerrain
“1 3 N”
=> #<Continuation:0xb7466240>

> c.call(Position.new(2, 3, “E”))
“2 4 E”
=> #<Continuation:0xb7466240>

Normal Control Flow with Continuation invoked
Control Flow Continuations with different parameters

continuation
continuation With different
parameters

def continuation
puts "i do some computation " +
"& save the state of execution."
m = callcc { |cc| return cc }
puts "then i restart the execution from " +
"the saved state & pass custom " +
"values such as "#{m[0]}" and "#{m[1]}""
end

> c = continuation

i do some computation & save the state of execution.

> c.call("so..", "then..")

then i restart the execution from the saved state & pass custom
values such as "so.." and "then.."

Normal Control Flow with Continuation reinvoked
Control Flow Continuations paused with parameters

continuation
With parameters

continuation

Continuation?
●
A continuation reifies the control flow of a
program as just another data value

●
Reify – To consider an abstract concept to be
real. To make control flow as first class
functions/objects.

Continuation
●
Continuation is an object that takes a
snapshot copy of the running process.

●
The current process is copied and saved.

●
Current process – execution context and
all local variables

Normal Control Flow marked with
Control Flow continuation

Memory

Local variables
#

continuation

A new process that
invokes a continuation

Memory

#

The current execution is abandoned

Note: Global data does not
get stored in a continuation

Continuations
●
A Continuation stores the execution context, its
references and pointers.

●
However, it does not store the heap data or
data that is outside the scope of local
references and pointers.

●
So any change made to them will stay changed
even after invoking the continuation.

Continuations Sandwich Analogy

call-with-current-continuation
●
call-with-current-continuation, in short
called as “call/cc” is a procedure first
introduced in Scheme.

●
Now exists in several other programming
languages such as Ruby, known as
“callcc”

● call/cc takes in only one argument, which is a
function f.
● It takes the current continuation as an object and
applies f to it.

– continuation = new Continuation()
– function(continuation)

●
When, callcc gets invoked, the program flow will
continue from the point it was captured and the
argument to the continuation will become its
return value

Method definition:
function: call/cc(function: f) {
c = new: Continuation
f(c)
}

JavaScript (Rhino):
function callcc(f) {
var c = new Continuation();
return f(c);
}

Ruby:
callcc {|cont|} cont => Continuation object
cont.call(*args)

function callcc(fun) {
var continuation = new Continuation()
return fun(continuation)
}

var cc
print(3 * (8 + callcc(function(c) {
cc = c
return 4
})))

js> 36

function callcc(fun) {
var continuation = new Continuation()
return fun(continuation)
}

var cc
print(3 * (8 + callcc(function(c) {
cc = c
return 4
})))

js> 36

js> cc(6)
42

def continue_example
i = 0
print "Before callcc #{i}n"
callcc do |cc|
for i in 1..10 do
print "In callcc #{i}n"
return cc if i > 5
end
end
print "After callcc #{i}n"
end

i = 0
callcc do |cc|
for i in 1..10 do
print "In callcc #{i}n"
return cc if i > 5
end
end
print "After callcc #{i}n"
end

puts "begin control flow."
c = continue_example()
puts "after invoking the method"
if c
c.call
puts "this will not get executed"
end

i = 0
callcc do |cc| begin control flow.
for i in 1..10 do Before callcc 0
print "In callcc #{i}n" In callcc 1
return cc if i > 5 In callcc 2
end In callcc 3
end In callcc 4
print "After callcc #{i}n" In callcc 5
end In callcc 6
after invoking the method
After callcc 6
after invoking the method

puts "begin control flow."
c = continue_example()
puts "after invoking the method"
if c
c.call
puts "this will not get executed"
end

●
Continuations are sort of goto labels. But
more powerful.
●
We can store the continuation in a variable or
object field, unlike a goto label.
●
Goto labels are restricted within a functional
scope, continuations are not limited by that.

The following code is strictly for session purposes
only. Implementing this in usable code may cause
harm to life and limb.

Continuation as a GOTO
def goto_in_ruby_1
i = 0
callcc {|@label|}
i = i + 1
print "#{i}t"
@label.call if i < 10
end

def goto_in_ruby_1
i = 0
callcc {|@label|}
i = i + 1
print "#{i}t"
@label.call if i < 10
end

output:
1 2 3 4 5 6 7 8 9 10

def goto_in_ruby_1 def goto_in_ruby_2
i = 0 i = 0
callcc {|@label|} callcc do |label|
i = i + 1 label.call
print "#{i}t" i = 99
@label.call if i < 10 end
end print i
end

output:
1 2 3 4 5 6 7 8 9 10

def goto_in_ruby_1 def goto_in_ruby_2
i = 0 i = 0
callcc {|@label|} callcc do |label|
i = i + 1 label.call
print "#{i}t" i = 99
@label.call if i < 10 end
end print i
end

output: output:
1 2 3 4 5 6 7 8 9 10 0

Summary
●
Continuation is a procedure that represents
the remaining steps in a computation.

●
It is a "snapshot" of the current control
context or control state of the program

●
The continuation can be later invoked
(repeatedly) with custom arguments

Continuations
●
Languages that support first class
continuations
●
Scheme (call/cc)
●
Ruby (callcc)
●
Smalltalk (Continuation currentDo:)
●
Rhino [JavaScript] (new Continuation())
●
Scala (Responder & shift, reset)
●
Perl (Coro)
●
Haskell (Control.Monad.Cont)

Continuations Types

●
First class continuations
●
Continuation Passing Style (CPS)
●
Delimited Continuations

Continuation Passing Style
●
A function, instead of returning an explict
value, it takes an explict 'continuation'
argument – another function which is
meant to receive the result of the
computation

Continuation Passing Style

function (arg) { function (arg, k) {
//computation //computation
return result; k(result);
} }

CPS - Example
function add(x, y) {
return x + y
}

function square(x) {
return x * x
}

function calculate(x, y) {
return add(square(x), square(y))
}

js> print(calculate(3, 4))
=> 25

CPS - Example
function add_cps(x, y, c) {
c(x + y)
}

function square_cps(x, c) {
c(x * x)
}

CPS - Example
c(x + y)
}

c(x * x)
}

function calculate_cps(x, y, c) {
square_cps(x, function(v) {
square_cps(y, function(r){
add_cps(v,r,c)
})
})
}

CPS - Example
c(x + y)
}

c(x * x)
}

function calculate_cps(x, y, c) {
square_cps(x, function(v) {
square_cps(y, function(r){
add_cps(v,r,c)
})
})
}

js> calculate_cps(3, 4, print)
=> 25
js> calculate_cps(3, 4, function(v) {print(sqrt(v))});
=> 5

CPS
● CPS is used in programming languages that do not
have first – class continuations, but have first –
class functions

● Every call is a tail call and if used without Tail Call
Optimization (TCO), the continuation would grow
during recursion

● Using CPS and TCO, compilers and interperters of
some programming languages managed to
eliminate the need for a runtime stack.

● Delimited (or partial, or composable) continuations are
more like regular functions and less like GOTOs. They
do not embody the entire rest of the computation, but
just a partial rest, up to a programmer-defined outer
bound.

● delimited continuations will return control to the caller
after they are invoked, and they may also return values

– Tiark Rompf, Ingo Maier, Martin Odersky

Normal Control Flow with
Control Flow Delimited Continuations Continuation invoked

Continuation -
start

Delimited
continuation

Continuation -
end

●
Delimited Continuations are manipulated
by means of two primitives, shift and
reset.

●
Calling shift captures the current
continuation, and reset defines the
boundary up to which the continuation
reaches.

reset {
shift { k: (Int=>Int) =>
k(7)
} + 1
} * 2

scala>
16

CPS transformed
2 * (1 + 7)

reset {
shift { k: (Int=>Int) =>
k(7)
} + 1
} * 2

scala>
16
__________________________________________________________

reset { def foo() {
foo() + 1 shift { k: (Int=>Int) =>
} * 2 k(7)
}
}

CPS transformed
2 * (1 + 7)

reset{
1 + shift{ k: (Int=>Int) =>
k(k(7))
}
} * 3

scala>
27

reset{
k(k(7))
} * 3
}

scala>
67

reset{
k(k(7))
}
} * 3

scala>
27 CPS Transform: (1 + (1 + 7)) * 3 = 27

reset{
k(k(7))
} * 3
}

scala>
67 CPS Transform: (1 + (1 + 7 * 3) * 3) = 67

So... ummm..
Whats the real world use of this thing?

Applications of Continuations
●
Pause and Play e.g. User Interface design
●
Backtracking
●
Coroutines
●
Exception Handling
●
Web Frameworks
●
Compiler & Interpreter implementation

Problems
●
Page centric
●
HTTP Stateless-ness causes heavy use of
session, cookies
●
'Back' button in user interaction pages
●
Browser cloning in user interaction pages
●
Inversion of control

def add_two_numbers
n1 = callcc { |cc| return ask("1st number:", cc) }
n2 = callcc { |cc| return ask("2nd number:", cc) }
print "sum of two numbers is #{n1 + n2}n"
end

def add_two_numbers
end

irb> add_two_numbers
1st number:
respond with: submit(25, ...): => #<Continuation:0xb76e0e58>
irb> submit(25, 30)
2nd number:
respond with: submit(37, ...): => #<Continuation:0xb76ddde8>
irb> submit(37, 12)
sum of two numbers is 42
=> nil

def add_two_numbers
end

@session = Hash.new

def ask(str, cc)
r = rand(100).to_i
print "#{str}nrespond with: submit(#{r}, ...): "
@session[r] = cc
cc
end

def submit(id, number)
@session[id].call(number)
End

Continuations lookup table (session)

Server renders the
page and saves the The continuation
continuation in the object is looked up
session with a uuid based on the uuid The page is
1 4 in the url and rendered with the
executed uuid given in the
url

server
2
call: page a
call: page b client
call: page c
3

Client makes a
server response
with required
information

Web Frameworks
●
Seaside in Smalltalk (v 3.1)
●
Wee in Ruby (v 2.1)
●
Nagare in Python (v 0.3)
●
Apache Cocoon in Java (v 2.2)
●
RIFE in Java (v 1.6)
●
Ocsigen in OCaml (v 1.3)
●
Weblocks in Common Lisp

Web Servers with Continuations
support
● Continuity (Perl)
● Jetty 6 (Java)
● Plt-Scheme web server (Scheme)

Seaside
● Written in Smalltalk

● Often called the 'heretic' framework because it doesn't
follow the 'conventional' web wisdom

– Clean urls
– Templates to separate model and view
– Share as little state as possible

– Urls contain session identifiers
– There is no 'view' or html templates. Views are part of smalltalk objects
– Seaside continuations & callbacks give a stateful behavior

Seaside - Installation
One – click install experience

Seaside - Workspace
●
Smalltalk deals with image snapshots for
saving the workspace.

●
There is no concept of 'files' and 'editors'

●
Class objects are created through 'object
browser' and code is executed through a
'workspace'

Seaside - Components
● Components represent the whole of a page or portion of
pages

● The inherit from 'WAComponent' class

● Components maintain state

● Component's response loop
– Renders the component
– Waits for user's input
– Executes a block of code when a link or button is pressed
– Re-renders itself or another component in place

Image from - Seaside – A Multiple Control Flow Web Application Framework, Stéphane Ducasse

Seaside - callbacks
● Every user interaction on the web page triggers a
callback.

●
A callback can be on a link, button or any other
form elements

●
The callback will execute a block of code whose
result will end in re-rendering the component or
a new component

SimpleCounter>>initialize
super initialize.
counter := 0.

SimpleCounter>>renderContentOn: html
html heading: 'the counter is ', counter asString.
html anchor
callback: [ counter := counter + 1 ];
with: 'increase'.

Seaside - Continuations
● Call / Answer
● call: takes a component as a parameter and it swaps
the current component for the new component

● The new component will be rendered and user action
will cause a callback to run that'll send an answer:

● The value passed to answer: will be returned to call:
and the program will continue from there.

Problem with full continuation

Response 1 Response 2

Continuation saved Continuation invoked

Request 1 Request 2

Solved with delimited
continuation
Response 1

Response 2
Delimited
Continuation saved

Delimited
Continuation invoked

Request 1 Request 2

Challenges
●
Understanding continuations as a concept
●
Debugging
●
Buying into the 'heretic' idea

References
● http://en.wikipedia.org/wiki/Continuation
● http://idea-log.blogspot.com/2005/10/why-are-continuations-so-confusing-
and.html
● http://www.iam.unibe.ch/~scg/Archive/Papers/Duca04eSeaside.pdf
● http://vimeo.com/13304075
● http://www.intertwingly.net/blog/2005/04/13/Continuations-for-Curmudgeons
● http://www.cs.indiana.edu/~dfried/appcont.pdf
● http://lambda-the-ultimate.org/node/86
● http://en.wikipedia.org/wiki/Continuation-passing_style
● http://lamp.epfl.ch/~rompf/continuations-icfp09.pdf
● http://book.seaside.st/book
● http://en.wikipedia.org/wiki/Call-with-current-continuation
● http://www.ibm.com/developerworks/java/library/j-contin.html
● http://www.cs.brown.edu/~sk/Publications/Papers/Published/khmgpf-impl-use-
plt-web-server-journal/
● http://double.co.nz/pdf/continuations.pdf

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