How Many Continuations can Dance on the Head of a Pin

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How Many Continuations can Dance on the Head of a
Pin

• Matthias Felleisen
• King of Continuations
• and
• Professor of Computer Science

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What does this mean?
Academia and the Real World

• The theory of continuations (85-95)
• Phil Here is the king of continuations.
• The practice of PLT Scheme at ICFP (95-
• Dick Gabriel Theoreticians just count how many
  continuations
• Its really all about the Web.

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What does this mean?
Common Lisp and PLT Scheme

• Functions are values like all other values.
• Continuations are fine values, too.
• Tail calls are jumps.
• Macros dont surprise you.

• Functions live on a different planet.
• Whats a continuation?
• We have do.
• Who needs protection?

Its really about the Web.
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What does this mean?

• The Orbitz Problem
• Programming the Interactive Web
• Continuations, closures and the Web
• Academia and the Real World, Again

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The Interactive Web and the Orbitz Problem
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The Orbitz Problem

• The same problem found on Web sites of
• Microsoft / Apple /
• Continental Airlines / Orbitz / Hertz /
• the National Science Foundation
• and many other companies

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Programming the Interactive Web
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The Interactive Web

• Numerous Web APIs
• The Common Gateway Interface (CGI)
• Java Servlets
• Active Server Pages, Java Server Pages
• Scripting languages (Perl, PHP, etc)
• Microsofts Web Services

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Printing a Message (Console)

• (print Hello, World\n)
• (exit)

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Printing a Message (Web)

• (print
• lthtmlgt
• ltheadgtlttitlegtTestlt/titlegt
• lt/headgt
• ltbodygt
• ltpgtHello, World!lt/pgt
• lt/bodygt
• lt/htmlgt )
• (exit)

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Do it! .. In DrScheme
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Printing Uptime (Console)

• (print Uptime s\n
• (system uptime))
• (exit)

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Printing Uptime (Web)

• (print
• lthtmlgt
• ltheadgtlttitlegtUptimelt/titlegt
• lt/headgt
• ltbodygt
• ltpgt(system uptime)lt/pgt
• lt/bodygt
• lt/htmlgt)
• (exit)

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Do it! In DrScheme
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Area of Circle (Console)

• (define r (read Enter radius )
• (print area is d\n
• ( 3.14 r r))
• (exit)

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Area of Circle (Web)
(define r (get_binding radius
bindings)) (print ltpgtarea is (3.14rr)lt/pgt
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Adding Two Numbers (Console)

• (define n1
• (prompt-read Enter first))
• (define n2
• (prompt-read Enter second))
• (print sum d\n( n1 n2))
• (exit)

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Adding Two Numbers User Interfaces
Enter first
Enter second
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Adding Two Numbers Web Interactions
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Adding Two Numbers Web Interactions
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Adding Two Numbers Web Interactions
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Adding Two Numbers Web Interactions
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Adding Two Numbers Web Interactions
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Adding Two Numbers Web Interactions
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Adding Two Numbers (Web)
(define n1 (get n1 bindings)) ltformgtlt/formgt
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Adding Two Numbers The Problem

• Web scripts write a page, then terminate
• When the user replies, another script reads the
  forms bindings and performs the next step

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Adding Two Numbers(Web)
(define n1 (get_binding n1
bindings)) ltformgtlt/formgt
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In Practice

• System halts cleanly with an error
• The user doesnt get a useful answer
• The user may not understand the error
• User expended a lot of effort and time
• Program captures variable by accident (i.e., it
  implements dynamic scope!), or
• internal server error

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Adding Two Numbers(Web)
(define n1 (get_binding n1
bindings)) ltformgtlt/formgt
(define n1 (get_binding n1
bindings)) (define n2 (get_binding n2
bindings)) ltpgtsum ( n1 n2)lt/pgt
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The Actual Form

• lthtmlgt
• ltheadgt
• lttitlegtThe Addition Pagelt/titlegt
• ltbodygt
• ltpgtEnter the second numberlt/pgt
• ltform method"get"
• action"http//www. .../cgi-second.ss"gt
• ltinput type"hidden" namen1" value1729"gt
• ltinput type"text" namen2" value"0"gt
• lt/formgt
• lt/htmlgt

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General Web Programming Problems

• Invert the program
• Manually tracks hidden fields
• Its difficult and mistakes have painful
  consequences.

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Bad News

• Thats the easy part!

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The Real Picture
The script and the user are
Event lines
coroutines!
script
user
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Control Flow Back Button
A silent action!
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Control Flow Cloning
script
user
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Control Flow Bookmarks
script
user
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What Programmers Need

• Multiply-resumable and
• restartable coroutines
• No language has exactly this the new control
  operator for the Web
• How do we implement it?

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How to Reengineer Programs for the Web

• Automated Software Engineering 2002
• J. Automated Software Engineering 2003

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The Legacy Code

• (define n1
• (read Enter first ))
• (define n2
• (read Enter second ))
• (print sum d\n
• ( n1 n2))
• (exit)

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How we should take this code to the Web

• (define n1
• (read
• Enter first ))
• (define n2
• (read
• Enter second ))
• (print
• sum d\n
• ( n1 n2)
• (exit)

(define n1 (read/web ltformgtEnter first
lt/formgt)) (define n2 (read/web ltformgtEnter
second lt/formgt)) (print ltpgtsum ( n1
n2)lt/pgt) (exit)
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But the program structure is destroyed

• (define n1
• (read/web
• ltformgtEnter first lt/formgt))
• (define n2
• (read/web
• ltformgtEnter second lt/formgt))
• (print
• ltpgtsum ( n1 n2)lt/pgt ))
• (exit)

• (define (main)
• (print
• ltform actionf1gt
• Enter first
• ltinput namen1gt
• lt/formgt ))
• (define (f1 form)
• (print
• ltform actionf2gt
• ltinput hidden namen1
• value (form-n1 form)gt
• Enter second
• ltinput namen2gt
• lt/formgt))
• (define (f2 form)
• (print
• The sum is

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The Reengineering Challenge

• Web interfaces have grown up
• from scripts to programs
• (or services)
• Need debugging, maintenance, evolution,
• We would like a Web compiler that automatically
• splits programs into procedures by form
• propagates fields as needed
• preserves behavior yet accommodates bizarre
  control

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The Key Insight

• The manual conversion
• simply implements the
• continuation-passing style
• transformation!

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Step 1 Create Function for the Rest of the
Computation

• (define n1
• (read/web ltformgtEnter first lt/formgt))
• n2

(read/web/k ltformgtEnter first lt/formgt
(lambda(n1) n2
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The Result

• (read/web/k
• ltformgtEnter first lt/formgt
• (lambda (n1)
• (read/web/k
• ltformgtEnter second lt/formgt
• (lambda(n2)
• (print ltpgtsum
• ( n1 n2)lt/pgt)

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Lift Functions

• (define (main)
• (read/web/k
• ltformgtEnter first lt/formgt f1))
• (define (f1 n1)
• (read/web/k
• ltformgtEnter second lt/formgt f2))
• (define (f2 n2)
• (print ltpgtsum
• ( n1 n2)lt/pgt

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Step 2 Propagate Free Variables

• (define (main)
• (read/web/k
• ltformgtEnter first lt/formgt f1))
• (define (f1 n1)
• (read/web/k/args n1
• ltformgtEnter second lt/formgt f2))
• (define (f2 n1 n2)
• (print ltpgtsum
• ( n1 n2)lt/pgt)

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Convert to Web API

• (define (f1 n1)
• (read/web/k/args n1
• ltformgtEnter second lt/formgt f2))

(define (f1 form) (print ltform actionf2gt
ltinput hidden namen1
value(form-n1 form)gt Enter second
ltinput namen2gt lt/formgt))
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Resulting Web Application

• (define (main)
• (print
• ltform actionf1gt
• Enter first
• ltinput namen1gt
• lt/formgt))

(define (f1 form) (print ltform actionf2gt
ltinput hidden namen1
value(form-n1 form)gt Enter second
ltinput namen2gt lt/formgt))
(define (f2 form) (print ltpgtsum ( (form-n1
form) (form-n2 form))lt/pgt))
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Summary

• Three transformations
• Make all value receivers explicit functions
• Make all functions top-level, replace free
  variables with explicit parameters
• Replace first-class functions with first-order
  representations

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A Remarkable Coincidence

• These are known as
• Continuation-passing style transformation
• Lambda lifting
• Closure conversion (to first-order values)
• Youve studied them in academic compilers
• languages courses, especially when functions are
• ordinary values.

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Cant Languages do Better?

• European Symposium on Programming 2001
• J. Higher-Order Symbolic Logic 2004
• see also
• Hughes (1999), Queinnec (ICFP 2000), Thielman
  (ESOP 2002)

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Program Structure Reinstatement
What we have
What we want

• (define n1
• (read
• Enter first ))
• (define n2
• (read
• Enter second ))
• (print
• sum d\n
• ( n1 n2))
• (exit)

• (define n1
• (read/web
• ltformgtEnter firstlt/formgt))
• (define n2
• (read/web
• ltformgtEnter secondlt/formgt))
• (print
• ltpgtsum
• ( n1 n2)lt/pgt
• (exit)

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Now What?

• APIs offer form, cookie, c primitives
• Why not an API with read/web ?

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Now what?
Programmers Stand up for your rights make
server implementers work harder!
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The Real Primitive

• read/web is a small lie
• (define n1
• (read/web
• ltform action???gtEnter firstlt/formgt))
• We provide send/suspend
• (define n1
• (send/suspend
• (lambda (k)
• ltform actionkgtEnter firstlt/formgt))

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Generated URLs

• send/suspend generates a URL
• http//host/servlets/add.ssid281k2-95799725
• When a consumer uses this URL, the Web
• server delivers the filled out form as the result
• of the call to send/suspend .

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And send/suspend is what
a plain old call-with-current-contiuation.
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The PLT Web Server

• send/suspend associates urls with continuation
  objects
• reading and writing becomes a coroutine action
• the rest is (interesting) engineering

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The Moral of the Story
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How Many Continuations can Dance on the Head of a
Pin
933262154439441526816992388 5626670049071596826438
16214 685929638952175999932299156 0894146397615651
82862536979 208272237582511852109168640 0000000000
0000000000000
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So what does this mean?
Because we have closures and continuations in
Scheme and because this motivates us to think
about CPS and friends, we can design, implement,
and teach robust interactive Web programs
easily.
Can Common LISPers do it? Paul Graham did it.
But with closures and hygienic macros, things
would have been so much easier and cleaner.
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The End
http//www.plt-scheme.org/
Credits

• Shriram Krisnamurthi (Brown)
• Robert Bruce Findler (Chicago)
• Matthew Flatt (Utah)
• Paul T. Graunke (Northeastern)