How Many Continuations can Dance on the Head of a Pin Matthias Felleisen King of Continuations and Professor of Computer Science.

How Many Continuations can Dance on the Head of a Pin

Matthias FelleisenKing of Continuations

and Professor of Computer Science

What does this mean?

• 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 …”

• It’s really all about the Web.

Academia and the Real World


• Functions are values like all other values.

• Continuations are fine values, too.

• Tail calls are jumps. • Macros don’t surprise

you.

• Functions live on a different planet.

• What’s a continuation?

• We have do. • Who needs

protection? It’s really about the Web.

Common Lisp and PLT Scheme


• The “Orbitz” Problem

• Programming the Interactive Web

• Continuations, closures and the Web

• Academia and the Real World, Again

The Interactive Web and the “Orbitz Problem”

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

Programming the Interactive Web

The Interactive Web

Numerous Web APIs:• The Common Gateway Interface (CGI)• Java Servlets• Active Server Pages, Java Server

Pages• Scripting languages (Perl, PHP, etc)• Microsoft’s Web Services

Printing a Message (Console)

(print “Hello, World\n”)

(exit)

Printing a Message (Web)

(print <html> <head><title>Test</title> </head> <body> <p>Hello, World!</p> </body> </html> )(exit)

Do it! .. In DrScheme

Printing Uptime (Console)

(print “Uptime: %s\n”

(system “uptime”))

(exit)

Printing Uptime (Web)

(print <html> <head><title>Uptime</title> </head> <body> <p>(system “uptime”)</p> </body> </html>)(exit)

Do it! … In DrScheme …

Area of Circle (Console)

(define r (read “Enter radius:” )

(print “area is %d\n”

(* 3.14 r r))

(exit)

Area of Circle (Web)

Enter radius:

(define r (get_binding “radius” bindings))(print <p>area is (3.14*r*r)</p>

Adding Two Numbers (Console)

(define n1

(prompt-read “Enter first:”))

(define n2

(prompt-read “Enter second:”))

(print “sum: %d\n”(+ n1 n2))

(exit)

Adding Two Numbers: User Interfaces

Enter first:

Enter second:

Enter first:

Enter second:

Adding Two Numbers: Web Interactions






Adding Two Numbers (Web)

Enter first: (define n1

(get “n1 bindings))

<form>…</form>

Adding Two Numbers: The Problem

• Web scripts write a page, then terminate

• When the user replies, another script reads the form’s bindings and performs the next step

Adding Two Numbers(Web)

Enter first:

(define n1 (get_binding “n1” bindings))<form>…</form>

Enter second:

(define n2 (get_binding “n2” bindings))

<p>sum: (+ n1 n2)</p>

In Practice

• System halts cleanly with an error– The user doesn’t 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”

Enter second:

n1::

Adding Two Numbers(Web)

Enter first:

(define n1 (get_binding “n1” bindings))<form>…</form>

Enter second:

(define n1 (get_binding “n1” bindings))(define n2 (get_binding “n2” bindings))<p>sum: (+ n1 n2)</p>

The Actual Form

<html><head><title>The Addition Page</title><body><p>Enter the second number:</p><form method="get" action="http://www. .../cgi-second.ss"><input type="hidden" name=“n1" value=“1729"><input type="text" name=“n2" value="0"></form></html>

General Web Programming Problems

• “Invert” the program

• Manually tracks hidden fields

• It’s difficult and mistakes have painful consequences.

Bad News

That’s the easy part!

The Real Picture

The scriptand the user

are

script user

Event lines

coroutines!

Control Flow: Back Button

script user script user

A silent action!

Control Flow: Cloning


Control Flow: Bookmarks


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?

How to Reengineer Programs for the Web

Automated Software Engineering 2002J. Automated Software Engineering

2003

The Legacy Code

(define n1

(read “Enter first: ”))

(define n2

(read “Enter second: ”))

(print “sum: %d\n”

(+ n1 n2))

(exit)

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 <form>Enter first: </form>))(define n2 (read/web <form>Enter second: </form>))(print <p>sum: (+ n1 n2)</p>)(exit)

But: the program structure is destroyed

(define n1

(read/web

<form>Enter first: </form>))

(define n2

(read/web

<form>Enter second: </form>))

(print

<p>sum: (+ n1 n2)</p> ))

(exit)

(define (main) (print <form action=“f1”> Enter first: <input name=“n1”> </form> ))

(define (f1 form) (print <form action=“f2”> <input hidden name=“n1” value= (form-n1 form)> Enter second: <input name=“n2”> </form>))

(define (f2 form) (print The sum is (+ (form-n1 form) (form-n2 form))

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

The Key Insight

The manual conversionsimply implements the

continuation-passing style transformation!

Step 1: Create Function for the “Rest of the Computation”

(define n1

(read/web <form>Enter first: </form>))

… n2 …

(read/web/k

<form>Enter first: </form>

(lambda(n1)

… n2 …

The Result

(read/web/k <form>Enter first: </form>

(lambda (n1)

(read/web/k <form>Enter second: </form>

(lambda(n2) (print <p>sum: (+ n1 n2)</p>)

Lift Functions

(define (main) (read/web/k <form>Enter first: </form> f1))(define (f1 n1) (read/web/k <form>Enter second: </form> f2))(define (f2 n2) (print <p>sum: (+ n1 n2)</p>

Step 2: Propagate Free Variables

(define (main) (read/web/k <form>Enter first: </form> f1))(define (f1 n1) (read/web/k/args n1 <form>Enter second: </form> f2))(define (f2 n1 n2) = (print <p>sum: (+ n1 n2)</p>)

Convert to Web API

(define (f1 n1)

(read/web/k/args n1

<form>Enter second: </form> f2))

(define (f1 form)

(print

<form action=“f2”>

<input hidden name=“n1”

value=(form-n1 form)>

Enter second:

<input name=“n2”>

</form>))

Resulting Web Application

(define (main) (print <form action=“f1”> Enter first: <input name=“n1”> </form>))

(define (f1 form)

(print

<form action=“f2”>

<input hidden name=“n1”

value=(form-n1 form)>

Enter second:

<input name=“n2”>

</form>))

(define (f2 form)

(print

<p>sum: (+ (form-n1 form) (form-n2 form))</p>))

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

A Remarkable Coincidence

These are known as:• Continuation-passing style transformation• Lambda lifting • Closure conversion (to first-order values)

You’ve studied them in academic compilers & languages courses, especially when functions areordinary values.

Can’t Languages do Better?

European Symposium on Programming 2001

J. Higher-Order Symbolic Logic 2004

see also: Hughes (1999), Queinnec (ICFP 2000),

Thielman (ESOP 2002)

Program Structure Reinstatement

(define n1

(read

“Enter first: ”))

(define n2

(read

“Enter second: ”))

(print

“sum: %d\n”

(+ n1 n2))

(exit)

(define n1

(read/web

<form>Enter first:</form>))

(define n2

(read/web

<form>Enter second:</form>))

(print

<p>sum:

(+ n1 n2)</p>

(exit)

What we have: What we want:

Now What?

APIs offer form, cookie, &c primitives

Why not an API with read/web ?

Now what?

Programmers: Stand up for your rights –

make server implementers work harder!

The Real Primitive

read/web is a small lie:(define n1 (read/web <form action=???>Enter first:</form>))

We provide send/suspend:(define n1 (send/suspend (lambda (k) <form action=k>Enter first:</form>))

send/suspend generates theURL that resumes computation

Generated URLs

send/suspend generates a URL:

http://host/servlets/add.ss;id281*k2-95799725

When a consumer uses this URL, the Webserver delivers the filled out form as the

resultof the call to send/suspend .

And send/suspend is what …

… a plain old call-with-current-contiuation.

The PLT Web Server

• send/suspend associates url’s with continuation objects

• reading and writing becomes a coroutine action

• the rest is (interesting) engineering

The Moral of the Story

How Many Continuations can Dance on the Head of a Pin

93326215443944152681699238856266700490715968264381621468592963895217599993229915608941463976156518286253697920827223758251185210916864000000000000000000000000

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, andteach robust interactive Webprograms 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.

The End

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

Credits …http://www.plt-scheme.org/

How Many Continuations can Dance on the Head of a Pin Matthias Felleisen King of Continuations and Professor of Computer Science.

Documents