Class 10: Continuation Lab

Held: Monday, February 13, 2006

Summary: Today we ground your understanding of continuations in a laboratory experience.

Related Pages:

• EBoard.

Assignments

• Read section 1-6 of The Revised⁵ Report on the Algorithmic Language Scheme.

Notes:

• Don't forget that Math/CS lunch today!
• Office hours end early today.
• There will be time for questions on the homework.

Overview:

• Lab!

Lab on Continuations

This laboratory was designed by Ogechi Nnadi and was edited by Samuel A. Rebelsky.

Summary: In this lab, you will familiarize yourself with continuations in Scheme.

Preparation

a. What is the continuation captured in the code below? Predict the value of the expression.

(+ 1 
   (let/cc k
         (k 5)))

b. What is the value the program? If it was different from your prediction, explain why.

c. What do you expect the following expression to return? Why?

(+ 1 
   (let/cc k
     5))

d. Evaluate the expression. If it was different from your prediction, explain why.

e. What do you expect the following expressions to return? Why?

; Expression 1
(+ 5
   (let/cc k1
     (* 2
        (let/cc k2
          3))))
; Expression 2
(+ 5
   (let/cc k1
     (* 2
        (let/cc k2
          (k2 3)))))
; Expression 3
(+ 5
   (let/cc k1
     (* 2
        (let/cc k2
          (k1 3)))))

f. Confirm or refute your answers experimentally. If your predictions were incorrect, determine why.

Escape Procedures Using Continuations

(The following two exercises are based on chapter13 of Teach Yourself Scheme in Fixnum Days. )

The following is a definition for a program that computes the product of a list of numbers.

(define list-product
  (lambda (s)
    (let recur ((s s))
      (if (null? s) 1
          (* (car s) (recur (cdr s)))))))

a. Rewrite list-product using continuations so that if a0 is encountered during the recursive calls, 0 is returned immediately without further calls to recur.

Hint! To make sure that the return is immediate, without applying the cached multiplications, see what happens if you return the symbol zero when you encounter a 0.

b. Explain why one might want to to immediately return 0 when a 0 is encountered.

c. Explain how to solve the problem without continuations.

Generators

Write a procedure cyc that takes no arguments and successively generates the "cycles" of the string "abc" in order. For example,

> (cyc)
"abc"
> (cyc)
"bca"
> (cyc)
"cab"
> (cyc)
"abc"

Hint: You can do this with continuations by running a loop that first returns the subsequent cycle, and then stores the rest of the loop (captured as a continuation) in a variable.