From Principles to Practice with Class in the First YearMatthew Flatt, University of Utah, Salt Lake...

From Principles to Practicewith Classin the First Year

Sam Tobin-HochstadtDavid Van Horn

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Industrial co-op

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Industrial co-op

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Under consideration for publication in J. Functional Programming 1

EDUCATIONAL PEARL

The Structure and Interpretation of the

Computer Science Curriculum

Matthias Felleisen, Northeastern University, Boston, MA, USA

Robert Bruce Findler, University of Chicago, Chicago, IL, USA

Matthew Flatt, University of Utah, Salt Lake City, UT, USA

Shriram Krishnamurthi, Brown University, Providence, RI, USA

Email: {matthias,robby,mflatt,shriram}@plt-scheme.org

Abstract

Twenty years ago Abelson and Sussman’s Structure and Interpretation of Computer Pro-grams radically changed the intellectual landscape of introductory computing courses. In-stead of teaching some currently fashionable programming language, it employed Schemeand functional programming to teach important ideas. Introductory courses based on thebook showed up around the world and made Scheme and functional programming popular.Unfortunately, these courses quickly disappeared again due to shortcomings of the bookand the whimsies of Scheme. Worse, the experiment left people with a bad impression ofScheme and functional programming in general.

In this pearl, we propose an alternative role for functional programming in the first-yearcurriculum. Specifically, we present a framework for discussing the first-year curriculumand, based on it, the design rationale for our book and course, dubbed How to Design

Programs. The approach emphasizes the systematic design of programs. Experience showsthat it works extremely well as a preparation for a course on object-oriented programming.

1 History and critique

The publication of Abelson and Sussman’s Structure and Interpretation of Com-

puter Programs (sicp) (Abelson et al., 1985) revolutionized the landscape of theintroductory computing curriculum in the 1980s. Most importantly, the book lib-erated the introductory course from the tyranny of syntax. Instead of arranging acourse around the syntax of a currently fashionable programming language, sicp

focused the first course on the study of important ideas in computing: functional ab-straction, data abstraction, streams, data-directed programming, implementationof message-passing objects, interpreters, compilers, and register machines.

Over a short period, many universities in the US and around the world switchedtheir first course to sicp and Scheme. The book became a major bestseller for MITPress.1 Along with sicp, the Scheme programming language (Sussman & Steele Jr.,

1 According to Bob Prior (editor at MIT Press), sicp sold 45,000 copies in its first five years[personal communication, 9 June 2003].

EDUCATIONAL PEARL

Abstract

EDUCATIONAL PEARL

Abstract

1. Introduce only those language constructs that are necessary to teach programming principles

EDUCATIONAL PEARL

Abstract

2. Choose a language with as few language constructs as possible, and one in which they can be introduced one at a time

regular, minimal syntax

regular, minimal syntax complicated irregular syntax

untyped

untyped typed

pedagogical environment

untyped typed

pedagogical environment industrial environment

untyped typed

mathematical numbers

untyped typed

mathematical numbers machine numbers

untyped typed

images as values

untyped typed

images as values ?

untyped typed

images as values ?

interaction

untyped typed

images as values ?

interaction compilation

untyped typed

images as values ?

untyped typed

images as values ?

functions and structures

untyped typed

images as values ?

functions and structures objects

regular, minimal syntax

untyped

pedagogical environment

mathematical numbers

images as values

interaction

functions and structures objects

objects

S. Tobin-Hochstadt & D. Van Horn 5

included with each method definition, following the principles of the design recipe studied in the firstsemester. In fact, the check-expect mechanism works exactly as it did before.

Methods can be defined to consume any number of arguments, but they are implicitly parameterizedover this, the object that was sent the message.

3.2 Where did the cond go?

Unions, and recursive unions in particular, are a fundamental kind of data definition that students arewell-versed in from the previous semester. A fundamental early lesson is how to represent (recursive)unions using classes and how to write recursive methods. As an example, figure 1 defines binary trees ofnumbers (an archetypal recursive union data definition) using the BSL language and the Class language.

#lang bsl

;; A Tree is one of:

;; - (make-leaf Number)

;; - (make-node Tree Number Tree)

(define-struct leaf (v))

(define-struct node (left v right))

;; sum : Tree -> Number

;; sums the elements of the given tree

(define (sum a-tree)

(cond [(leaf? a-tree) (leaf-v a-tree)]

(+ (sum (node-left a-tree))

(node-v a-tree)

(sum (node-right a-tree)))]))

(check-expect (sum (make-leaf 7)) 7)

(check-expect

(sum (make-node

(make-leaf 1)

(make-node (make-leaf 0)

(make-leaf 0))))

#lang class/1

;; - (new leaf Number)

;; - (new node Tree Number Tree)

;; and implements

;; sum : -> Number

;; sums the elements of this tree

(define-class leaf

(fields v)

(define (sum) (this . v)))

(define-class node

(fields left v right)

(define (sum)

(+ (this . left . sum)

(this . v)

(this . right .sum))))

(check-expect ((new leaf 7) . sum) 7)

(check-expect

((new node

(new leaf 1)

(new node (new leaf 0)

(new leaf 0))))

. sum)

Figure 1: Binary tree sum in Beginning Student and in the Class language

The structure of this data definition is analogous to the approach of the previous semester but thisexample brings to light an important difference with the functional approach. The method for computingthe sum of the empty tree is defined in the leaf class, while the method for computing the sum of a node

objects

#lang class/0(define-class posn (fields x y))(new posn 3 4)(send (new posn 3 4) x) ;=> 3(send (new posn 3 4) y) ;=> 4

objects

#lang class/1(define-class posn (fields x y))(new posn 3 4)((new posn 3 4) . x) ;=> 3((new posn 3 4) . y) ;=> 4

objects

#lang class/1(define-class posn (fields x y) ;; Posn -> Number ;; Distance between this posn and that posn (check-expect ((new posn 0 0) . dist (new posn 3 4)) 5) (define (dist that) (sqrt (+ (sqr (- (this . x) (that . x))) (sqr (- (this . y) (that . y)))))) ;; -> Number ;; Distance of this posn from the origin (check-expect ((new posn 0 0) . dist-origin) 0) (check-expect ((new posn 3 4) . dist-origin) 5) (define (dist-origin) (this . dist (new posn 0 0))))

objects

#lang class/1 ;;A Tree is one of:;; - (new leaf Number);; - (new node Tree Number Tree);; and implements;; sum : -> Number;; sums the elements of this tree(define-class leaf (fields v) (define (sum) (this . v)))

(define-class node (fields left v right) (define (sum) (+ (this . left . sum) (this . v) (this . right . sum))))

(check-expect ((new leaf 7) . sum) 7)(check-expect ((new node (new leaf 1) 5 (new node (new leaf 0) 10 (new leaf 0))) . sum) 16)

#lang class/1 ;; A Tree is one of:;; - (make-leaf Number);; - (make-node Tree Number Tree)(define-struct leaf (v))(define-struct node (left v right))

;; sum : Tree -> Number;; sums the elements of the given tree(define (sum a-tree) (cond [(leaf? a-tree) (leaf-v a-tree)] [else (+ (sum (node-left a-tree)) (node-v a-tree) (sum (node-right a-tree)))]))

objects

#lang class/1(require 2htdp/image class/universe)

;; A World is a (new world Number)(define-class world (fields n)

;; on-tick : -> World (define (on-tick) (new world (add1 (this . n))))

;; to-draw : -> Image (define (to-draw) (rotate (modulo (this . n) 360)

;; on-key : KeyEvent -> World (define (on-key k) (new world 10)))

(big-bang (new world 0))

#lang class/1(require 2htdp/image 2htdp/universe)

;; A World is a Number(define-struct world (n))

;; tock : World -> World(define (tock w) (make-world (add1 (world-n w))))

;; draw : World -> Image(define (draw w) (rotate (modulo (world-n w) 360)

;; : KeyEvent World -> World(define (press k w) (make-world 0))

(big-bang (make-world 0) [on-tick tock] [on-draw draw] [on-key press])

objects

#lang bsl

(node-v a-tree)

(check-expect

(sum (make-node

(make-leaf 1)

(make-leaf 0))))

#lang class/1

;; and implements

;; sum : -> Number

(define-class leaf

(fields v)

(define-class node

(define (sum)

(this . v)

(check-expect

((new node

(new leaf 1)

(new leaf 0))))

. sum)

0. objects

#lang bsl

(node-v a-tree)

(check-expect

(sum (make-node

(make-leaf 1)

(make-leaf 0))))

#lang class/1

;; and implements

;; sum : -> Number

(define-class leaf

(fields v)

(define-class node

(define (sum)

(this . v)

(check-expect

((new node

(new leaf 1)

(new leaf 0))))

. sum)

0. objects

#lang bsl

(node-v a-tree)

(check-expect

(sum (make-node

(make-leaf 1)

(make-leaf 0))))

#lang class/1

;; and implements

;; sum : -> Number

(define-class leaf

(fields v)

(define-class node

(define (sum)

(this . v)

(check-expect

((new node

(new leaf 1)

(new leaf 0))))

. sum)

0. objects

1. shorthand method call

#lang bsl

(node-v a-tree)

(check-expect

(sum (make-node

(make-leaf 1)

(make-leaf 0))))

#lang class/1

;; and implements

;; sum : -> Number

(define-class leaf

(fields v)

(define-class node

(define (sum)

(this . v)

(check-expect

((new node

(new leaf 1)

(new leaf 0))))

. sum)

0. objects

2. inheritance

#lang bsl

(node-v a-tree)

(check-expect

(sum (make-node

(make-leaf 1)

(make-leaf 0))))

#lang class/1

;; and implements

;; sum : -> Number

(define-class leaf

(fields v)

(define-class node

(define (sum)

(this . v)

(check-expect

((new node

(new leaf 1)

(new leaf 0))))

. sum)

0. objects

2. inheritance

3. overriding

#lang bsl

(node-v a-tree)

(check-expect

(sum (make-node

(make-leaf 1)

(make-leaf 0))))

#lang class/1

;; and implements

;; sum : -> Number

(define-class leaf

(fields v)

(define-class node

(define (sum)

(this . v)

(check-expect

((new node

(new leaf 1)

(new leaf 0))))

. sum)

0. objects

2. inheritance

3. overriding

4. first-class classes

#lang bsl

(node-v a-tree)

(check-expect

(sum (make-node

(make-leaf 1)

(make-leaf 0))))

#lang class/1

;; and implements

;; sum : -> Number

(define-class leaf

(fields v)

(define-class node

(define (sum)

(this . v)

(check-expect

((new node

(new leaf 1)

(new leaf 0))))

. sum)

0. objects

2. inheritance

3. overriding

5. mutation

0. objects

2. inheritance

3. overriding

5. mutation

#lang bsl

(node-v a-tree)

(check-expect

(sum (make-node

(make-leaf 1)

(make-leaf 0))))

#lang class/1

;; and implements

;; sum : -> Number

(define-class leaf

(fields v)

(define-class node

(define (sum)

(this . v)

(check-expect

((new node

(new leaf 1)

(new leaf 0))))

. sum)

8 From Principles to Practice with Class in the First Year

From this point, almost any language that students might use in future co-op positions and courseswould be an appropriate follow-up. Our course transitions to Java, but C#, Python, Ruby, Eiffel, orJavaScript would all work naturally. The key lesson of the transition is that the fundamental principlesunderlying object-oriented programming remain the same between languages, and that learning a newlanguage is primarily a matter of mapping these concepts to specific constructs in the new language.Of course, particular languages also use unique specific mechanisms which need to be taught to use thelanguage effectively, but these are rarely as vital as the cross-language principles.

4.1 Functional Java

The transition begins with replicating the object-oriented style of our teaching languages in Java. In par-ticular, we do not introduce mutation, for loops, or mutable data structures such as arrays or ArrayListsuntil later in the semester. Instead, students design data representations using classes, with interfacesrepresenting unions of data. Additionally, we avoid mention of the distinction between primitive andother values in Java, which is made easier by not using standard libraries early. An example of this styleof programming is presented in figure 3, repeating the binary tree sum from the previous section.

Comparing this figure to the previous example illustrates a number of the differences that studentsare exposed to upon transition to Java.

1. Explicit representation of unions and interfaces in the language. Previously, interfaces were simplydescribed in stylized comments, following the How to Design Programs approach.

2. Types are now specified as part of the program, and are now enforced.3. Java syntax is substantially different and more verbose. For example, constructors must be defined

explicitly.4. The testing environment is somewhat different, and requires additional boilerplate, although we

are able to use the JavaLib framework [19] to support simple testing by structural equality.Of course, there are other differences which cannot be seen from a code snippet.

5. Students must use a new development environment and compiler. In class, we primarily developin a text editor and run the Java compiler at the command line. In labs and on homeworks, studentstypically use the Eclipse IDE.

6. Installing and configuring libraries is now required. Because we use a custom library for testing,students must cope with library installation and class paths on the first day.

Of course, all but the first two of these changes are unrelated to the fundamental lessons we hope toteach—the rest merely present additional hurdles for students.

4.2 Traditional Java

Thanks to the the preparation in the first half of the course, we can cover functional OO programming inJava in a just a few lectures. We then increase the subset of the language we use to encompass mutation,loops, and mutable data structures. We present ArrayLists, followed briefly by arrays. Students use,and then implement, hash tables as well as other mutable and immutable data structures. Conventionalinput and output are treated only very briefly, as we focus instead of both fundamentals and exercisesmaking use of real APIs such as hashing functions or Twitter posting. Finally, while, for, and for-eachloops are presented, following the methodology of How to Design Classes which connects loops tostylized use of recursive functions with accumulators, a technique the students now have two semestersof practice with.

import tester.*;

interface Tree {

// sums the elements of this tree

Integer sum();

class Leaf {

Integer v;

Leaf(Integer v) { this.v = v; }

public Integer sum() { return this.v; }

class Node {

Tree left; Integer v; Tree right;

Node(Tree l, Integer v, Tree r) {

this.left = l;

this.v = v;

this.right = r;

public Integer sum() {

return this.left.sum() + this.v + this.right.sum();

class Examples {

void test_tree(Tester t) {

t.checkExpect(new Leaf(7).sum(), 7);

t.checkExpect(new Node(new Leaf(1),

new Node(new Leaf(0), 10, new Leaf(0))).sum(),

Figure 3: Binary tree sum in How to Design Classes

Principles

Practice

Industrial co-op

#lang bsl

(node-v a-tree)

(check-expect

(sum (make-node

(make-leaf 1)

(make-leaf 0))))

#lang class/1

;; and implements

;; sum : -> Number

(define-class leaf

(fields v)

(define-class node

(define (sum)

(this . v)

(check-expect

((new node

(new leaf 1)

(new leaf 0))))

. sum)

DanBrown

Asumu Takikawa

Nicholas Labich

Principles

Practice

Industrial co-op

#lang bsl

(node-v a-tree)

(check-expect

(sum (make-node

(make-leaf 1)

(make-leaf 0))))

#lang class/1

;; and implements

;; sum : -> Number

(define-class leaf

(fields v)

(define-class node

(define (sum)

(this . v)

(check-expect

((new node

(new leaf 1)

(new leaf 0))))

. sum)

http://www.ccs.neu.edu/course/cs2510h/

From Principles to Practice with Class in the First YearMatthew Flatt, University of Utah, Salt Lake...

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