# Epilogue

 ROS: I mean, what exactly do you do? PLAYER: We keep to our usual stuff, more or less, only inside out. We do on stage things that are supposed to happen off. Which is a kind of integrity, if you look on every exit as being an entrance somewhere else.       -- Tom Stoppard, Rosencrantz and Guildenstern are Dead

We have reached the end of this introduction to computing and program design. While there is more to learn about both subjects, this is a good point to stop, to summarize, and to look ahead.

## Computing

From elementary school to high school we learn to compute with one form of data: numbers. Our first use of numbers is to count real things, say, three apples, five friends, twelve bagels. Later we use numbers without any appeal to concrete objects, but we have learned that numbers represent information in the real world.

Computing with software is algebra for all kinds of data, not just numbers. Nowadays, computer programs process representations of music, molecules, law cases, electrical diagrams, architectures of houses, and poems. Fortunately, we have learned to represent information with other forms of data than just numbers. Otherwise, computing and programming would become extremely tedious tasks.

Above all, we shouldn't forget that computing means manipulating data through proper basic operations. Some operations create new values. Others extract values from values. Yet others modify values. Finally, there are also basic operations for determining to which class a piece of data belongs. Built-in operations and functions are of course just another class of data. Definition is value creation; application is a form of value extraction.79

When we define a function, we combine basic data operations. There are two fundamental mechanisms for combining functions: function composition and conditional expressions. The former means that the result of one function becomes the argument of another one. The latter represents a choice among several possibilities. When we eventually apply a function, we trigger a computation.

In this book we have studied the laws of basic operations and the laws of operation combination. Using these laws we can understand, in principle, how any function processes its input data and how it produces its results and effects. Because the computer is extremely fast and good at using these laws, it can perform such evaluations for more data and for larger programs than we can do with paper and pencil.

## Programming

Programs consist of definitions and expressions. Large programs consist of hundreds and thousands of definitions and expressions. Programmers design functions, use other programmer's functions, leave, start on the project. Without a strong discipline we cannot hope to produce software of high quality. The key to programming discipline is to understand the design of programs as a means to describe computations, which, in turn, is to manipulate data through combinations of basic operations.

For that reason, the design of every program -- whether it is small and for personal use or large and for business use -- must start with an analysis of the surrounding world of information and a description of the classes of data that represent the relevant information. If the classes are unusual or new, we make up examples so we understand the structure of the class description. After we understand the world of information surrounding our project and its data representation, we make a plan.

A project plan identifies what data we wish to produce from the data that the program will be given. In many cases, though, a program doesn't process data in just one way but in many ways. For example, a program for managing bank accounts must handle deposits, withdrawals, interest calculations, tax form generation, and many other tasks. In other cases, a program may have to compute complex relationships. For example, a program for simulating a ping-pong game must compute the movement of the ball, bounces on the table, bounces from the paddle, paddle movements, etc. In either case, we need to describe what the various ways of processing data are and how they relate to each other. Then we rank them and start with the most important one. We develop a working product, make sure that it meets our specifications, and refine the product by adding more functions or taking care of more cases or both.

Designing a function requires a rigorous understanding of what it computes. Unless we can describe its purpose and its effect with concise statements, we can't produce the function. In almost all cases, it helps to make up examples and work through the function's computation by hand. For complicated functions or for functions that use generative recursion, we should include some examples with the purpose statements. The examples illustrate the purpose and effect statements for others who may have to read or modify the program.

Studying examples tends to suggest the basic design recipe. In most cases, the design of a function is structural, even if it uses an accumulator or structure mutation. In a few others, we must use generative recursion. For these cases, it is important to explain the method for generating new problems and to sketch why the computation terminates.

When the definition is complete, we must test the function. Testing discovers mistakes, which we are bound to make due to all kinds of reasons. The best testing process turns independently developed examples into test suites, that is, a bunch of expressions that apply the function to select input examples and compare its results and effects with expected results and effects (mostly) automatically. If a mismatch is discovered, the test suite reports a problem. The test suite should never be discarded, only commented out. Every time we modify the function, we must use the test suite to check that we didn't introduce mistakes. If we changed the underlying process, we may have to adapt the test suite mutatis mutandis.

No matter how hard we work, a function (or program) isn't done the first time it works for our test suite. We must consider whether the development of the function revealed new interesting examples and turn such examples into additional tests. And we must edit the program. In particular, we must use abstraction properly to eliminate all patterns wherever possible.

If we respect these guidelines, we will produce decent software. It will work because we understand why and how it works. Others who must modify or enhance this software will understand it, because we include sufficient information on its development process. Still, to produce great software, we must practice following these guidelines and also learn a lot more about computing and programming than a first book can teach.

## Moving On

The knowledge and design skills from this book are a good foundation for learning more about programming, computing, and even practical work on software. First, the skills are good for learning the currently fashionable collection of object-oriented languages, especially Java. The two languages share a philosophy of programming. In both settings, computing means dealing with data, and programming means describing classes of values and functions on them. Unlike Scheme, however, Java requires programmers to spell out the class descriptions in Java, not just in English, and to place function definitions with class descriptions. As a result, Java requires programmers to learn a lot of syntactic conventions and is unsuitable as a first language.

Second, a programmer must study the fundamental ideas of computing. Thus far, our studies have focused on the laws of computing for data-oriented programming languages. Using the programming skills from this book, we can design and implement a simulation of how the hardware computes. By doing so we see the laws of computing from a radically different perspective. The contrast points to a number of interesting questions:

1. The two mechanisms of computing are rather different. Can one mechanism compute what the other one can compute and vice versa?

2. The laws we have used are mathematical and abstract. They do not take into account any real-world limitations. Does this mean that we can compute whatever we wish?

3. The (simulated) hardware shows that computers have limitations. How do these limitations affect what we can compute?

Research on these questions created the discipline of computing and still guides the design of most computing curricula.

 Full Scheme

Finally, the design knowledge of this book is enough to build some real-world programs in Scheme. DrScheme with its built-in Web browser and email capabilities is such a program. Building large real-world programs, however, requires some more knowledge about the functions that Scheme uses to create GUIs, to connect computers on a network, to script things such as shells, common gateway interfaces (CGI), COM objects, and so on.

Material on all three topics is available from this book's Web site in a form that extends the coverage and the style of the book. The book's Web site is

`http://www.htdp.org/`

Check in with this site on a regular basis and continue to study computing and programming.

79 An object in a language such as Java is a function with many different bodies. Each method represents a different way of extracting data from an object.