The magic of a computer lies in its ability to become almost anything you can imagine, as long as you can explain exactly what that is. The hitch is in explaining what you want. With the right programming, a computer can become a theater, a musical instrument, a reference book, a chess opponent. No other entity in the world except a human being has such an adaptable, universal nature. Ultimately all these functions are implemented by the Boolean logic blocks and finite-state machines described in the previous chapter, but the human computer programmer rarely thinks about these elements; instead, programmers work with a more convenient tool called a programming language.
Just as Boolean logic and finite-state machines are the building blocks of computer hardware, a programming language is a set of building blocks for constructing computer software. Like a human language, a programming language has a vocabulary and a grammar, but unlike a human language there is an exact meaning in the programming language for every word and sentence. Most programming languages are universal, in the same sense that Boolean logic is universal: they can be used to describe anything a computer can do. Anyone who has ever written a program—or debugged a program—knows that telling a computer what you want it to do is not as easy as it sounds. Every detail of the computer’s desired operation must be precisely described. For instance, if you tell an accounting program to bill your clients for the amount that each owes, then the computer will send out a weekly bill for $0.00 to clients who owe nothing. If you tell the computer to send a threatening letter to clients who have not paid, then clients who owe nothing will receive threatening letters until they send in payments of $0.00. Avoiding this kind of misunderstanding is what computer programming is all about. The programmer’s art is the art of saying exactly what you want. In this example, it means making a distinction between clients who have not sent in any money and clients who actually owe money. To paraphrase Mark Twain, the difference between the right program and the almost-right program is like the difference between lightning and a lightning bug—the difference is just a bug.
A skilled programmer is like a poet who can put into words those ideas that others find inexpressible. If you are a poet, you assume a certain amount of shared knowledge and experience on the part of your reader. The knowledge and experience that the programmer and the computer have in common is the meaning of the programming language. How the computer “knows” the meaning of the programming language will be described later; first, we will discuss the grammar, vocabulary, and idioms of such languages.