In Chapter 1 we defined a procedure as a description of a process. Scheme provides a way to define procedures that take inputs, carry out a sequence of actions, and produce an output. Section 3.4.1 introduced some of Scheme’s primitive procedures. To construct complex programs, however, we need to be able to create our own procedures.
Procedures are similar to mathematical functions in that they provide a mapping between inputs and outputs, but they differ from mathematical functions in two important ways:
State. In addition to producing an output, a procedure may access and modify state. This means that even when the same procedure is applied to the same inputs, the output produced may vary. Because mathematical functions do not have external state, when the same function is applied to the same inputs it always produces the same result. State makes procedures much harder to reason about. We will ignore this issue until Chapter 9, and focus until then only on procedures that do not involve any state.state
Resources. Unlike an ideal mathematical function, which provides an instantaneous and free mapping between inputs and outputs, a procedure requires resources to execute before the output is produced. The most important resources are space (memory) and time. A procedure may need space to keep track of intermediate results while it is executing. Each step of a procedure requires some time to execute. Predicting how long a procedure will take to execute and finding the fastest procedure possible for solving some problem are core problems in computer science. We consider this throughout this book, and in particular in Chapter 7.
For the rest of this chapter, we view procedures as idealized mathematical functions: we consider only procedures that involve no state and do not worry about the resources required to execute our procedures.