The expression includes the parentheses. When it is typed in, it is evaluated immediately. When we say that Scheme is an interactive programming language, we mean that it evaluates expressions immediately. Later on, I show how we define this expression. It is by defining such expressions that we write our simulation program.
Every expression has a value.
The value of the expression above is the new data structure itself. This expression illustrates two Scheme features. The first is that expressions can contain sub-expressions inside them. The inside expressions are called nested . Scheme figures out which expressions are nested by counting parentheses.