Structure and Interpretation of Computer Programs

Naming and the Environment

A critical aspect of a programming language is the means it provides for using names to refer to computational objects. We say that the name identifies a variable whose value is the object.

In the LFE dialect of Lisp, we name things using functions (macros, really; we'll get to that later). In the REPL, we can assign values to variables using set:

> (set size 2)
2

causes the interpreter to associate the value 2 with the name size. Once the name size has been associated with the number 2, we can refer to the value 2 by name:

> size
2
> (* 5 size)
10

Here are further examples of the use of set:

> (set pi 3.14159)
3.14159
> (set radius 10)
10
> (* pi (* radius radius))
314.159
> (set circumference (* 2 pi radius))
62.8318
> circumference
62.8318

When not in the REPL, we can use such things as the (let ...) form to locally assign values to variables.

Though one could also use set do define a function in the REPL

> (set identity (lambda (x) x))
#Fun<lfe_eval.12.86468545>
> (funcall identity 2)
2

it is more idiomatic to use the defun macro:

> (defun identity (x) x)
identity
> (identity 2)
2

And, as you can see, it's a bit easier to use.

The defun macro is a convenience macro for define-function. These two, and in the REPL, set, are examples of the sorts of basic tools of abstraction available to the LFE programmer. These allow us to use simple names to refer to the results of compound operations, such as the circumference computed above. In general, computational objects may have very complex structures, and it would be extremely inconvenient to have to remember and repeat their details each time we want to use them. Indeed, complex programs are constructed by building, step by step, computational objects of increasing complexity. The interpreter makes this step-by-step program construction particularly convenient because name-object associations can be created incrementally in successive interactions. This feature encourages the incremental development and testing of programs and is largely responsible for the fact that an LFE program usually consists of a large number of relatively simple functions.

It should be clear that the possibility of associating values with symbols and later retrieving them means that the interpreter must maintain some sort of memory that keeps track of the name-object pairs. This memory is called the environment (more precisely the global environment, since we will see later that a computation may involve a number of different environments).¹