Chapter 1.8

Variables, Parameters, and Constants

"In a dilemma, it is helpful to change any variable, then reexamine the problem."

By this point, you may be pretty tired of having to manually re-type all your data every time you want to use them---but don't worry, you won't have to do that anymore. Whenever you want to use or re-use data---say, a string, number, list, or whatever else your program might need to pass around---you can bind a symbol and assign a value to it.

In keeping with the idea that "Lisp is different," variables are very different in Lisp from other programming languages. Some Lisp Hackers like to compare them to pointers in C and C++, but I have found that such a comparison overcomplicates a much simpler system. Once you know that the variable's symbol is not the variable itself, it is easy to see that the variable's assignment is a literal pointer to a memory address, without worrying about the extra complications that come with pointers in C or C++.

You may have also heard that Common Lisp is a "Lisp-2"; while all Lisp-family languages have lexical scope---a neat feature that we will cover in this chapter's exercises as well as extend upon in the following chapter---other Lisps such as Clojure, Scheme, Racket, and Emacs Lisp don't have separate namespaces for variables and functions within the same lexical scope. So, for example, in Common Lisp you can have both a function and a variable with the same name in the same scope; context tells the Lisp reader which definition to use.

This chapter will contain exercises on:

• Symbols
• Binding and Assignment
• (Global) Dynamic Variables
• (Local) Lexical Variables
• Re-assigning Values to Variables
• Parameters
• Constants

Exercise 1.8.1

Symbols

(intern "FOO")
FOO
NIL

Exercise 1.8.2

Symbol Tables and Inspecting Objects

Exercise 1.8.3

More Symbols: Binding and Assignment

Exercise 1.8.4

Even More Symbols: Namespaces

Exercise 1.8.5

DEFVAR: Dynamic Binding of Variables

Exercise 1.8.6

LET: Lexical Binding of Variables

Exercise 1.8.7

SETQ vs SETF

Exercise 1.8.8

DEFPARAMETER: Dynamic Binding Revisited

Exercise 1.8.9

DEFCONSTANT

Exercise 1.8.10

Function Parameters