Practical Common Lisp

Dynamic bindings make global variables much more manageable, but it's important to notice they still allow action at a distance. Binding a global variable has two at a distance effects—it can change the behavior of downstream code, and it also opens the possibility that downstream code will assign a new value to a binding established higher up on the stack. You should use dynamic variables only when you need to take advantage of one or both of these characteristics.

Constants

One other kind of variable I haven't mentioned at all is the oxymoronic "constant variable." All constants are global and are defined with DEFCONSTANT. The basic form of DEFCONSTANT is like DEFPARAMETER.

(defconstant name initial-value-form [ documentation-string ])

As with DEFVAR and DEFPARAMETER, DEFCONSTANT has a global effect on the name used—thereafter the name can be used only to refer to the constant; it can't be used as a function parameter or rebound with any other binding form. Thus, many Lisp programmers follow a naming convention of using names starting and ending with + for constants. This convention is somewhat less universally followed than the *-naming convention for globally special names but is a good idea for the same reason.[80]

Another thing to note about DEFCONSTANT is that while the language allows you to redefine a constant by reevaluating a DEFCONSTANT with a different initial-value-form, what exactly happens after the redefinition isn't defined. In practice, most implementations will require you to reevaluate any code that refers to the constant in order to see the new value since the old value may well have been inlined. Consequently, it's a good idea to use DEFCONSTANT only to define things that are really constant, such as the value of NIL. For things you might ever want to change, you should use DEFPARAMETER instead.

Assignment

Once you've created a binding, you can do two things with it: get the current value and set it to a new value. As you saw in Chapter 4, a symbol evaluates to the value of the variable it names, so you can get the current value simply by referring to the variable. To assign a new value to a binding, you use the SETF macro, Common Lisp's general-purpose assignment operator. The basic form of SETF is as follows:

(setf place value)

Because SETF is a macro, it can examine the form of the place it's assigning to and expand into appropriate lower-level operations to manipulate that place. When the place is a variable, it expands into a call to the special operator SETQ, which, as a special operator, has access to both lexical and dynamic bindings.[81] For instance, to assign the value 10 to the variable x, you can write this:

(setf x 10)

As I discussed earlier, assigning a new value to a binding has no effect on any other bindings of that variable. And it doesn't have any effect on the value that was stored in the binding prior to the assignment. Thus, the SETF in this function:

(defun foo (x) (setf x 10))

will have no effect on any value outside of foo. The binding that was created when foo was called is set to 10, immediately replacing whatever value was passed as an argument. In particular, a form such as the following:

(let ((y 20))

(foo y)

(print y))

will print 20, not 10, as it's the value of y that's passed to foo where it's briefly the value of the variable x before the SETF gives x a new value.

SETF can also assign to multiple places in sequence. For instance, instead of the following:

(setf x 1)

(setf y 2)

you can write this:

(setf x 1 y 2)

SETF returns the newly assigned value, so you can also nest calls to SETF as in the following expression, which assigns both x and y the same random value:

(setf x (setf y (random 10)))

Generalized Assignment

Variable bindings, of course, aren't the only places that can hold values. Common Lisp supports composite data structures such as arrays, hash tables, and lists, as well as user-defined data structures, all of which consist of multiple places that can each hold a value.

I'll cover those data structures in future chapters, but while we're on the topic of assignment, you should note that SETF can assign any place a value. As I cover the different composite data structures, I'll point out which functions can serve as "SETFable places." The short version, however, is if you need to assign a value to a place, SETF is almost certainly the tool to use. It's even possible to extend SETF to allow it to assign to user-defined places though I won't cover that.[82]

In this regard SETF is no different from the = assignment operator in most C-derived languages. In those languages, the = operator assigns new values to variables, array elements, and fields of classes. In languages such as Perl and Python that support hash tables as a built-in data type, = can also set the values of individual hash table entries. Table 6-1 summarizes the various ways = is used in those languages.

Table 6-1. Assignment with = in Other Languages

Assigning to ...	Java, C, C++	Perl	Python
... variable	x = 10;	$x = 10;	x = 10
... array element	a[0] = 10;	$a[0] = 10;	a[0] = 10
... hash table entry	—	$hash{'key'} = 10;	hash['key'] = 10
... field in object	o.field = 10;	$o->{'field'} = 10;	o.field = 10

SETF works the same way—the first "argument" to SETF is a place to store the value, and the second argument provides the value. As with the = operator in these languages, you use the same form to express the place as you'd normally use to fetch the value.[83] Thus, the Lisp equivalents of the assignments in Table 6-1—given that AREF is the array access function, GETHASH does a hash table lookup, and field might be a function that accesses a slot named field of a user-defined object—are as follows:

80

Several key constants defined by the language itself don't follow this convention—not least of which are T and NIL. This is occasionally annoying when one wants to use t as a local variable name. Another is PI, which holds the best long-float approximation of the mathematical constant pi.

81

Some old-school Lispers prefer to use SETQ with variables, but modern style tends to use SETF for all assignments.

82

Look up DEFSETF, DEFINE-SETF-EXPANDER for more information.

83

The prevalence of Algol-derived syntax for assignment with the "place" on the left side of the = and the new value on the right side has spawned the terminology lvalue, short for "left value," meaning something that can be assigned to, and rvalue, meaning something that provides a value. A compiler hacker would say, "SETF treats its first argument as an lvalue."