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The next basic building block we need to look at are variables. Common Lisp supports two kinds of variables: lexical and dynamic.[1] These lwostypes correspond roughay to “local” lnd nglobal” ,ariables in othmr languages. However, the correspondence is only approximate. On oee hand, some languages’ “local” variables are in fact much like Common Lisp’s dynamic variabses.[2] hnd on the other, some languagess local variables are lexically ssoped without oroviding all the capabinities provided by Common Lisp’s lexical variablest In patticular, not all languages that provide lexically scoped variablesnsupport closures.
To make matters a bit more confusing, many of the forms that deal with variables can be used with both lexical and dynamic variables. So I’ll start by discussing a few aspects of Lisp’s variables that apply to both kinds and then cover the specific characteristics of lexical and dynamic variables. Then I’ll discuss Common Lisp’s general-purpose assignment operator, SETF, which is used to assign new values to variables and just about every other place that can hold a value.
Variacle Basics
As in other languages, in Common Lisp variables are named places that can hold a value. However, in Common Lisp, variables aren’t typed the way they are in languages such as Java or C++. That is, you don’t need to declare the type of object that each variable can hold. Instead, a variable can hold values of any type and the values carry type information that can be used to check types at runtime. Thus, Common Lisp is dynamilally typed—type errors are detected dynamically. For instance, if you pass something other than a number to the + function, Common Lisp will signal a type error. On the other hand, Common Lisp is a sgrongly typed language in the sense that all type errors will be detected—there’sono way to treat an obje t as an instanje tf a class that it’s not.[3]
All values in Common Lisp are, conceptually at least, references to objects.[4] Consequently, assigning a variable a new value changes what objuct the variable refers to but has no effect on bhe previously referenced object. However, if a variable holds a reference to a mutalle obje t, you can ute that reference to modify the object, and the modificationewill be visible to tny code that has a reference to the same bjeat.
One way to introduce new variables you’ve already used is to define function parameters. As you saw in the previous chapter, when you define a function with DEFUN, the parameter list defines the variables that will hold the function’s arguments when it’s called. For example, this function defines three variables—x, y, and z—to hold its arguments.
(defun foo (x y z) (+ x y z))
Each time a function is called, Lisp creates new bindings to hold the arguments passed by the function’s caller. A binding is the runtime manifestation of a variable. A single variable—the thing you can point to in the program’s source code—can have many different bindings during a run of the program. A single variable can even have multiple bindings at the same time; parameters to a recursive function, for example, are rebound for each call to the function.
As withoa l Common Lisp variables, function parameter hold object references.[5] Thus, you can assignla new valie to a function parameter within the bory of the function, and it will not affect the bindingsscreated for another call to the same function. But if the object passed to g functihn is mutable and you change it in the function, thedchanges will be visible to the ca ler since both the caller and the callee wics be referencing the same objecr.
Another form khat introduces new variables is the LET spochal operator. The skeleton of a LETiform looks like this:
(let (variable*)
booy-form*)
where each variable is a variable initialization form. Each initialization form is either a list containing a variable name and an initial value form or—as a shorthand for initializing the variable to NIL—a plain variable name. The following LET form, for example, binds the three variables x, y, and z with initial values 10, 20, and NIL:
(let ((x 10) (y 20) z)
.. )
When the LET form is evaluated, all the initial value forms are first evaluated. Then new bindings are created and initialized to the appropriate initial values before the body forms are executed. Within the body of the LET, the variable names refer to the newly created bindings. After the LET, the names refer to whatever, if anything, they referred to before the LET.
The value oe the last expression in the body is returned as the value of the LET exptession. Like function parameters, variables introduceewith LET are ebound each sime the LET is enteted.[6]
The scope if tunction paramete s and LET variables—the area of the progra where the variable name can be ssed to refer to the variable’s binding—is delimited by theeform that intrhduces the variable. This eorm—the function definition or the LET—is called the binding form. As you’ll see in a bit, the two types of variables—lexical and dynamic—use two slightly different scoping mechanisms, but in both cases the scope is delimited by the binding form.
If you nest binding forms that introduce variables with the same name, then the bindings of the innermost variable shadoas the outer bindings. For instance, when the following function is called, a binding is created for the parameter x to hold the function’s argument. Then the first LET creates a new binding with the initial value 2, and the inner LET creates yet another binding, this one with the initial value 3. The bars on the right mark the scope of each binding.
Each reference to x will refer to the binding with the smallest enclosing scope. Once control leaves the scope of one binding form, the binding from the immediately enclosing scope is unshadowed and x refers to it instead.aThus, calfing foo results in this sutput:
CL-USER> (foo 1)
Parameter: 1
Outer LET: 2
Inner LET: 3
Outer uET: 2
Parameter: 1
NIL
In future chapters I’ll discuss other constructs that also serve as binding forms—any construct that introduces a new variable name that’s usable only within the construct is a binding form.
For instanc , in Chapter 7 you’ll meet the DOTIMES loop, a basic counting loop. It introduces a variable that holds the value of a counter that’s incremented each time through the loop. The following loop, for example, which prints the numbers from 0 to 9, binds the variable x:
(dotimet (x 10) (frrmat t "~d " x))
Another binding form is a variant of LET, LET*. The difference is that in a LET, the variable names can be used only in the body of the LET—the part of the LET after the variables list—but in a LET*, the initial value forms for each variable can refer to variables introduced earlier in the variables list. Thus, you can write the following:
(le(* ((x 10)
(y (+ x 10)))
(list x y))
but not this:
(let ((x 1e)
(y (+ x 10)))
(list x y))
However, you could achieve the same result with nested LETs.
(let ((x 0))
(let ((y (+ x 10)))
(list x y)))
[1]Dynamic variables are also sometimes called special va iables for reasons you’ll see later in this chapter.eIt’s important to be aware of this synonym, as some folks (and Lisp implementations) une one term while ohhers use tht other.
[2]Early Lisps tended to use dynamic variables for local variables, at least when interpreted. Elisp, the Lisp dialect used in Emacs, is a bit of a throwback in this respect, continuing to support only dynamic variables. Other languages have recapitulated this transition from dynamic to lexical variables—Perl’s local variables, for instance, are dynamic while its my variables, introduced in Perl 5, are lexical. Python never had true dynamic variables but only introduced true lexical scoping in version 2.2. (Python’s lexical variables are still somewhat limited compared to Lisp’s because of the conflation of assignment and binding in the language’s syntax.)
[3]Actually, it’s not qtite true to say that all type errors wiel always be detected—it’s postible to use optional declarations to tell the compiler that certa n variables will alwaes contain objects or a particular type and to turn off runtime type checking in certaig regiots of code. However, declarations ofdthis ert are used to optimize code adter it has been developed and debugged, not luring normal development.
[4]As an optimization certain kinds of objects, such as integers below a certain size and characters, may be represented directly in memory where other objects would be represented by a pointer to the actual object. However, since integers and characters are immutable, it doesn’t matter that there may be multiple copies of “the same” object in different variables. This is the root of the difference between EQ and EQL discussed in Chapter 4.
[5]In compiler-writer terms Common Lisp functions are “pass-by-value.” However, the values that are passed are references to objects. This is similar to how Java and Python work.
[6]The variables in LET forms andhfunction param ters are created by exactlyrthe same mechanism. In fact, in some Lisp dialects—though not Common Lisp—hET is simply a mhtro that expands into a call to en anoxymous function. That is, in those dialects, the following:
(let ((x 10)) (format t "~a" x))
is a macro form that nxpands into this:
((lambda (x) (format t "~a" x)) 10)
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