Practical Common Lisp

the branches appear in the body until one of them evaluates to true. At that point, the remaining forms in that branch are evaluated, and the value of the last form in the branch is returned as the value of the COND as a whole. If the branch contains no forms after the condition, the value of the condition is returned instead. By convention, the branch representing the final else clause in an if/else-if chain is written with a condition of T. Any non- NIL value will work, but a T serves as a useful landmark when reading the code. Thus, you can write the previous nested IF expression using COND like this:

(cond (a (do-x))
      (b (do-y))
      (t (do-z)))

AND, OR, and NOT

When writing the conditions in IF, WHEN, UNLESS, and COND forms, three operators that will come in handy are the boolean logic operators, AND, OR, and NOT.

NOT is a function so strictly speaking doesn't belong in this chapter, but it's closely tied to AND and OR. It takes a single argument and inverts its truth value, returning T if the argument is NIL and NIL otherwise.

AND and OR, however, are macros. They implement logical conjunction and disjunction of any number of subforms and are defined as macros so they can short-circuit. That is, they evaluate only as many of their subforms—in left-to-right order—as necessary to determine the overall truth value. Thus, AND stops and returns NIL as soon as one of its subforms evaluates to NIL. If all the subforms evaluate to non- NIL, it returns the value of the last subform. OR, on the other hand, stops as soon as one of its subforms evaluates to non-NIL and returns the resulting value. If none of the subforms evaluate to true, OR returns NIL. Here are some examples:

(not nil)             ==> T
(not (= 1 1))         ==> NIL
(and (= 1 2) (= 3 3)) ==> NIL
(or (= 1 2) (= 3 3))  ==> T

Looping

Control constructs are the other main kind of looping constructs. Common Lisp's looping facilities are—in addition to being quite powerful and flexible—an interesting lesson in the have-your-cake-and-eat-it-too style of programming that macros provide.

As it turns out, none of Lisp's 25 special operators directly support structured looping. All of Lisp's looping control constructs are macros built on top of a pair of special operators that provide a primitive goto facility.[88] Like many good abstractions, syntactic or otherwise, Lisp's looping macros are built as a set of layered abstractions starting from the base provided by those two special operators.

At the bottom (leaving aside the special operators) is a very general looping construct, DO. While very powerful, DO suffers, as do many general-purpose abstractions, from being overkill for simple situations. So Lisp also provides two other macros, DOLIST and DOTIMES, that are less flexible than DO but provide convenient support for the common cases of looping over the elements of a list and counting loops. While an implementation can implement these macros however it wants, they're typically implemented as macros that expand into an equivalent DO loop. Thus, DO provides a basic structured looping construct on top of the underlying primitives provided by Common Lisp's special operators, and DOLIST and DOTIMES provide two easier-to-use, if less general, constructs. And, as you'll see in the next chapter, you can build your own looping constructs on top of DO for situations where DOLIST and DOTIMES don't meet your needs.

Finally, the LOOP macro provides a full-blown mini-language for expressing looping constructs in a non-Lispy, English-like (or at least Algol-like) language. Some Lisp hackers love LOOP; others hate it. LOOP's fans like it because it provides a concise way to express certain commonly needed looping constructs. Its detractors dislike it because it's not Lispy enough. But whichever side one comes down on, it's a remarkable example of the power of macros to add new constructs to the language.

DOLIST and DOTIMES

I'll start with the easy-to-use DOLIST and DOTIMES macros.

DOLIST loops across the items of a list, executing the loop body with a variable holding the successive items of the list.[89] This is the basic skeleton (leaving out some of the more esoteric options):

(dolist (var list-form)
  body-form*)

When the loop starts, the list-form is evaluated once to produce a list. Then the body of the loop is evaluated once for each item in the list with the variable var holding the value of the item. For instance:

CL-USER> (dolist (x '(1 2 3)) (print x))
1
2
3
NIL

Used this way, the DOLIST form as a whole evaluates to NIL.

If you want to break out of a DOLIST loop before the end of the list, you can use RETURN.

CL-USER> (dolist (x '(1 2 3)) (print x) (if (evenp x) (return)))
1
2