(defun make-comparison-expr (field value)
(list 'equal (list 'getf 'cd field) value))
You can test it out in the REPL.
CL-USER> (make-comparison-expr :rating 10)
(EQUAL (GETF CD :RATING) 10)
CL-USER> (make-comparison-expr :title "Give Us a Break")
(EQUAL (GETF CD :TITLE) "Give Us a Break")
It turns out that there's an even better way to do it. What you'd really like is a way to write an expression that's mostly not evaluated and then have some way to pick out a few expressions that you do want evaluated. And, of course, there's just such a mechanism. A back quote (`) before an expression stops evaluation just like a forward quote.
CL-USER> `(1 2 3)
(1 2 3)
CL-USER> '(1 2 3)
(1 2 3)
However, in a back-quoted expression, any subexpression that's preceded by a comma is evaluated. Notice the effect of the comma in the second expression:
`(1 2 (+ 1 2)) ==> (1 2 (+ 1 2))
`(1 2 ,(+ 1 2)) ==> (1 2 3)
Using a back quote, you can write make-comparison-expr like this:
(defun make-comparison-expr (field value)
`(equal (getf cd ,field) ,value))
Now if you look back to the hand-optimized selector function, you can see that the body of the function consisted of one comparison expression per field/value pair, all wrapped in an AND expression. Assume for the moment that you'll arrange for the arguments to the where macro to be passed as a single list. You'll need a function that can take the elements of such a list pairwise and collect the results of calling make-comparison-expr on each pair. To implement that function, you can dip into the bag of advanced Lisp tricks and pull out the mighty and powerful LOOP macro.
(defun make-comparisons-list (fields)
(loop while fields
collecting (make-comparison-expr (pop fields) (pop fields))))
A full discussion of LOOP will have to wait until Chapter 22; for now just note that this LOOP expression does exactly what you need: it loops while there are elements left in the fields list, popping off two at a time, passing them to make-comparison-expr, and collecting the results to be returned at the end of the loop. The POP macro performs the inverse operation of the PUSH macro you used to add records to *db*.
Now you just need to wrap up the list returned by make-comparison-list in an AND and an anonymous function, which you can do in the where macro itself. Using a back quote to make a template that you fill in by interpolating the value of make-comparisons-list, it's trivial.
(defmacro where (&rest clauses)
`#'(lambda (cd) (and ,@(make-comparisons-list clauses))))
This macro uses a variant of , (namely, the ,@) before the call to make-comparisons-list. The ,@ "splices" the value of the following expression—which must evaluate to a list—into the enclosing list. You can see the difference between , and ,@ in the following two expressions:
`(and ,(list 1 2 3)) ==> (AND (1 2 3))
`(and ,@(list 1 2 3)) ==> (AND 1 2 3)
You can also use ,@ to splice into the middle of a list.
`(and ,@(list 1 2 3) 4) ==> (AND 1 2 3 4)
The other important feature of the where macro is the use of &rest in the argument list. Like &key, &rest modifies the way arguments are parsed. With a &rest in its parameter list, a function or macro can take an arbitrary number of arguments, which are collected into a single list that becomes the value of the variable whose name follows the &rest. So if you call where like this:
(where :title "Give Us a Break" :ripped t)
the variable clauses will contain the list.
(:title "Give Us a Break" :ripped t)
This list is passed to make-comparisons-list, which returns a list of comparison expressions. You can see exactly what code a call to where will generate using the function MACROEXPAND-1. If you pass MACROEXPAND-1, a form representing a macro call, it will call the macro code with appropriate arguments and return the expansion. So you can check out the previous where call like this:
CL-USER> (macroexpand-1 '(where :title "Give Us a Break" :ripped t))
#'(LAMBDA (CD)
(AND (EQUAL (GETF CD :TITLE) "Give Us a Break")
(EQUAL (GETF CD :RIPPED) T)))
T
Looks good. Let's try it for real.
CL-USER> (select (where :title "Give Us a Break" :ripped t))
((:TITLE "Give Us a Break" :ARTIST "Limpopo" :RATING 10 :RIPPED T))
It works. And the where macro with its two helper functions is actually one line shorter than the old where function. And it's more general in that it's no longer tied to the specific fields in our CD records.
Wrapping Up
Now, an interesting thing has happened. You removed duplication and made the code more efficient and more general at the same time. That's often the way it goes with a well-chosen macro. This makes sense because a macro is just another mechanism for creating abstractions—abstraction at the syntactic level, and abstractions are by definition more concise ways of expressing underlying generalities. Now the only code in the mini-database that's specific to CDs and the fields in them is in the make-cd, prompt-for-cd, and add-cd functions. In fact, our new where macro would work with any plist-based database.
However, this is still far from being a complete database. You can probably think of plenty of features to add, such as supporting multiple tables or more elaborate queries. In Chapter 27 we'll build an MP3 database that incorporates some of those features.
The point of this chapter was to give you a quick introduction to just a handful of Lisp's features and show how they're used to write code that's a bit more interesting than "hello, world." In the next chapter we'll begin a more systematic overview of Lisp.
4. Syntax and Semantics
After that whirlwind tour, we'll settle down for a few chapters to take a more systematic look at the features you've used so far. I'll start with an overview of the basic elements of Lisp's syntax and semantics, which means, of course, that I must first address that burning question. . .
What's with All the Parentheses?
Lisp's syntax is quite a bit different from the syntax of languages descended from Algol. The two most immediately obvious characteristics are the extensive use of parentheses and prefix notation. For whatever reason, a lot of folks are put off by this syntax. Lisp's detractors tend to describe the syntax as "weird" and "annoying." Lisp, they say, must stand for Lots of Irritating Superfluous Parentheses. Lisp folks, on the other hand, tend to consider Lisp's syntax one of its great virtues. How is it that what's so off-putting to one group is a source of delight to another?