Книга: Practical Common Lisp

Generating the Expansion

Generating the Expansion

Because do-primes is a fairly simple macro, after you've destructured the arguments, all that's left is to interpolate them into a template to get the expansion.

For simple macros like do-primes, the special backquote syntax is perfect. To review, a backquoted expression is similar to a quoted expression except you can "unquote" particular subexpressions by preceding them with a comma, possibly followed by an at (@) sign. Without an at sign, the comma causes the value of the subexpression to be included as is. With an at sign, the value—which must be a list—is "spliced" into the enclosing list.

Another useful way to think about the backquote syntax is as a particularly concise way of writing code that generates lists. This way of thinking about it has the benefit of being pretty much exactly what's happening under the covers—when the reader reads a backquoted expression, it translates it into code that generates the appropriate list structure. For instance, `(,a b) might be read as (list a 'b). The language standard doesn't specify exactly what code the reader must produce as long as it generates the right list structure.

Table 8-1 shows some examples of backquoted expressions along with equivalent list-building code and the result you'd get if you evaluated either the backquoted expression or the equivalent code.[94]

Table 8-1. Backquote Examples

It's important to note that backquote is just a convenience. But it's a big convenience. To appreciate how big, compare the backquoted version of do-primes to the following version, which uses explicit list-building code:

(defmacro do-primes-a ((var start end) &body body)
(append '(do)
(list (list (list var
(list 'next-prime start)
(list 'next-prime (list '1+ var)))))
(list (list (list '> var end)))
body))

As you'll see in a moment, the current implementation of do-primes doesn't handle certain edge cases correctly. But first you should verify that it at least works for the original example. You can test it in two ways. You can test it indirectly by simply using it—presumably, if the resulting behavior is correct, the expansion is correct. For instance, you can type the original example's use of do-primes to the REPL and see that it indeed prints the right series of prime numbers.

CL-USER> (do-primes (p 0 19) (format t "~d " p))
2 3 5 7 11 13 17 19
NIL

Or you can check the macro directly by looking at the expansion of a particular call. The function MACROEXPAND-1 takes any Lisp expression as an argument and returns the result of doing one level of macro expansion.[95] Because MACROEXPAND-1 is a function, to pass it a literal macro form you must quote it. You can use it to see the expansion of the previous call.[96]

CL-USER> (macroexpand-1 '(do-primes (p 0 19) (format t "~d " p)))
(DO ((P (NEXT-PRIME 0) (NEXT-PRIME (1+ P))))
((> P 19))
(FORMAT T "~d " P))
T

Or, more conveniently, in SLIME you can check a macro's expansion by placing the cursor on the opening parenthesis of a macro form in your source code and typing C-c RET to invoke the Emacs function slime-macroexpand-1, which will pass the macro form to MACROEXPAND-1 and "pretty print" the result in a temporary buffer.

However you get to it, you can see that the result of macro expansion is the same as the original handwritten expansion, so it seems that do-primes works.

Оглавление книги