Practical Common Lisp

       ,@body)))

In a moment I'll explain how the body generates the correct expansion; for now you can just note that the variables var, start, and end each hold a value, extracted from var-and-range, that's then interpolated into the backquote expression that generates do- primes's expansion.

However, you don't need to take apart var-and-range 'by hand' because macro parameter lists are what are called destructuring parameter lists. Destructuring, as the name suggests, involves taking apart a structure—in this case the list structure of the forms passed to a macro.

Within a destructuring parameter list, a simple parameter name can be replaced with a nested parameter list. The parameters in the nested parameter list will take their values from the elements of the expression that would have been bound to the parameter the list replaced. For instance, you can replace var-and-range with a list (var start end), and the three elements of the list will automatically be destructured into those three parameters.

Another special feature of macro parameter lists is that you can use &body as a synonym for &rest. Semantically &body and &rest are equivalent, but many development environments will use the presence of a &body parameter to modify how they indent uses of the macro—typically &body parameters are used to hold a list of forms that make up the body of the macro.

So you can streamline the definition of do-primes and give a hint to both human readers and your development tools about its intended use by defining it like this:

(defmacro do-primes ((var start end) &body body)
  `(do ((,var (next-prime ,start) (next-prime (1+ ,var))))
       ((> ,var ,end))
     ,@body))

In addition to being more concise, destructuring parameter lists also give you automatic error checking—with do-primes defined this way, Lisp will be able to detect a call whose first argument isn't a three- element list and will give you a meaningful error message just as if you had called a function with too few or too many arguments. Also, in development environments such as SLIME that indicate what arguments are expected as soon as you type the name of a function or macro, if you use a destructuring parameter list, the environment will be able to tell you more specifically the syntax of the macro call. With the original definition, SLIME would tell you do-primes is called like this:

(do-primes var-and-range &rest body)

But with the new definition, it can tell you that a call should look like this:

(do-primes (var start end) &body body)

Destructuring parameter lists can contain &optional, &key, and &rest parameters and can contain nested destructuring lists. However, you don't need any of those options to write do- primes.

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

Backquote Syntax	Equivalent List-Building Code	Result
`(a (+ 1 2) c)	(list 'a '(+ 1 2) 'c)	(a (+ 1 2) c)
`(a ,(+ 1 2) c)	(list 'a (+ 1 2) 'c)	(a 3 c)
`(a (list 1 2) c)	(list 'a '(list 1 2) 'c)	(a (list 1 2) c)
`(a ,(list 1 2) c)	(list 'a (list 1 2) 'c)	(a (1 2) c)
`(a ,@(list 1 2) c)	(append (list 'a) (list 1 2) (list 'c))	(a 1 2 c)

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)