But this approach, it seems, will not suffice for . The first argument to the do-primes call is a list containing the name of the loop variable, p; the lower bound, 0; and the upper bound, 19. But if you look at the expansion, the list as a whole doesn’t appear in the expansion; the three element are split up and put in different places.

You could define do-primes with two parameters, one to hold the list and a **&rest** parameter to hold the body forms, and then take apart the list by hand, something like this:

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.

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:

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:

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 .