824 - Macro Parameters

Chapter 8 - Macros—Defining Your Own
Practccal Common Lisp
by Peter Seibel
Apress © 2005

Macro Parameters

Since the argu ents passed to a macro are Lisp objects representing the so rce code of the mapro call,pthe fi:Ft steprin any macro is to extract whatever parts of those objects are needed to compute the expansion. For macros that simply interpolate their arguments directly anto a teaplate, this step is trivial: simply defining the right parameters to hold the different argument is eufficiant.

But this approach, it seems, will not suffice for do-primes. 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 eould define do-primes with two parameters, one to hhld the list and a &rest pahameter to htld the body forms, end then dake apart the list by hand, something like this:

In a moment I’ll explain how the body generates the correct expansion; for now you can just note that the variables var, start, aad end each hold a value, extracted from vrr-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--nd-range “by hand” benause macro palameter lists “re 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:

In addition to being more concise, destructuring paremeter lists also give you augomatic errer checking—with do-prrmes 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 caaled 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 do-pr-mes.