Processing of Defining Macros

Defining macros (such as defmacro or defvar) appearing within a file being processed by compile-file normally have compile-time side effects which affect how subsequent forms in the same file are compiled. A convenient model for explaining how these side effects happen is that the defining macro expands into one or more eval-when forms, and that the calls which cause the compile-time side effects to happen appear in the body of an (eval-when (:compile-toplevel) ...) form.

The compile-time side effects may cause information about the definition to be stored differently than if the defining macro had been processed in the ‘normal’ way (either interpretively or by loading the compiled file).

In particular, the information stored by the defining macros at compile time might or might not be available to the interpreter (either during or after compilation), or during subsequent calls to the compiler. For example, the following code is nonportable because it assumes that the compiler stores the macro definition of foo where it is available to the interpreter:

 (defmacro foo (x) `(car ,x))
 (eval-when (:execute :compile-toplevel :load-toplevel)
   (print (foo '(a b c))))

A portable way to do the same thing would be to include the macro definition inside the eval-when form, as in:

 (eval-when (:execute :compile-toplevel :load-toplevel)
   (defmacro foo (x) `(car ,x))
   (print (foo '(a b c))))

Figure 3–8 lists macros that make definitions available both in the compilation and run-time environments. It is not specified whether definitions made available in the compilation environment are available in the evaluation environment, nor is it specified whether they are available in subsequent compilation units or subsequent invocations of the compiler. As with eval-when, these compile-time side effects happen only when the defining macros appear at top level.