Like a function, a macro consists of a name, a parameter list, an optional documentation string, and a body of Lisp expressions.1 However, as I just discussed, the job of a macro isn’t to do anything directly—its job is to generate code that will later do what you want.

The job of a macro is to translate a macro form—in other words, a Lisp form whose first element is the name of the macro—into code that does a particular thing. Sometimes you write a macro starting with the code you’d like to be able to write, that is, with an example macro form. Other times you decide to write a macro after you’ve written the same pattern of code several times and realize you can make your code clearer by abstracting the pattern.

Once you have an example call and the desired expansion, you’re ready for the second step: writing the actual macro code. For simple macros this will be a trivial matter of writing a backquoted template with the macro parameters plugged into the right places. Complex macros will be significant programs in their own right, complete with helper functions and data structures.

To sum up, the steps to writing a macro are as follows: