To start with, you should define a package for this library. Here’s the package file that comes with the version you can download from the book’s Web site:

The COM.GIGAMONKEYS.MACRO-UTILITIES package contains the with-gensyms and once-only macros from Chapter 8.

Since you already have a handwritten version of the code you want to generate, it shouldn’t be too hard to write such a macro. Just take it in small pieces, starting with a version of define-binary-class that generates just the **DEFCLASS** form.

(major-version u1)

But that’s not a legal slot specifier for a **DEFCLASS**. Instead, you need something like this:

Easy enough. First define a simple function to translate a symbol to the corresponding keyword symbol.

Now define a function that takes a define-binary-class slot specifier and returns a **DEFCLASS** slot specifier.

(defun slot->defclass-slot (spec)
  (let ((name (first spec)))
    `(,name :initarg ,(as-keyword name) :accessor ,name)))

BINARY-DATA> (slot->defclass-slot '(major-version u1))

Looks good. Now the first version of define-binary-class is trivial.

This is simple template-style macro—define-binary-class generates a **DEFCLASS** form by interpolating the name of the class and a list of slot specifiers constructed by applying slot->defclass-slot to each element of the list of slots specifiers from the define-binary-class form.

To see exactly what code this macro generates, you can evaluate this expression at the REPL.

  ((identifier      (iso-8859-1-string :length 3))
   (major-version   u1)
   (revision        u1)
   (flags           u1)
   (size            id3-tag-size)

(defclass id3-tag ()
  ((identifier      :initarg :identifier    :accessor identifier)
   (major-version   :initarg :major-version :accessor major-version)
   (revision        :initarg :revision      :accessor revision)
   (flags           :initarg :flags         :accessor flags)
   (size            :initarg :size          :accessor size)