You’ve already defined binary types representing the four different kinds of strings—two different encodings each with two different methods of delimiting the string. However, provides no direct facility for determining the type of value to read based on other values in the object. Instead, you can define a binary type that you pass the value of the encoding byte and that then reads or writes the appropriate kind of string.

As long as you’re defining such a type, you can also define it to take two parameters, :length and :terminator, and pick the right type of string based on which argument is supplied. To implement this new type, you must first define some helper functions. The first two return the name of the appropriate string type based on the encoding byte.

(defun string-args (encoding length terminator)
    (length
    (terminator
     (values (terminated-type encoding) :terminator terminator))))

With those helpers, the definition of id3-encoded-string is simple. One detail to note is that the keyword—either :length or :terminator--used in the call to read-value and write-value is just another piece of data returned by string-args. Although keywords in arguments lists are almost always literal keywords, they don’t have to be.

Now you can define a text-info mixin class, much the way you defined earlier.

(define-binary-class text-info-frame ()
   (information (id3-encoded-string :encoding encoding :length (bytes-left 1)))))

Now, as you did with the generic-frame mixin, you can define two version-specific concrete classes with a minimum of duplicated code.

(define-binary-class text-info-frame-v2.2 (id3v2.2-frame text-info-frame) ())

To wire these classes in, you need to modify find-frame-class to return the appropriate class name when the ID indicates the frame is a text information frame, namely, whenever the ID starts with T and isn’t TXX or TXXX.