By abstracting out the durability layer, we can ignore (for the moment) the issues of data integrity and compression. We'll assume that the durability layer will provide a suitable level of assurance that the data returned by a call to read-bytes is exactly the same as the data previously written. Furthermore, we'll assume that the durability layer will compress the data if it has a need to.
For our purposes, we only need to serialize data in order to turn it into bytes that can be durably stored. There is one thing to keep in mind, though. We don't know how long we may end up keeping this data. We don't want future readers of the data to have to make too many assumptions about what it is we serialized. We should explicitly tag every datum we store with meta-data that tells us what it is.
Primitive data doesn't need much in the way of metadata. There should be a tag indicating the primitive type and some indication of the version of the serialization format. You might even dispense with the version tag (how many different ways will you be storing integers, anyway?). If you choose a reasonably standard serialization format, it is much more likely that you'll be able to get at your data in the future even if you lose all the information about how it is encoded.
Exercise 2: Pick a binary encoding format and implement an encoder and decoder for these Lisp/Scheme data types: integer (union of fixnum and bignum), character (Unicode code point), float (IEEE 754 double at the very least), UUID (RFC 4122), boolean, rational, complex, string (use UTF-8 unless you have a good reason not to). The decoder should be able to take any sequence of bytes generated by the encoder and return an object equivalent to the input (for an appropriate selection of equivalent). Don't do symbols or cons-cells yet.
For ChangeSafe, I used a single byte tag to encode the primitive serialized data. The tag was chosen by finding a mnemonic ascii code for the data:
(defconstant serialization-code/fixnum (char-code #\#)) (defconstant serialization-code/keyword (char-code #\:)) (defconstant serialization-code/string (char-code #\"))
This allowed me to read serialized data into Emacs and have a few visual hints as to what it meant.
Tip: When you set up an enumeration like this, it is a good idea to define code 0 and code 255 explicitly as illegal values. This will catch a whole slew of accidentally uninitialized data.