Best way to set object slots when loading from file?

From: Björn Lindberg (d95-bli_at_nada.kth.se)
Date: 12/19/03 

Date: 19 Dec 2003 00:11:35 +0100

Consider the following class:

(defclass foo ()
  ((alpha :initarg :alpha :accessor alpha)
   (beta :initarg :beta :accessor beta)))

Now, I want to initialise an instance of that class from a file. At
first, I simply chose the following format for my file:

(foo :alpha "A" :beta "B")
(...)
(...)

Then I could READ one object at a time, and just do
(apply #'make-instance read-object) for each instance. Now, for
various reasons, I want to change my file format to the following:

(foo (alpha "A")
     (beta "B"))
(... etc ...)

Ie, more markup-language like, and I am pondering the best way to
initialise the slots. I can think of a couple of different ways:

1) Convert the first symbol in each subelement (eg alpha, beta) to one
   in the keyword package, append the subelement lists, and then do as
   above. This seems a bit ugly, but is the only way I can see to
   initialise the slots at the same time as creating the object.

2) First create an "empty" instance, and then do
   (setf (slot-value instance slot) value) for each subelement, where
   slot = {alpha, beta}, and value = {"A", "B"} in turn. This has the
   disadvantage that it breaks encapsulation in a way, since
   now the slots are locked. If I later would like to change "alpha"
   so that it is computed instead of being a slot, I cannot.

3) Create an empty instance, and then for each subelement do
   (setf (accessor instance) value), where accessor is the car of the
   subelement (alpha, beta), and value the cadr (in turn). Here I am
   locking the names of the accessors, but it seems slightly better
   than (2).

Is there a "best" way to do what I want?

Björn

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