[elephant-devel] Quick sketch of type evolution system

[Leslie P. Polzer]

Hello everyone,

to clarify one of the points that are important to me,
I have prepared this working sketch:


(defmethod slot-value-using-class ((class persistent-metaclass) (instance
persistent-object) (slot-def persistent-slot-definition))
  "Get the slot value from the database."
  (let* ((name (slot-definition-name slot-def))
         (value (persistent-slot-reader (get-con instance) instance name))
         (data-type (type-of value)) ; type of data in store
         (slot-type (sb-mop::slot-definition-type slot-def))) ; type declared in defclass
    (format t "slot-value-using-class for slot ~A (declared type '~A');
               value '~A', type '~A'~%"
            name slot-type value data-type)
    (if (typep data-type slot-type)
        value
        #| here we would call the user-provided change-class for the
          old and new types |# )))

Notes:

  * it's obviously implementation dependent as of yet, but this
    is trivial.

  * we should provide a way for the user to specify that this type
    checking shouldn't take place, for performance reasons. This is
    simple, too.

  * as you can see, I don't care much about evolution at the class level
    (class renamed, substituted, ...) right now, but at the slot level.

  * I don't see any other way than slot-value-using-class to catch
    type changes. They can occur at every place in time between
    slot reads.

  * the replacement of slots (one slot discarded, new slot added -- how
    do we connect this?) could be done by a ":replaces" slot initarg.
    But this collides with multiple subsequent replacements, so I think
    the most sensible way would be a new class option named "version"
    of type integer.
    This would facilitate a user-level schema evolution protocol as
    outlined in the recent discussion:

    (defmethod elephant:class-evolve ((class myclass)
      (version (eql 1)) (version (eql 2)))

    [obviously would implement transition 1->2 for class "foo"]


And Ian, thanks for your concise, patient and quick answers.
I thought one could never get *all* three of those :)

  Leslie

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