# HG changeset patch
# User Steve Losh <steve@stevelosh.com>
# Date 1650331350 14400
# Node ID edeb31bc40ccb161386fa4353250481e1f139e89
# Parent  5510909795e69c4568a4a764e681bbefb75dbdeb
Rename draft post

diff -r 5510909795e6 -r edeb31bc40cc content/blog/2021/04/clos-dependent-maintenance.markdown
--- a/content/blog/2021/04/clos-dependent-maintenance.markdown	Mon Apr 18 21:22:22 2022 -0400
+++ /dev/null	Thu Jan 01 00:00:00 1970 +0000
@@ -1,496 +0,0 @@
-(:title "Using the CLOS Dependent Maintenance Protocol"
- :snip "Handling all the edge cases."
- :date "2021-04-26T16:10:00Z"
- :draft t)
-
-TODO: fix times!!!!!!!!!!!!!!!!!1
-
-Recently I've been working on a Common Lisp library that makes use of the
-[Metaobject Protocol](http://metamodular.com/CLOS-MOP/).  I ran into a few edge
-cases and it took a while for me to figure out how to solve them, so I wanted to
-write down what I learned in case anyone else might find it useful.  This post
-is an expanded version of [a Reddit thread][reddit] I posted on the topic
-a while ago.
-
-[reddit]: https://www.reddit.com/r/Common_Lisp/comments/kljyg1/need_advice_on_how_to_handle_metaclass_option/
-
-<div id="toc"></div>
-
-## Setting the Stage
-
-Before we can see the problem, we need a simple example.  We'll use the
-`monitored-class` metaclass from [The Art of the Metaobject Protocol][amop]
-(pages 96-97). Using this class as a metaclass will log all slot reads and
-writes, which could be useful for auditing access to certain objects.
-[amop]: https://en.wikipedia.org/wiki/The_Art_of_the_Metaobject_Protocol
-
-Before we get started, we'll need [Closer to MOP][closer-mop] as an
-implementation compatibility layer:
-
-[closer-mop]: https://github.com/pcostanza/closer-mop
-
-```lisp
-(ql:quickload :closer-mop)
-```
-
-First we define the metaclass:
-
-```lisp
-(defclass monitored-class (standard-class)
-  ())
-```
-
-Next we'll explicitly say that it's okay for a monitored class to have
-superclasses that are standard classes:
-
-```lisp
-(defmethod c2mop:validate-superclass
-    ((class monitored-class) (superclass standard-class))
-  t)
-```
-
-And now we can define the actual monitoring functionality.  We'll use `:before`
-methods on `slot-value-using-class` and its `setf` version to log the reads and
-writes of all slots:
-
-```lisp
-(defmethod c2mop:slot-value-using-class :before
-    ((class monitored-class) instance slot)
-  (format t "Reading slot ~A of ~A at ~A.~%"
-          (c2mop:slot-definition-name slot) instance (get-universal-time)))
-
-(defmethod (setf c2mop:slot-value-using-class) :before
-    (new-value (class monitored-class) instance slot)
-  (format t "Setting slot ~A of ~A to ~S at ~A.~%"
-          (c2mop:slot-definition-name slot) instance new-value (get-universal-time)))
-```
-
-With that complete, we can define a new monitored class:
-
-```lisp
-(defclass user ()
-  ((id   :initarg :id   :accessor id)
-   (name :initarg :name :accessor name))
-  (:metaclass monitored-class))
-```
-
-And now we can see it in action:
-
-```lisp
-(defparameter *u* (make-instance 'user :id 1 :name "sjl"))
-; => Setting slot ID of #<USER {10074DFD33}> to 1 at 3828527923.
-; => Setting slot NAME of #<USER {10074DFD33}> to "sjl" at 3828527923.
-; => *U*
-
-(id *u*)
-; => Reading slot ID of #<USER {10074DFD33}> at 3828527937.
-; => 1
-
-(setf (name *u*) "Steve")
-; => Setting slot NAME of #<USER {10074DFD33}> to "Steve" at 3828527946.
-; => "Steve"
-```
-
-## Adding More Flexibility
-
-Now that we have a toy example working, let's make it a little more flexible.
-Instead of always generating a string and writing it to standard out, we'll
-allow users to provide a `:monitoring-function` as a class option that will
-receive the data and can do whatever it wants, for example:
-
-* Logging to syslog instead of standard out.
-* Inserting a row into a Postgres database as an audit log.
-* Tracking read/write counts in a hash table to find slots that are written more
-  often than they're read and vice versa.
-
-A monitoring function will receive 2 arguments (the instance and slot name),
-plus an optional third argument when a slot is written (the new value).  We can
-make a default monitoring function that works the same way as before:
-
-```lisp
-(defun log-slot-access (instance slot-name &optional (new-value nil new-value?))
-  (if new-value?
-    (format t "Setting slot ~A of ~A to ~S at ~A.~%"
-              slot-name instance new-value (get-universal-time))
-    (format t "Reading slot ~A of ~A at ~A.~%"
-              slot-name instance (get-universal-time))))
-```
-
-Note the use of the extended `&optional` form with the
-[`supplied-p-parameter`][supplied] used to check whether a value was given,
-which ensures this works correctly even when setting a slot to `nil`.
-
-[supplied]: http://www.lispworks.com/documentation/HyperSpec/Body/03_dab.htm
-
-Also note how we called it `new-value?` and not `new-value-p` as you'll
-sometimes see people do.  The `-p` in `new-value-p` stands for "predicate", and
-a [predicate][predicate] is a *function* that returns a (generalized) boolean,
-*not* a boolean itself.  Using a name that ends in `-p` for a boolean value
-(rather than for a predicate) is a pet peeve of mine.  Unfortunately it happens
-in a couple of places (even in Common Lisp itself), so it's something to watch
-out for.
-
-[predicate]: http://www.lispworks.com/documentation/HyperSpec/Body/26_glo_p.htm#predicate
-
-Now we can update our `monitored-class` to add a slot to store the monitoring
-function for each class, and update the `slot-value-using-class` methods to use
-that instead of writing the string themselves:
-
-```lisp
-(defclass monitored-class (standard-class)
-  ((monitoring-function :initarg :monitoring-function
-                        :accessor monitoring-function)))
-
-(defmethod c2mop:slot-value-using-class :before
-    ((class monitored-class) instance slot)
-  (funcall (monitoring-function class)
-           instance
-           (c2mop:slot-definition-name slot)))
-
-(defmethod (setf c2mop:slot-value-using-class) :before
-    (new-value (class monitored-class) instance slot)
-  (funcall (monitoring-function class)
-           instance
-           (c2mop:slot-definition-name slot)
-           new-value))
-```
-
-
-When a user creates a new `monitored-class`, we need to set the
-`monitoring-function` slot appropriately.  We might initially consider doing
-this by having an `initform` for the `monitoring-function` slot in the
-metaclass, like this:
-
-```lisp
-(defclass monitored-class (standard-class)
-  ((monitoring-function :initarg :monitoring-function
-                        :accessor monitoring-function
-                        :initform #'log-slot-access))) ; default function
-```
-
-But this won't work for a number of reasons we'll see shortly.  Instead we'll
-need to handle the initialization ourselves.  We'll do it in `shared-initialize`
-so it will happen both when a class is first created and when it's reinitialized
-(e.g. after it's redefined):
-
-```lisp
-(defun parse-monitoring-function-class-option (arguments)
-  (case (length arguments)
-    (1 (eval (first arguments)))
-    (0 #'log-slot-access)
-    (t (error "Malformed class option ~S."
-              (list* :monitoring-function arguments)))))
-
-(defmethod shared-initialize :around
-  ((class monitored-class) slot-names
-   &rest initargs
-   &key monitoring-function &allow-other-keys)
-  (apply #'call-next-method class slot-names
-         :monitoring-function (parse-monitoring-function-class-option
-                                monitoring-function)
-         initargs))
-```
-
-If the user provided a `(:monitoring-function …)` class option we evaluate and
-use it, otherwise we default to our simple logging function.
-
-There are a couple of things to note here.
-
-First, when `defclass` gets a class option like `(:monitoring-function foo)`,
-what it *actually* passes to the `(re)initialize-instance` methods is the list
-`(foo)`.  This allows for class options with more than one argument.  In our
-case we only ever want a single argument, so we ensure the `length` of the
-argument is `0` or `1` and handle the cases individually.
-
-Second, `defclass` does not evaluate the class option's arguments.  If we say
-`(:monitoring-function (lambda (i s &optional v) (print (list i s v))))` what we
-get as the initarg will be `((lambda (i s &optional v) (print (list i s v))))`.
-That's a list of a list of three elements, *not* a list of an actual function
-object.  If we want the arguments to be evaluated, we have to do it ourselves.
-Unfortunately as far as I can tell there's no way to evaluate these arguments
-from `defclass` in their lexical environment — we have to fall back to `eval`
-and the null lexical environment.  That means that something like this will not
-work:
-
-```lisp
-(flet ((monitor (instance slot-name &optional new-value)
-          …))
-  (defclass foo ()
-    (…slots…)
-    (:monitoring-function #'monitor)))
-```
-
-I haven't managed to find a way to make this work with `defclass`.  If anyone
-knows of a solution, please let me know.
-
-Third, you might notice that we're `apply`ing with the full `initargs` list,
-which includes the original (unparsed) `monitoring-function`.  But that keyword
-argument will be shadowed by the `:monitoring-function` we add at the beginning,
-so there's no need to bother removing it from `initargs` before we apply (though
-it wouldn't hurt to do so).  This is another Common Lisp idiom you'll see here
-and there when someone wants to override a single keyword argument but preserve
-all the rest.
-
-Now we can now talk about all the reasons why `:initform #'log-slot-access`
-doesn't just magically solve all our problems.
-
-First, the `:initform` *would* work properly when you first define a class, but
-we still need all the code in `shared-initialize` to do the `eval`ing of the
-forms the user provides when they *don't* use it.
-
-Further, suppose a user runs:
-
-```lisp
-(defclass foo ()
-  ((some-slot :initarg :some-slot :accessor some-slot))
-  (:metaclass monitored-class)
-  (:monitoring-function monitor-foo))
-```
-
-Then later they *remove* the `:monitoring-function` from the `defclass` and
-reevaluate it:
-
-```lisp
-(defclass foo ()
-  ((some-slot :initarg :some-slot :accessor some-slot))
-  (:metaclass monitored-class))
-```
-
-What the user (probably) expects here is for the class to have the default
-monitoring function.  This is what will happen if they start a fresh Lisp image
-and load the current code into it.  But if we had just used `:initform`, the
-class would already have a value for the `monitoring-function` slot (the old
-function) and since there's no *new* value being specified, the `:initform`
-would never be used and nothing would get updated, so the class would continue
-to use the old monitoring function.  The user would have to clean things up
-manually by killing the class with `(setf (find-class 'foo) nil)` and
-reevaluating the `defclass`, or fixing the slot value up manually, or some other
-ugly alternative.
-
-With all that out of the way, we can now use a custom `monitoring-function` to
-do whatever we want:
-
-```lisp
-(defparameter *user-slot-reads* (make-hash-table))
-(defparameter *user-slot-writes* (make-hash-table))
-
-(defun track-user-slot-access
-    (instance slot-name &optional (new-value nil new-value?))
-  (declare (ignore instance new-value))
-  (incf (gethash slot-name
-                 (if new-value? *user-slot-writes* *user-slot-reads*)
-                 0)))
-
-(defclass user ()
-  ((id   :initarg :id   :accessor id)
-   (name :initarg :name :accessor name))
-  (:metaclass monitored-class)
-  (:monitoring-function #'track-user-slot-access))
-
-;; Two writes
-(defparameter *u* (make-instance 'user :id 1 :name "sjl"))
-
-;; A read
-(id *u*)
-
-;; Two more writes
-(setf (name *u*) "steve")
-(setf (name *u*) "sjl")
-
-;; Results
-(alexandria:hash-table-alist *user-slot-reads*)
-; => ((ID . 1))
-
-(alexandria:hash-table-alist *user-slot-writes*)
-; => ((NAME . 3) (ID . 1))
-```
-
-## Toy Example Disclaimer™
-
-The `monitored-class` example we've used so far is pretty small, and there are
-a number of other ways we could accomplish the same thing, some of which might
-not involve metaclasses at all.  This might make my example seem overly
-complicated.
-
-I wanted to keep the example small so I can focus on the actual problem I ran
-into without getting bogged down in too many irrelevant details about a specific
-implementation.  If you're bothered by how we're using metaclasses here when
-there are other ways to implement this toy example, feel free to implement
-a more extensive `monitored-class` variant as an exercise:
-
-```lisp
-(defclass user ()
-  ((id …)
-   ;; Never monitor this slot:
-   (session-id … :monitored nil)
-   ;; We only care when this slot *changes*:
-   (role … :monitored/reads nil)
-   ;; Names are PII, redact their values before logging:
-   (name … :monitored/redact-value t)
-   ;; Redact the user portion of the email address, logging only the domain:
-   (email … :monitored/redact-value #'scrub-email))
-  (:metaclass monitored-class)
-  (:monitoring-function #'log-to-syslog)
-  ;; Allow us to turn monitoring on/off globally:
-  (:monitor-when #'monitoring-enabled-p))
-```
-
-## The Problem
-
-```lisp
-(defclass paid-user (user)
-  ((plan :initarg :plan
-         :accessor plan
-         :type (member :bronze :silver :gold)))
-  (:metaclass monitored-class))
-
-(make-instance 'paid-user :id 2 :name "moneybags" :plan :gold)
-; =>
-; ??????????
-```
-
-
-```lisp
-(defclass user ()
-  (…slots…)
-  (:metaclass monitored-class)
-  (:monitoring-function yell))
-
-; inherits YELL as the monitoring-function from the superclass
-(defclass paid-user (user)
-  (…slots…)
-  (:metaclass monitored-class))
-
-; actually let's log user slot accesses to Postgres for more safety
-(defclass user ()
-  (…slots…)
-  (:metaclass monitored-class)
-  (:monitoring-function log-to-postgres))
-```
-
-## The Dependent Maintenance Protocol
-
-<http://metamodular.com/CLOS-MOP/dependent-maintenance-protocol.html>
-
-
-```lisp
-(ql:quickload :closer-mop)
-
-(defclass dep ()
-  ((val :accessor val :initarg :val)))
-
-(defclass mc (standard-class)
-  ((x :accessor x)
-   ;; We need to not only store the value, but also store what the user
-   ;; originally gave, so we can recompute later if one of our superclasses
-   ;; changes.
-   ;;
-   ;; This slot will be unbound if the user didn't give an (:x foo) option, or
-   ;; bound to (foo) if they did.
-   (given-x :initarg :x :accessor given-x)))
-
-(defun mcp (object)
-  (typep object 'mc))
-
-(defmethod c2mop:validate-superclass ((a mc) (b standard-class)) t)
-
-(defun dep= (d class)
-  (and (typep d 'dep) ; need to filter out any other deps other code might have added
-       (eql (val d) class)))
-
-(defun ensure-dep (superclass class)
-  (c2mop:map-dependents superclass
-                        (lambda (d)
-                          (when (dep= d class)
-                            (return-from ensure-dep))))
-  (c2mop:add-dependent superclass (make-instance 'dep :val class)))
-
-(defun ensure-no-dep (superclass class)
-  (c2mop:map-dependents superclass
-                        (lambda (d)
-                          (when (dep= d class)
-                            (c2mop:remove-dependent superclass d)
-                            (return-from ensure-no-dep)))))
-
-(defun recompute (&key class superclasses slot given-slot value value? default)
-  (format *debug-io* "~%Recomputing ~A of ~A." slot class)
-  (setf superclasses (remove-if-not #'mcp superclasses))
-  ;; We need to store whether the user gave an explicit value for later.
-  (if value?
-    (setf (slot-value class given-slot) value)
-    (slot-makunbound class given-slot))
-  ;; Set the actual value to the given value, or the superclass value, or the default.
-  ;;
-  ;; I THINK we only need to look at direct superclasses, not the entire class
-  ;; precedence list, because while it's possible for an MC to inherit from
-  ;; a standard-class, the reverse is not possible.  So the only way for an MC
-  ;; to get into the precedence list is to be there directly or come through
-  ;; another MC that IS direct (and which would come first in the full list
-  ;; anyway).
-  (setf (slot-value class slot)
-        (cond
-          (value? (first value))
-          (superclasses (slot-value (first superclasses) slot))
-          (t default))))
-
-
-(defmethod initialize-instance :around ((class mc) &key
-                                        (x nil x?)
-                                        direct-superclasses &allow-other-keys)
-  (recompute :class class
-             :superclasses direct-superclasses
-             :slot 'x
-             :given-slot 'given-x
-             :value x
-             :value? x?
-             :default :some-default)
-  (call-next-method)
-  ;; You might think we could get away with only having dependencies on
-  ;; superclasses that happen to be our specific metaclass, instead of on ALL
-  ;; direct superclasses.  Sadly this fails for forward-referenced classes, so
-  ;; we need to add dependencies on all of them and filter out the non-MC
-  ;; classes later.
-  (dolist (superclass (c2mop:class-direct-superclasses class))
-    (ensure-dep superclass class)))
-
-(defmethod reinitialize-instance :around ((class mc) &key
-                                          (x nil x?)
-                                          (direct-superclasses nil direct-superclasses?)
-                                          &allow-other-keys)
-  ;; We have to recompute X /before/ we call-next-method because the
-  ;; update-dependent calls happen as part of that next method.  If we wait
-  ;; until after call-next-method to patch up X, then the dependent will still
-  ;; see the old version when it's updated and won't get the new value until
-  ;; a second round of initialization.
-  (recompute :class class
-             :superclasses (if direct-superclasses?
-                             direct-superclasses
-                             (c2mop:class-direct-superclasses class))
-             :slot 'x
-             :given-slot 'given-x
-             :value x
-             :value? x?
-             :default :some-default)
-  (let ((before (c2mop:class-direct-superclasses class)))
-    (call-next-method)
-    (let* ((after (c2mop:class-direct-superclasses class))
-           (removed (set-difference before after))
-           (added (set-difference after before)))
-      (dolist (superclass removed)
-        (ensure-no-dep superclass class))
-      (dolist (superclass added)
-        (ensure-dep superclass class)))))
-
-(defmethod c2mop:update-dependent (obj (dep dep) &rest initargs)
-  (declare (ignore initargs))
-  (when (typep obj 'mc) ; We can ignore changes in non-MC superclasses here.
-    (let ((class (val dep)))
-      (format *debug-io* "~%Updating ~A because ~A changed." class obj)
-      ;; Need to call reinitialize-instance here (instead of just recomputing the
-      ;; slots) because otherwise transitive dependencies won't get updated
-      ;; properly.
-      (apply #'reinitialize-instance class
-             (if (slot-boundp class 'given-x)
-               (list :x (slot-value class 'given-x))
-               '())))))
-```
diff -r 5510909795e6 -r edeb31bc40cc content/blog/2022/04/clos-dependent-maintenance.markdown
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/content/blog/2022/04/clos-dependent-maintenance.markdown	Mon Apr 18 21:22:30 2022 -0400
@@ -0,0 +1,496 @@
+(:title "Using the CLOS Dependent Maintenance Protocol"
+ :snip "Handling all the edge cases."
+ :date "2022-04-26T16:10:00Z"
+ :draft t)
+
+TODO: fix times!!!!!!!!!!!!!!!!!1
+
+Recently I've been working on a Common Lisp library that makes use of the
+[Metaobject Protocol](http://metamodular.com/CLOS-MOP/).  I ran into a few edge
+cases and it took a while for me to figure out how to solve them, so I wanted to
+write down what I learned in case anyone else might find it useful.  This post
+is an expanded version of [a Reddit thread][reddit] I posted on the topic
+a while ago.
+
+[reddit]: https://www.reddit.com/r/Common_Lisp/comments/kljyg1/need_advice_on_how_to_handle_metaclass_option/
+
+<div id="toc"></div>
+
+## Setting the Stage
+
+Before we can see the problem, we need a simple example.  We'll use the
+`monitored-class` metaclass from [The Art of the Metaobject Protocol][amop]
+(pages 96-97). Using this class as a metaclass will log all slot reads and
+writes, which could be useful for auditing access to certain objects.
+[amop]: https://en.wikipedia.org/wiki/The_Art_of_the_Metaobject_Protocol
+
+Before we get started, we'll need [Closer to MOP][closer-mop] as an
+implementation compatibility layer:
+
+[closer-mop]: https://github.com/pcostanza/closer-mop
+
+```lisp
+(ql:quickload :closer-mop)
+```
+
+First we define the metaclass:
+
+```lisp
+(defclass monitored-class (standard-class)
+  ())
+```
+
+Next we'll explicitly say that it's okay for a monitored class to have
+superclasses that are standard classes:
+
+```lisp
+(defmethod c2mop:validate-superclass
+    ((class monitored-class) (superclass standard-class))
+  t)
+```
+
+And now we can define the actual monitoring functionality.  We'll use `:before`
+methods on `slot-value-using-class` and its `setf` version to log the reads and
+writes of all slots:
+
+```lisp
+(defmethod c2mop:slot-value-using-class :before
+    ((class monitored-class) instance slot)
+  (format t "Reading slot ~A of ~A at ~A.~%"
+          (c2mop:slot-definition-name slot) instance (get-universal-time)))
+
+(defmethod (setf c2mop:slot-value-using-class) :before
+    (new-value (class monitored-class) instance slot)
+  (format t "Setting slot ~A of ~A to ~S at ~A.~%"
+          (c2mop:slot-definition-name slot) instance new-value (get-universal-time)))
+```
+
+With that complete, we can define a new monitored class:
+
+```lisp
+(defclass user ()
+  ((id   :initarg :id   :accessor id)
+   (name :initarg :name :accessor name))
+  (:metaclass monitored-class))
+```
+
+And now we can see it in action:
+
+```lisp
+(defparameter *u* (make-instance 'user :id 1 :name "sjl"))
+; => Setting slot ID of #<USER {10074DFD33}> to 1 at 3828527923.
+; => Setting slot NAME of #<USER {10074DFD33}> to "sjl" at 3828527923.
+; => *U*
+
+(id *u*)
+; => Reading slot ID of #<USER {10074DFD33}> at 3828527937.
+; => 1
+
+(setf (name *u*) "Steve")
+; => Setting slot NAME of #<USER {10074DFD33}> to "Steve" at 3828527946.
+; => "Steve"
+```
+
+## Adding More Flexibility
+
+Now that we have a toy example working, let's make it a little more flexible.
+Instead of always generating a string and writing it to standard out, we'll
+allow users to provide a `:monitoring-function` as a class option that will
+receive the data and can do whatever it wants, for example:
+
+* Logging to syslog instead of standard out.
+* Inserting a row into a Postgres database as an audit log.
+* Tracking read/write counts in a hash table to find slots that are written more
+  often than they're read and vice versa.
+
+A monitoring function will receive 2 arguments (the instance and slot name),
+plus an optional third argument when a slot is written (the new value).  We can
+make a default monitoring function that works the same way as before:
+
+```lisp
+(defun log-slot-access (instance slot-name &optional (new-value nil new-value?))
+  (if new-value?
+    (format t "Setting slot ~A of ~A to ~S at ~A.~%"
+              slot-name instance new-value (get-universal-time))
+    (format t "Reading slot ~A of ~A at ~A.~%"
+              slot-name instance (get-universal-time))))
+```
+
+Note the use of the extended `&optional` form with the
+[`supplied-p-parameter`][supplied] used to check whether a value was given,
+which ensures this works correctly even when setting a slot to `nil`.
+
+[supplied]: http://www.lispworks.com/documentation/HyperSpec/Body/03_dab.htm
+
+Also note how we called it `new-value?` and not `new-value-p` as you'll
+sometimes see people do.  The `-p` in `new-value-p` stands for "predicate", and
+a [predicate][predicate] is a *function* that returns a (generalized) boolean,
+*not* a boolean itself.  Using a name that ends in `-p` for a boolean value
+(rather than for a predicate) is a pet peeve of mine.  Unfortunately it happens
+in a couple of places (even in Common Lisp itself), so it's something to watch
+out for.
+
+[predicate]: http://www.lispworks.com/documentation/HyperSpec/Body/26_glo_p.htm#predicate
+
+Now we can update our `monitored-class` to add a slot to store the monitoring
+function for each class, and update the `slot-value-using-class` methods to use
+that instead of writing the string themselves:
+
+```lisp
+(defclass monitored-class (standard-class)
+  ((monitoring-function :initarg :monitoring-function
+                        :accessor monitoring-function)))
+
+(defmethod c2mop:slot-value-using-class :before
+    ((class monitored-class) instance slot)
+  (funcall (monitoring-function class)
+           instance
+           (c2mop:slot-definition-name slot)))
+
+(defmethod (setf c2mop:slot-value-using-class) :before
+    (new-value (class monitored-class) instance slot)
+  (funcall (monitoring-function class)
+           instance
+           (c2mop:slot-definition-name slot)
+           new-value))
+```
+
+
+When a user creates a new `monitored-class`, we need to set the
+`monitoring-function` slot appropriately.  We might initially consider doing
+this by having an `initform` for the `monitoring-function` slot in the
+metaclass, like this:
+
+```lisp
+(defclass monitored-class (standard-class)
+  ((monitoring-function :initarg :monitoring-function
+                        :accessor monitoring-function
+                        :initform #'log-slot-access))) ; default function
+```
+
+But this won't work for a number of reasons we'll see shortly.  Instead we'll
+need to handle the initialization ourselves.  We'll do it in `shared-initialize`
+so it will happen both when a class is first created and when it's reinitialized
+(e.g. after it's redefined):
+
+```lisp
+(defun parse-monitoring-function-class-option (arguments)
+  (case (length arguments)
+    (1 (eval (first arguments)))
+    (0 #'log-slot-access)
+    (t (error "Malformed class option ~S."
+              (list* :monitoring-function arguments)))))
+
+(defmethod shared-initialize :around
+  ((class monitored-class) slot-names
+   &rest initargs
+   &key monitoring-function &allow-other-keys)
+  (apply #'call-next-method class slot-names
+         :monitoring-function (parse-monitoring-function-class-option
+                                monitoring-function)
+         initargs))
+```
+
+If the user provided a `(:monitoring-function …)` class option we evaluate and
+use it, otherwise we default to our simple logging function.
+
+There are a couple of things to note here.
+
+First, when `defclass` gets a class option like `(:monitoring-function foo)`,
+what it *actually* passes to the `(re)initialize-instance` methods is the list
+`(foo)`.  This allows for class options with more than one argument.  In our
+case we only ever want a single argument, so we ensure the `length` of the
+argument is `0` or `1` and handle the cases individually.
+
+Second, `defclass` does not evaluate the class option's arguments.  If we say
+`(:monitoring-function (lambda (i s &optional v) (print (list i s v))))` what we
+get as the initarg will be `((lambda (i s &optional v) (print (list i s v))))`.
+That's a list of a list of three elements, *not* a list of an actual function
+object.  If we want the arguments to be evaluated, we have to do it ourselves.
+Unfortunately as far as I can tell there's no way to evaluate these arguments
+from `defclass` in their lexical environment — we have to fall back to `eval`
+and the null lexical environment.  That means that something like this will not
+work:
+
+```lisp
+(flet ((monitor (instance slot-name &optional new-value)
+          …))
+  (defclass foo ()
+    (…slots…)
+    (:monitoring-function #'monitor)))
+```
+
+I haven't managed to find a way to make this work with `defclass`.  If anyone
+knows of a solution, please let me know.
+
+Third, you might notice that we're `apply`ing with the full `initargs` list,
+which includes the original (unparsed) `monitoring-function`.  But that keyword
+argument will be shadowed by the `:monitoring-function` we add at the beginning,
+so there's no need to bother removing it from `initargs` before we apply (though
+it wouldn't hurt to do so).  This is another Common Lisp idiom you'll see here
+and there when someone wants to override a single keyword argument but preserve
+all the rest.
+
+Now we can now talk about all the reasons why `:initform #'log-slot-access`
+doesn't just magically solve all our problems.
+
+First, the `:initform` *would* work properly when you first define a class, but
+we still need all the code in `shared-initialize` to do the `eval`ing of the
+forms the user provides when they *don't* use it.
+
+Further, suppose a user runs:
+
+```lisp
+(defclass foo ()
+  ((some-slot :initarg :some-slot :accessor some-slot))
+  (:metaclass monitored-class)
+  (:monitoring-function monitor-foo))
+```
+
+Then later they *remove* the `:monitoring-function` from the `defclass` and
+reevaluate it:
+
+```lisp
+(defclass foo ()
+  ((some-slot :initarg :some-slot :accessor some-slot))
+  (:metaclass monitored-class))
+```
+
+What the user (probably) expects here is for the class to have the default
+monitoring function.  This is what will happen if they start a fresh Lisp image
+and load the current code into it.  But if we had just used `:initform`, the
+class would already have a value for the `monitoring-function` slot (the old
+function) and since there's no *new* value being specified, the `:initform`
+would never be used and nothing would get updated, so the class would continue
+to use the old monitoring function.  The user would have to clean things up
+manually by killing the class with `(setf (find-class 'foo) nil)` and
+reevaluating the `defclass`, or fixing the slot value up manually, or some other
+ugly alternative.
+
+With all that out of the way, we can now use a custom `monitoring-function` to
+do whatever we want:
+
+```lisp
+(defparameter *user-slot-reads* (make-hash-table))
+(defparameter *user-slot-writes* (make-hash-table))
+
+(defun track-user-slot-access
+    (instance slot-name &optional (new-value nil new-value?))
+  (declare (ignore instance new-value))
+  (incf (gethash slot-name
+                 (if new-value? *user-slot-writes* *user-slot-reads*)
+                 0)))
+
+(defclass user ()
+  ((id   :initarg :id   :accessor id)
+   (name :initarg :name :accessor name))
+  (:metaclass monitored-class)
+  (:monitoring-function #'track-user-slot-access))
+
+;; Two writes
+(defparameter *u* (make-instance 'user :id 1 :name "sjl"))
+
+;; A read
+(id *u*)
+
+;; Two more writes
+(setf (name *u*) "steve")
+(setf (name *u*) "sjl")
+
+;; Results
+(alexandria:hash-table-alist *user-slot-reads*)
+; => ((ID . 1))
+
+(alexandria:hash-table-alist *user-slot-writes*)
+; => ((NAME . 3) (ID . 1))
+```
+
+## Toy Example Disclaimer™
+
+The `monitored-class` example we've used so far is pretty small, and there are
+a number of other ways we could accomplish the same thing, some of which might
+not involve metaclasses at all.  This might make my example seem overly
+complicated.
+
+I wanted to keep the example small so I can focus on the actual problem I ran
+into without getting bogged down in too many irrelevant details about a specific
+implementation.  If you're bothered by how we're using metaclasses here when
+there are other ways to implement this toy example, feel free to implement
+a more extensive `monitored-class` variant as an exercise:
+
+```lisp
+(defclass user ()
+  ((id …)
+   ;; Never monitor this slot:
+   (session-id … :monitored nil)
+   ;; We only care when this slot *changes*:
+   (role … :monitored/reads nil)
+   ;; Names are PII, redact their values before logging:
+   (name … :monitored/redact-value t)
+   ;; Redact the user portion of the email address, logging only the domain:
+   (email … :monitored/redact-value #'scrub-email))
+  (:metaclass monitored-class)
+  (:monitoring-function #'log-to-syslog)
+  ;; Allow us to turn monitoring on/off globally:
+  (:monitor-when #'monitoring-enabled-p))
+```
+
+## The Problem
+
+```lisp
+(defclass paid-user (user)
+  ((plan :initarg :plan
+         :accessor plan
+         :type (member :bronze :silver :gold)))
+  (:metaclass monitored-class))
+
+(make-instance 'paid-user :id 2 :name "moneybags" :plan :gold)
+; =>
+; ??????????
+```
+
+
+```lisp
+(defclass user ()
+  (…slots…)
+  (:metaclass monitored-class)
+  (:monitoring-function yell))
+
+; inherits YELL as the monitoring-function from the superclass
+(defclass paid-user (user)
+  (…slots…)
+  (:metaclass monitored-class))
+
+; actually let's log user slot accesses to Postgres for more safety
+(defclass user ()
+  (…slots…)
+  (:metaclass monitored-class)
+  (:monitoring-function log-to-postgres))
+```
+
+## The Dependent Maintenance Protocol
+
+<http://metamodular.com/CLOS-MOP/dependent-maintenance-protocol.html>
+
+
+```lisp
+(ql:quickload :closer-mop)
+
+(defclass dep ()
+  ((val :accessor val :initarg :val)))
+
+(defclass mc (standard-class)
+  ((x :accessor x)
+   ;; We need to not only store the value, but also store what the user
+   ;; originally gave, so we can recompute later if one of our superclasses
+   ;; changes.
+   ;;
+   ;; This slot will be unbound if the user didn't give an (:x foo) option, or
+   ;; bound to (foo) if they did.
+   (given-x :initarg :x :accessor given-x)))
+
+(defun mcp (object)
+  (typep object 'mc))
+
+(defmethod c2mop:validate-superclass ((a mc) (b standard-class)) t)
+
+(defun dep= (d class)
+  (and (typep d 'dep) ; need to filter out any other deps other code might have added
+       (eql (val d) class)))
+
+(defun ensure-dep (superclass class)
+  (c2mop:map-dependents superclass
+                        (lambda (d)
+                          (when (dep= d class)
+                            (return-from ensure-dep))))
+  (c2mop:add-dependent superclass (make-instance 'dep :val class)))
+
+(defun ensure-no-dep (superclass class)
+  (c2mop:map-dependents superclass
+                        (lambda (d)
+                          (when (dep= d class)
+                            (c2mop:remove-dependent superclass d)
+                            (return-from ensure-no-dep)))))
+
+(defun recompute (&key class superclasses slot given-slot value value? default)
+  (format *debug-io* "~%Recomputing ~A of ~A." slot class)
+  (setf superclasses (remove-if-not #'mcp superclasses))
+  ;; We need to store whether the user gave an explicit value for later.
+  (if value?
+    (setf (slot-value class given-slot) value)
+    (slot-makunbound class given-slot))
+  ;; Set the actual value to the given value, or the superclass value, or the default.
+  ;;
+  ;; I THINK we only need to look at direct superclasses, not the entire class
+  ;; precedence list, because while it's possible for an MC to inherit from
+  ;; a standard-class, the reverse is not possible.  So the only way for an MC
+  ;; to get into the precedence list is to be there directly or come through
+  ;; another MC that IS direct (and which would come first in the full list
+  ;; anyway).
+  (setf (slot-value class slot)
+        (cond
+          (value? (first value))
+          (superclasses (slot-value (first superclasses) slot))
+          (t default))))
+
+
+(defmethod initialize-instance :around ((class mc) &key
+                                        (x nil x?)
+                                        direct-superclasses &allow-other-keys)
+  (recompute :class class
+             :superclasses direct-superclasses
+             :slot 'x
+             :given-slot 'given-x
+             :value x
+             :value? x?
+             :default :some-default)
+  (call-next-method)
+  ;; You might think we could get away with only having dependencies on
+  ;; superclasses that happen to be our specific metaclass, instead of on ALL
+  ;; direct superclasses.  Sadly this fails for forward-referenced classes, so
+  ;; we need to add dependencies on all of them and filter out the non-MC
+  ;; classes later.
+  (dolist (superclass (c2mop:class-direct-superclasses class))
+    (ensure-dep superclass class)))
+
+(defmethod reinitialize-instance :around ((class mc) &key
+                                          (x nil x?)
+                                          (direct-superclasses nil direct-superclasses?)
+                                          &allow-other-keys)
+  ;; We have to recompute X /before/ we call-next-method because the
+  ;; update-dependent calls happen as part of that next method.  If we wait
+  ;; until after call-next-method to patch up X, then the dependent will still
+  ;; see the old version when it's updated and won't get the new value until
+  ;; a second round of initialization.
+  (recompute :class class
+             :superclasses (if direct-superclasses?
+                             direct-superclasses
+                             (c2mop:class-direct-superclasses class))
+             :slot 'x
+             :given-slot 'given-x
+             :value x
+             :value? x?
+             :default :some-default)
+  (let ((before (c2mop:class-direct-superclasses class)))
+    (call-next-method)
+    (let* ((after (c2mop:class-direct-superclasses class))
+           (removed (set-difference before after))
+           (added (set-difference after before)))
+      (dolist (superclass removed)
+        (ensure-no-dep superclass class))
+      (dolist (superclass added)
+        (ensure-dep superclass class)))))
+
+(defmethod c2mop:update-dependent (obj (dep dep) &rest initargs)
+  (declare (ignore initargs))
+  (when (typep obj 'mc) ; We can ignore changes in non-MC superclasses here.
+    (let ((class (val dep)))
+      (format *debug-io* "~%Updating ~A because ~A changed." class obj)
+      ;; Need to call reinitialize-instance here (instead of just recomputing the
+      ;; slots) because otherwise transitive dependencies won't get updated
+      ;; properly.
+      (apply #'reinitialize-instance class
+             (if (slot-boundp class 'given-x)
+               (list :x (slot-value class 'given-x))
+               '())))))
+```