Chez Phase 2 (inc S): atom watches + validators (jolt-mn9o)

The Chez atom record gains watches (an alist of key->fn) and validator
slots. swap!/reset! now validate the candidate value before storing and
notify watches after, in the seed's order (core_refs.janet) — the watch fn
is called (key ref old new). compare-and-set!/swap-vals!/reset-vals! route
through reset!/swap! so they validate + notify too.

add-watch/remove-watch/set-validator!/get-validator are native here and
re-asserted in post-prelude.ss: the clojure.core overlay implements them via
jolt.host/ref-put! on (get atom :watches), a Janet-table mutation a Chez atom
record can't answer, so its def-var! would otherwise clobber these. set-
validator! validates the current value immediately (Clojure throws if already
invalid).

Parity 2150 -> 2154, 0 new divergences. New test/chez/_atomwatch.janet 10/10.

The (class x) native and the clojure.walk port were explored this increment
but deferred: class alone makes the String-class-token corpus cases emit-and-
run with a wrong value (the bare token doesn't resolve yet) — filed jolt-13zk
to land the native together with token resolution; clojure.walk is blocked on
list? + map-entry-as-vector (jolt-75sv).

host/chez/atoms.ss

@@ -11,11 +11,36 @@
 ;; self-sufficient for atoms without the full prelude (the overlay versions, when
 ;; the full prelude loads, override these but compose the same native kernel).
 
-(define-record-type jolt-atom (fields (mutable val)) (nongenerative jolt-atom-v1))
+;; watches is an alist of (key . watch-fn); validator is a jolt fn or jolt-nil.
+;; The overlay's add-watch/set-validator! drive these via jolt.host/ref-put! on a
+;; Janet table, which a Chez atom record is not — so the peripheral ops + the
+;; notify/validate behaviour live natively here, and post-prelude.ss re-asserts
+;; them over the overlay's def-var! (jolt-mn9o).
+(define-record-type jolt-atom
+  (fields (mutable val) (mutable watches) (mutable validator))
+  (nongenerative jolt-atom-v2))
 
-;; (atom init) — extra :meta/:validator opts are accepted and ignored for now
-;; (watches/validators are overlay features layered via jolt.host/ref-put!).
-(define (jolt-atom-new v . _opts) (make-jolt-atom v))
+;; (atom init) / (atom init :validator f :meta m): scan the trailing keyword opts
+;; for :validator (the only one with runtime behaviour; :meta is accepted/ignored).
+(define (jolt-atom-new v . opts)
+  (let loop ((o opts) (validator jolt-nil))
+    (cond
+      ((or (null? o) (null? (cdr o))) (make-jolt-atom v '() validator))
+      ((and (keyword-t? (car o)) (string=? (keyword-t-name (car o)) "validator"))
+       (loop (cddr o) (cadr o)))
+      (else (loop (cddr o) validator)))))
+
+;; validate a candidate value: a non-nil validator that returns falsey rejects.
+(define (jolt-atom-validate a v)
+  (let ((vf (jolt-atom-validator a)))
+    (when (and (not (jolt-nil? vf)) (jolt-not (jolt-invoke vf v)))
+      (error #f "Invalid reference state"))))
+
+;; notify each watch (k ref old new), in insertion order (alist is reverse-built,
+;; so walk it reversed to match add order — matches the seed's :pairs iteration).
+(define (jolt-atom-notify a old new)
+  (for-each (lambda (kv) (jolt-invoke (cdr kv) (car kv) a old new))
+            (reverse (jolt-atom-watches a))))
 
 ;; deref reads an atom; it also unwraps a `reduced` (Clojure @(reduced x) => x,
 ;; which the overlay's `unreduced` relies on). The reduced record is in seq.ss.
@@ -25,31 +50,61 @@
     ((jolt-reduced? x) (jolt-reduced-val x))
     (else (error #f "deref: unsupported reference type" x))))
 
-;; (swap! a f arg*) -> (reset! a (f @a arg*)); f is invoked through jolt-invoke
-;; (a jolt fn value, keyword, or invokable collection).
+;; (swap! a f arg*) -> (reset! a (f @a arg*)); f is invoked through jolt-invoke.
+;; Validate the new value BEFORE storing, notify watches AFTER (the seed order).
 (define (jolt-swap! a f . args)
-  (let ((nv (apply jolt-invoke f (jolt-atom-val a) args)))
+  (let* ((old (jolt-atom-val a))
+         (nv (apply jolt-invoke f old args)))
+    (jolt-atom-validate a nv)
     (jolt-atom-val-set! a nv)
+    (jolt-atom-notify a old nv)
     nv))
 
-(define (jolt-reset! a v) (jolt-atom-val-set! a v) v)
+(define (jolt-reset! a v)
+  (let ((old (jolt-atom-val a)))
+    (jolt-atom-validate a v)
+    (jolt-atom-val-set! a v)
+    (jolt-atom-notify a old v)
+    v))
 
 (define (jolt-compare-and-set! a oldv newv)
   (if (jolt= (jolt-atom-val a) oldv)
-      (begin (jolt-atom-val-set! a newv) #t)
+      (begin (jolt-reset! a newv) #t)
       #f))
 
 (define (jolt-swap-vals! a f . args)
   (let* ((old (jolt-atom-val a))
          (nv (apply jolt-invoke f old args)))
-    (jolt-atom-val-set! a nv)
+    (jolt-reset! a nv)
     (jolt-vector old nv)))
 
 (define (jolt-reset-vals! a v)
   (let ((old (jolt-atom-val a)))
-    (jolt-atom-val-set! a v)
+    (jolt-reset! a v)
     (jolt-vector old v)))
 
+;; --- watches / validators (jolt-mn9o) ---------------------------------------
+;; add-watch interns (key . fn) (replacing any existing key, keeping order);
+;; remove-watch drops it; both return the atom. set-validator! installs a
+;; validator and validates the CURRENT value immediately (Clojure throws if it's
+;; already invalid); get-validator reads the slot.
+(define (jolt-add-watch a key f)
+  (jolt-atom-watches-set! a
+    (cons (cons key f)
+          (remp (lambda (kv) (jolt=2 (car kv) key)) (jolt-atom-watches a))))
+  a)
+(define (jolt-remove-watch a key)
+  (jolt-atom-watches-set! a
+    (remp (lambda (kv) (jolt=2 (car kv) key)) (jolt-atom-watches a)))
+  a)
+(define (jolt-set-validator! a f)
+  (let ((vf (if (jolt-nil? f) jolt-nil f)))
+    (when (and (not (jolt-nil? vf)) (jolt-not (jolt-invoke vf (jolt-atom-val a))))
+      (error #f "Invalid reference state"))
+    (jolt-atom-validator-set! a vf)
+    jolt-nil))
+(define (jolt-get-validator a) (jolt-atom-validator a))
+
 (def-var! "clojure.core" "atom" jolt-atom-new)
 (def-var! "clojure.core" "deref" jolt-deref)
 (def-var! "clojure.core" "swap!" jolt-swap!)
@@ -58,3 +113,9 @@
 (def-var! "clojure.core" "swap-vals!" jolt-swap-vals!)
 (def-var! "clojure.core" "reset-vals!" jolt-reset-vals!)
 (def-var! "clojure.core" "atom?" jolt-atom?)
+;; peripheral ops: the overlay (20-coll) re-defs these over jolt.host/ref-put!,
+;; which fails on a Chez atom record — post-prelude.ss re-asserts the natives.
+(def-var! "clojure.core" "add-watch" jolt-add-watch)
+(def-var! "clojure.core" "remove-watch" jolt-remove-watch)
+(def-var! "clojure.core" "set-validator!" jolt-set-validator!)
+(def-var! "clojure.core" "get-validator" jolt-get-validator)