add-watch/remove-watch/set-validator!/get-validator were atom-only; the
atom ctor ignored :meta and :validator; watching a var crashed. Now the
ARef contract is one seam: atoms keep their record slots (hot path
unchanged), every other reference type registers a predicate and stores
watches/validators in identity-keyed side tables, and notifies at its
mutation points. Vars notify on root changes (def on a watched var,
var-set outside a thread binding, alter-var-root — thread-binding sets
don't notify, like the JVM); agents notify per action. The def-var! wrap
costs two weak-table probes per def and does IRef work only on a watched
var.
Ctor options follow ARef: the validator gates the initial value
(IllegalStateException 'Invalid reference state' — also the class for
rejected swap!/reset!), :meta must be a map (else ClassCastException),
nil allowed. meta reads any reference through the identity side-table
(the type-gated fall-through is gone); alter-meta!/reset-meta! work on
non-var references.
Runtime-only (no re-mint). 9 JVM-certified corpus rows; spec entry; cts
baseline 5781 -> 5805 pass, 73 baselined namespaces (the residual error
in the watch namespaces is their STM ref section — refs stay out of
scope).
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