Change the value of a Clojure var without touching the metadata - clojure

Question

I've been working on the assumption that a var's metadata is "stable," that is, I can change the var's value without changing the var's metadata. Now I see there's something wrong with my understanding. Code:
(def ^{:Metadata "metaA"} A 1) ;; Define A with value 1 and metadata.
=> #'thic.core/A
(def ^{:Metadata "metaB"} B 2) ;; Define B with value 2 and metadata.
=> #'thic.core/B
A ;; A's value is 1.
=> 1
B ;; B's value is 2.
=> 2
(meta (var A))
=>
{:Metadata "metaA", ;; A has the defined metadata.
:line 1,
:column 1,
:file "C:\\Users\\Joe User\\AppData\\Local\\Temp\\form-init2487748963910096550.clj",
:name A,
:ns #object[clojure.lang.Namespace 0x147c445 "thic.core"]}
(meta (var B))
=>
{:Metadata "metaB", ;; So does B.
:line 1,
:column 1,
:file "C:\\Users\\Joe User\\AppData\\Local\\Temp\\form-init2487748963910096550.clj",
:name B,
:ns #object[clojure.lang.Namespace 0x147c445 "thic.core"]}
(def B A) ;; Give B A's value,
=> #'thic.core/B
A
=> 1
B ;; which it now has,
=> 1
(meta (var B)) ;; and B's previously-defined metadata is gone.
=>
{:line 1,
:column 1,
:file "C:\\Users\\Joe User\\AppData\\Local\\Temp\\form-init2487748963910096550.clj",
:name B,
:ns #object[clojure.lang.Namespace 0x147c445 "thic.core"]}
(meta (var A)) ;; A's remains unchanged.
=>
{:Metadata "metaA",
:line 1,
:column 1,
:file "C:\\Users\\Joe User\\AppData\\Local\\Temp\\form-init2487748963910096550.clj",
:name A,
:ns #object[clojure.lang.Namespace 0x147c445 "thic.core"]}
Is there a way I can assign the value of A to B without wiping out the metadata in B?
Or maybe I just don't understand the difference between "binding" and "assignment"?
Clojure in Action 2nd Edition says "When new values are created from those that have metadata, the metadata is copied over to the new data." (p. 57) My example is not creating a new value. Is that the problem? I still want to modify a var's value without modifying its metadata.
I humbly beseech thee, ClojureGods.

Answer 1

alter-var-root can do that.
-------------------------
clojure.core/alter-var-root
([v f & args])
Atomically alters the root binding of var v by applying f to its
current value plus any args
E.g.:
; Clojure 1.10.3
(def ^{:Metadata "metaA"} A 1)
; #'user/A
(def ^{:Metadata "metaB"} B 2)
; #'user/B
A
; 1
B
; 2
(meta #'A)
; {:Metadata "metaA", :line 1, :column 1, :file "NO_SOURCE_PATH", :name A, :ns #object[clojure.lang.Namespace 0x2189e7a7 "user"]}
(meta #'B)
; {:Metadata "metaB", :line 1, :column 1, :file "NO_SOURCE_PATH", :name B, :ns #object[clojure.lang.Namespace 0x2189e7a7 "user"]}
(alter-var-root #'B (constantly A))
; 1
B
; 1
(meta #'B)
; {:Metadata "metaB", :line 1, :column 1, :file "NO_SOURCE_PATH", :name B, :ns #object[clojure.lang.Namespace 0x2189e7a7 "user"]}