[electric] early detect unwatchable argument to e/watch

Author: xificurC

src/hyperfiddle/electric3.cljc

@@ -140,13 +140,16 @@ Syntax :
 Returns the current state of current continuous flow `cf`.
 " [& flows] `(join (r/fixed-signals ~@flows)))
 
+(cc/defn watchable! [ref]
+  (ca/is ref #?(:clj #(instance? clojure.lang.IRef %) :cljs #(satisfies? IWatchable %)) (str "Not watchable: " ref)))
+
 (defmacro watch "
 Syntax :
 ```clojure
 (watch !ref)
 ```
 Returns the current state of current reference `!ref`.
-" [ref] `(check-electric watch (input (m/watch ~ref))))
+" [ref] `(check-electric watch (input (m/watch (watchable! ~ref)))))
 
 (defmacro diff-by "
 Syntax :

test/hyperfiddle/electric3_test.cljc

@@ -2900,3 +2900,8 @@
       % := [_ 1]
       (hash (first *1)) := (hash (first *3)) ; not the same hash – pure got rebuilt
       )))
+
+(tests
+  "nice error message on non-watchable `e/watch` call"
+  (with ((l/single {} (e/watch :foo)) tap tap)
+    (str/includes?  (ex-message %) "Not watchable: :foo") := true))