middleware.format: print otherwise non-serializable objects as strings

Fixes #722

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Bugs fixed
 
 * [#719](https://github.com/clojure-emacs/cider-nrepl/issues/719): `middleware.test`: gracefully handle exceptions thrown within fixtures.
+* [#722](https://github.com/clojure-emacs/cider-nrepl/issues/722): `middleware.format`: print otherwise non-serializable objects as strings.
 
 ## 0.27.2 (2021-10-03)
 

src/cider/nrepl/middleware/format.clj

@@ -40,7 +40,10 @@
   (let [reader (readers/string-push-back-reader s)
         sentinel (Object.)]
     (loop [forms []]
-      (let [form (edn/read {:eof sentinel} reader)]
+      (let [form (edn/read {:eof sentinel
+                            :default (fn [_tag value]
+                                       (pr-str value))}
+                           reader)]
         (if (= sentinel form)
           forms
           (recur (conj forms form)))))))

test/clj/cider/nrepl/middleware/format_test.clj

@@ -113,6 +113,19 @@
       (is (= formatted-edn-sample formatted-edn))
       (is (= #{"done"} status))))
 
+  ;; See: https://github.com/clojure-emacs/cider-nrepl/issues/722
+  (testing "Objects of classes without an associated data-reader function are converted to strings via `pr-str`"
+    (let [{:keys [formatted-edn status]
+           :as response} (session/message {:op "format-edn"
+                                           :edn (pr-str [1 2 (Object.) 3 4])})
+          [a b ^String c d e] (read-string formatted-edn)]
+      (testing (pr-str response)
+        (is (= [1 2 3 4]
+               [a b d e]))
+        (is (string? c))
+        (is (.contains c "java.lang.Object@"))
+        (is (= #{"done"} status)))))
+
   (testing "format-edn works for multiple forms"
     (let [{:keys [formatted-edn status]} (session/message {:op "format-edn"
                                                            :edn ugly-edn-forms-sample})]