Introduce middleware.test/*test-error-handler*

This allows users to attach arbitrary functionality when tests fail due to an exception.

For example, I used this to add an Expound-backed `println` for exceptions that were caused by Spec failures.

...While CIDER already prints Spec failures nicely via an Emacs UI, not all cider-nrepl clients necessarily have such an UI.

One might also appreciate a println with Expound's format, which is familiar for many, and will be consistent with printlns placed elsewhere in a given codebase.

Author: vemv

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### New features
 
 * [#698](https://github.com/clojure-emacs/cider-nrepl/pull/698): Add `undef-all` op to undefine all symbols and aliases in namespace
+* Introduce `cider.nrepl.middleware.test/*test-error-handler*` var which you can override with arbitrary functions.
 
 ### Bugs fixed
 

src/cider/nrepl/middleware/test.clj

@@ -73,6 +73,14 @@
                    println)]
     (with-out-str (print-fn object))))
 
+(def ^:dynamic *test-error-handler*
+  "A function you can override via `binding`, or safely via `alter-var-root`.
+  On test `:error`s, the related Throwable be invoked as the sole argument
+  passed to this var.
+  For example, you can use this to add an additional `println`,
+  for pretty-printing Spec failures. Remember to `flush` if doing so."
+  identity)
+
 (defn test-result
   "Transform the result of a test assertion. Append ns, var, assertion index,
   and 'testing' context. Retain any exception. Pretty-print expected/actual or
@@ -81,7 +89,7 @@
   (let [{:keys [actual diffs expected fault]
          t :type} m
         c (when (seq test/*testing-contexts*) (test/testing-contexts-str))
-        i (count (get-in (@current-report :results) [ns (:name (meta v))]))
+        i (count (get-in (:results @current-report {}) [ns (:name (meta v))]))
         gen-input (:gen-input @current-report)]
 
     ;; Errors outside assertions (faults) do not return an :expected value.
@@ -99,6 +107,7 @@
            (when (#{:error} t)
              (let [e actual
                    f (or (:test (meta v)) @v)] ; test fn or deref'ed fixture
+               (*test-error-handler* e)
                {:error e
                 :line (:line (stack-frame e f))})))))
 

test/clj/cider/nrepl/middleware/test_test.clj

@@ -151,3 +151,17 @@
            (#'test/print-object (with-meta {:not :printed} {:type ::custom}))))
     (is (= "{:a :b, :c :d}\n"
            (#'test/print-object {:a :b :c :d})))))
+
+(deftest test-result-test
+  (testing "It passes `:error`s to `test/*test-error-handler*`"
+    (let [proof (atom [])
+          exception (ex-info "." {::unique (rand)})]
+      (binding [test/*test-error-handler* (fn [e]
+                                            (swap! proof conj e))]
+        (with-out-str
+          (test/test-result 'some-ns
+                            #'+
+                            {:type :error
+                             :actual exception}))
+        (is (= [exception]
+               @proof))))))