test fn? (#104)

* test `fn?`

* test fn? for protocols and multimethods

This is arguably an implementation detail and not worth testing. Included for
completeness nonetheless.

* don't test fn? for multimethods or protocols

* `testing` groups > comments

Author: daveliepmann

test/clojure/core_test/fn_qmark.cljc

@@ -0,0 +1,34 @@
+(ns clojure.core-test.fn-qmark
+  (:require clojure.core
+            [clojure.test :as t :refer [deftest testing is are]]
+            [clojure.core-test.portability #?(:cljs :refer-macros :default :refer)  [when-var-exists]]))
+
+(defn foo [x] (str "hello " x))
+
+(when-var-exists clojure.core/fn?
+  (deftest test-fn?
+    (testing "`fn?`"
+      (testing "functions, functions from HOFs, transducers, #() reader macro, `fn`, `defn`"
+        (is (fn? juxt))
+        (is (fn? (juxt inc dec)))
+        (is (fn? (map inc)))
+        (is (fn? #(str "hello " %)))
+        (is (fn? (fn [x] (str "hello " x))))
+        (is (fn? foo)))
+
+      ;; Note: we intentionally do not test multimethods or protocols, because
+      ;; behavior differs across dialects due to implementation decisions that
+      ;; don't matter for our purposes.
+
+      (testing "atomic values are not functions"
+        (is (not (fn? nil)))
+        (is (not (fn? 12345678)))
+        (is (not (fn? "string")))
+        (is (not (fn? :keyword))) ; note: also IFn
+        (is (not (fn? 'symbol)))
+        (is (not (fn? \space))))
+
+      (testing "implementing IFn is not the same as implementing Fn"
+        (is (not (fn? {:a :b})))
+        (is (not (fn? #{:a :b :c})))
+        (is (not (fn? [:a :b])))))))