Add basic tests for when-first.

Includes a test ensuring the seq is run only once, since that's a core
promise when-first makes.

Author: alysbrooks

test/clojure/core_test/when_first.cljc

@@ -0,0 +1,23 @@
+(ns clojure.core-test.when-first
+  (:require [clojure.test :as t :refer [deftest testing is are]]
+            [clojure.core-test.portability #?(:cljs :refer-macros :default :refer) [when-var-exists]]))
+
+
+(when-var-exists when-first
+  (deftest test-when-first
+    (testing "Basic single-binding tests using vectors or nil"
+      (is (= 0 (when-first [x [0 1 2 3 4] ] x)))
+      (is (nil? (when-first [x [nil]] x)))
+      (is (nil? (when-first [x []] x)))
+      (is (nil? (when-first [x nil] x))))
+    (testing "Basic single-binding tests using seqs"
+      (is (= 0 (when-first [x (range 5) ] x)))
+      (is (nil? (when-first [x (repeat nil)] x))))
+    (testing "Seq only called once"
+      (let [calls (atom 0)
+            seq-fn (fn s [] (lazy-seq
+                              (swap! calls inc)
+                              (cons 1 (s))))
+            s (seq-fn)]
+        (is (= 1 (when-first [x s] x)))
+        (is (= @calls 1))))))