wip: add if-let

Author: alysbrooks

test/clojure/core_test/if_let.cljc

@@ -0,0 +1,37 @@
+(ns clojure.core-test.if-let
+(:require [clojure.test :as t :refer [are deftest is testing]]
+            [clojure.core-test.portability #?(:cljs :refer-macros :default :refer) [when-var-exists]]))
+
+(when-var-exists if-let
+                 (deftest test-if-let
+                   (testing "basic single-binding tests using vectors or nil"
+                     (is (= [0 1 2 3 4] (if-let [x [0 1 2 3 4] ] x :else)))
+                     (is (not (nil? (if-let [x [nil]] x :else))))
+                     (is (= [] (if-let [x []] x :else)))
+                     (is (= :else (if-let [x nil] x :else))))
+                   (testing "basic single-binding tests using seqs"
+                     (is (= '(0 1 2 3 4) (if-let [x (range 5)] x :else))))
+                   (testing "unlike when-some, we're looking for logical false specifically, so false doesn't evaluate"
+                     (is (= :else (if-let [x false] x :else))))
+                   (testing "seq is only called once"
+                     (let [calls (atom 0)
+                           seq-fn (fn s [] (lazy-seq
+                                             (swap! calls inc)
+                                             (cons 1 (s))))
+                           s (take 5 (seq-fn))]
+                       (is (= '(1 1 1 1 1) (if-let [x s] x :else)))
+                       (is (= @calls 5))))
+                   (comment (testing "without a body, truth doesn't matter"
+                              (is (nil? (if-let [x nil])))
+                              (is (nil? (if-let [x [false]])))
+                              (is (nil? (if-let [x [true]])))))
+                   #?(:cljs nil ; Skipped due to ClojureScript's atypical macro expansion.
+                      :default (testing "if-let accepts exactly two bindings"
+                                 (is (thrown? Exception
+                                              (macroexpand
+                                                '(if-let [x (range 5) y (range 5)]))))))
+                   #?(:cljs nil ; Skipped due to ClojureScript's atypical macro expansion.
+                      :default (testing "if-let accepts exactly two bindings"
+                                 (is (thrown? Exception
+                                              (macroexpand
+                                                '(if-let [x (range 5) y (range 5)]))))))))