add implementation for the optional goal (lists)

exercises/practice/list-ops/.meta/src/example_lists.clj

@@ -0,0 +1,46 @@
+(ns list-ops)
+
+(declare foldl)
+(declare reverse-order)
+
+(defn append
+  [coll1 coll2]
+  (reverse-order (foldl conj coll2 (reverse-order coll1))))
+
+(defn concatenate
+  [colls]
+  (foldl append colls ()))
+
+(defn select-if
+  [pred coll]
+  (let [reducer (fn [acc el]
+                  (if (pred el)
+                    (conj acc el)
+                    acc))]
+    (reverse-order (foldl reducer coll ()))))
+
+(defn length
+  [coll]
+  (let [reducer (fn [acc _]
+                  (inc acc))]
+    (foldl reducer coll 0)))
+
+(defn apply-to-each
+  [f coll]
+  (let [reducer (fn [acc el]
+                  (conj acc (f el)))]
+    (reverse-order (foldl reducer coll ()))))
+
+(defn foldl
+  [f coll acc]
+  (if (seq coll)
+    (recur f (rest coll) (f acc (first coll)))
+    acc))
+
+(defn foldr
+  [f coll acc]
+  (foldl f (reverse-order coll) acc))
+
+(defn reverse-order
+  [coll]
+  (foldl conj coll ()))