Add a recursive list processing example.

ideaboard.txt

@@ -3,13 +3,11 @@ Concepts / Language Features
 Agents
 Vars
 state identity lifetime
-lazy sequences
 Metadata
 Tuples - syntax
 Pattern Matching
 immutability/side effects
 memoization
-recursive list processing
 trampolining
 reflection
 Java interop

src/koans/recursion.clj

@@ -4,10 +4,15 @@
       (___ (is-even? (dec n)))))
 
 (defn is-even-bigint? [n]
-  (loop [n n acc true]
-    (if (= n 0) __
+  (loop [n   n
+         acc true]
+    (if (= n 0)
+        __
         (recur (dec n) (not acc)))))
 
+(defn recursive-reverse [coll]
+  __)
+
 (defn factorial [n]
   __)
 
@@ -21,6 +26,12 @@
   "Having too many stack frames requires explicit tail calls with recur"
   (= false (is-even-bigint? 100003N))
 
+  "Reversing directions is easy when you have not gone far"
+  (= '(1) (recursive-reverse [1]))
+
+  "Yet more difficult the more steps you take"
+  (= '(5 4 3 2 1) (recursive-reverse [1 2 3 4 5]))
+
   "Simple things may appear simple."
   (= 1 (factorial 1))
 

src/path_to_answer_sheet.clj

@@ -115,8 +115,15 @@
                                'multiply-by-5
                                '(comp dec square)]}
 
-   "recursion" {"__" [true 'acc
-                      '(loop [n n acc 1]
+   "recursion" {"__" [true
+                      'acc
+                      '(loop [coll coll
+                              acc ()]
+                         (if (seq coll)
+                           (recur (rest coll) (conj acc (first coll)))
+                           acc))
+                      '(loop [n n
+                              acc 1]
                          (if (zero? n)
                              acc
                              (recur (dec n) (* acc n))))]