Add exclusive or logical operator

Added xor to complement and, or, not operators. Standard python
falsey/truthy semantics are followed. This implementation works for
two or more parameters.

docs/language/api.rst

@@ -1511,6 +1511,28 @@ expands to:
    Section :ref:`using-gensym`
 
 
+xor
+---
+
+.. versionadded:: 0.12.0
+
+``xor`` is used in logical expressions to perform exclusive or. It takes at
+least two parameters. It returns ``True`` if exactly one of the parameters
+evaluates to ``True``. In all other cases ``False`` is returned. Example
+usage:
+
+.. code-block:: clj
+
+    => (xor True False)
+    True
+
+    => (xor True True)
+    False
+
+    => (xor [] [] [0])
+    True
+
+
 yield
 -----
 

hy/core/macros.hy

@@ -237,3 +237,8 @@
   (let [[decorators (slice expr nil -1)]
         [fndef (get expr -1)]]
     `(with-decorator ~@decorators ~fndef)))
+
+(defmacro xor [&rest args]
+  "perform exclusive or comparison between all arguments"
+  (when (< (len args) 2) (macro-error nil "xor requires at least two arguments."))
+  `(= (reduce (fn [a b] (if b (inc a) a)) ~args 0) 1))

tests/native_tests/language.hy

@@ -813,6 +813,23 @@
   (assert (= a 1)))
 
 
+(defn test-xor []
+  "NATIVE: test the xor macro"
+  (let [[xor-both-true (xor true true)]
+        [xor-both-false (xor false false)]
+        [xor-true-false (xor true false)]
+        [xor-one-true (xor false true false)]
+        [xor-one-false (xor true false true)]
+        [xor-all-true (xor true true true)]
+        [xor-all-false (xor false false false)]]
+    (assert (= xor-both-true false))
+    (assert (= xor-both-false false))
+    (assert (= xor-true-false true))
+    (assert (= xor-one-true true))
+    (assert (= xor-one-false false))
+    (assert (= xor-all-true false))
+    (assert (= xor-all-false false))))
+
 (defn test-if-return-branching []
   "NATIVE: test the if return branching"
                                 ; thanks, algernon