Make xor return single true inputs

The documentation should now be correct (#1214).

NEWS

@@ -1,3 +1,13 @@
+Changes from 0.12.1
+
+   [ Language Changes ]
+   * xor: If exactly one argument is true, return it
+
+Changes from 0.12.0
+
+   [ Bug Fixes ]
+   * Allow installation without Git
+
 Changes from 0.11.0
 
 This release brings some quite significant changes on the language and as a

docs/language/api.rst

@@ -1713,20 +1713,15 @@ xor
 
 .. versionadded:: 0.12.0
 
-``xor`` is used in logical expressions to perform exclusive or. It takes two
-parameters. It returns ``True`` if only of the parameters is ``True``. In all
-other cases ``False`` is returned. Example usage:
+``xor`` performs the logical operation of exclusive OR. It takes two arguments.
+If exactly one argument is true, that argument is returned. If neither is true,
+the second argument is returned (which will necessarily be false). Otherwise,
+when both arguments are true, the value ``False`` is returned.
 
 .. code-block:: clj
 
-    => (xor True False)
-    True
-
-    => (xor True True)
-    False
-
-    => (xor [] [0])
-    True
+    => [(xor 0 0) (xor 0 1) (xor 1 0) (xor 1 1)]
+    [0, 1, 1, False]
 
 
 yield

hy/core/language.hy

@@ -481,8 +481,9 @@
 
 (defn xor [a b]
   "Perform exclusive or between two parameters"
-  (or (and a (not b))
-      (and b (not a))))
+  (if (and a b)
+    False
+    (or a b)))
 
 (def *exports*
   '[*map accumulate butlast calling-module-name chain coll? combinations

tests/native_tests/language.hy

@@ -1019,12 +1019,23 @@
 
 (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)]
-    (assert (= xor-both-true False))
-    (assert (= xor-both-false False))
-    (assert (= xor-true-false True))))
+
+  ; Test each cell of the truth table.
+  (assert (is (xor False  False) False))
+  (assert (is (xor False True)  True))
+  (assert (is (xor True  False) True))
+  (assert (is (xor True  True)  False))
+
+  ; Same thing, but with numbers.
+  (assert (is (xor 0 0) 0))
+  (assert (is (xor 0 1) 1))
+  (assert (is (xor 1 0) 1))
+  (assert (is (xor 1 1) False))
+
+  ; Of two distinct false values, the second is returned.
+  (assert (is (xor False 0) 0))
+  (assert (is (xor 0 False) False)))
+
 
 (defn test-if-return-branching []
   "NATIVE: test the if return branching"