Add a `unless' macro

Signed-off-by: Julien Danjou <julien@danjou.info>

hy/core/bootstrap.py

@@ -181,3 +181,15 @@ def when_macro(tree):
         test,
         HyExpression([HySymbol("do")]) + tree,
     ])
+
+
+@macro("unless")
+def unless_macro(tree):
+    tree.pop(0)  # "unless"
+    test = tree.pop(0)
+    return HyExpression([
+        HySymbol('if'),
+        test,
+        HySymbol('None'),
+        HyExpression([HySymbol("do")]) + tree,
+    ])

tests/__init__.py

@@ -4,4 +4,5 @@
 import hy  # noqa
 from .native_tests.math import *  # noqa
 from .native_tests.language import *  # noqa
+from .native_tests.unless import *  # noqa
 from .native_tests.when import *  # noqa

tests/native_tests/unless.hy

@@ -0,0 +1,8 @@
+(defn test-unless []
+  "NATIVE: test unless"
+  (assert (= (unless false 1) 1))
+  (assert (= (unless false 1 2) 2))
+  (assert (= (unless false 1 3) 3))
+  (assert (= (unless true 2) null))
+  (assert (= (unless (!= 1 2) 42) null))
+  (assert (= (unless (!= 2 2) 42) 42)))