Add a `when' macro

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

hy/core/bootstrap.py

@@ -170,3 +170,14 @@ def drop_macro(tree):
     return HyExpression([HySymbol('slice'),
                          ret,
                          HyInteger(n)])
+
+
+@macro("when")
+def when_macro(tree):
+    tree.pop(0)  # "when"
+    test = tree.pop(0)
+    return HyExpression([
+        HySymbol('if'),
+        test,
+        HyExpression([HySymbol("do")]) + tree,
+    ])

tests/__init__.py

@@ -4,3 +4,4 @@
 import hy  # noqa
 from .native_tests.math import *  # noqa
 from .native_tests.language import *  # noqa
+from .native_tests.when import *  # noqa

tests/native_tests/when.hy

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