Added lisp-if (or lif), with tests and documentation.

Now everyone can get along and everyone can have a pony, and that pony is
actually a unicorn that shoots rainbows from its horn.

docs/language/api.rst

@@ -726,6 +726,34 @@ any numeric type, empty sequence and empty dictionary are considered `False`.
 Everything else is considered `True`.
 
 
+lisp-if / lif
+-------------
+
+For those that prefer a more lisp-y if clause, we have lisp-if, or lif.  This
+*only* considers None/nil as false!  All other values of python
+"falseiness" are considered true.
+
+
+.. code-block:: clj
+
+    => (lisp-if True "true" "false")
+    "true"
+    => (lisp-if False "true" "false")
+    "true"
+    => (lisp-if 0 "true" "false")
+    "true"
+    => (lisp-if nil "true" "false")
+    "false"
+    => (lisp-if None "true" "false")
+    "false"
+
+    ; And, same thing
+    => (lif True "true" "false")
+    "true"
+    => (lif nil "true" "false")
+    "false"
+
+
 import
 ------
 

hy/core/macros.hy

@@ -145,6 +145,11 @@
     `(if (not ~test) ~not-branch ~yes-branch)))
 
 
+(defmacro-alias [lisp-if lif] [test &rest branches]
+  "Like `if`, but anything that is not None/nil is considered true."
+  `(if (is-not ~test nil) ~@branches))
+
+
 (defmacro when [test &rest body]
   "Execute `body` when `test` is true"
   `(if ~test (do ~@body)))

tests/native_tests/native_macros.hy

@@ -191,6 +191,25 @@
              :yes)))
 
 
+(defn test-lisp-if []
+  "test that lisp-if works as expected"
+  ; nil is false
+  (assert (= (lisp-if None "true" "false") "false"))
+  (assert (= (lisp-if nil "true" "false") "false"))
+
+  ; But everything else is True!  Even falsey things.
+  (assert (= (lisp-if True "true" "false") "true"))
+  (assert (= (lisp-if False "true" "false") "true"))
+  (assert (= (lisp-if 0 "true" "false") "true"))
+  (assert (= (lisp-if "some-string" "true" "false") "true"))
+  (assert (= (lisp-if "" "true" "false") "true"))
+  (assert (= (lisp-if (+ 1 2 3) "true" "false") "true"))
+
+  ; Just to be sure, test the alias lif
+  (assert (= (lif nil "true" "false") "false"))
+  (assert (= (lif 0 "true" "false") "true")))
+
+
 (defn test-defn-alias []
   (defn-alias [tda-main tda-a1 tda-a2] [] :bazinga)
   (defun-alias [tda-main tda-a1 tda-a2] [] :bazinga)