hy/core/macros.hy: Add an (if-not) macro

Sometimes it is better to start with the false condition, sometimes that
makes the code clearer. For that, the (if-not) macro, which simply
reverses the order of the condition blocks, can be of great use.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>

docs/language/api.rst

@@ -679,13 +679,16 @@ would thrown a `NameError`.
     (set-a 5)
     (print-a)
 
-if
+if / if-not
---
+-----------
 
 the `if` form is used to conditionally select code to be executed. It has to
 contain the condition block and the block to be executed if the condition
 evaluates `True`. Optionally it may contain a block that is executed in case
-the evaluation of the condition is `False`.
+the evaluation of the condition is `False`. The `if-not` form (*new in
+0.9.13*) is similar, but the first block after the test will be
+executed when the test fails, while the other, conditional one, when
+the test succeeds - opposite of the order of the `if` form.
 
 Example usage:
 
@@ -695,6 +698,10 @@ Example usage:
       (print "lets go shopping")
       (print "lets go and work"))
 
+    (if-not (money-left? account)
+      (print "lets go and work")
+      (print "lets go shopping"))
+
 Truth values of Python objects are respected. Values `None`, `False`, zero of
 any numeric type, empty sequence and empty dictionary are considered `False`.
 Everything else is considered `True`.

hy/core/macros.hy

@@ -138,6 +138,13 @@
   ret)
 
 
+(defmacro if-not [test not-branch &optional [yes-branch nil]]
+  "Like `if`, but execute the first branch when the test fails"
+  (if (nil? yes-branch)
+    `(if (not ~test) ~not-branch)
+    `(if (not ~test) ~not-branch ~yes-branch)))
+
+
 (defmacro when [test &rest body]
   "Execute `body` when `test` is true"
   `(if ~test (do ~@body)))

tests/native_tests/native_macros.hy

@@ -176,3 +176,12 @@
   (assert (in ":res_" s1))
   (assert (in ":res_" s2))
   (assert (not (= s1 s2))))
+
+(defn test-if-not []
+  (assert (= (if-not True :yes :no)
+             :no))
+  (assert (= (if-not False :yes :no)
+             :yes))
+  (assert (nil? (if-not True :yes)))
+  (assert (= (if-not False :yes)
+             :yes)))