Modify cond to support single argument branches

NEWS

@@ -32,6 +32,7 @@ Changes from 0.12.1
      `(try 1 (except [ValueError] 2) (else 3))` returns `3`.
    * xor: If exactly one argument is true, return it
    * hy.core.reserved is now hy.extra.reserved
+   * cond now supports single argument branches
 
    [ Bug Fixes ]
    * All shadowed operators have the same arities as real operators

docs/language/api.rst

@@ -374,6 +374,18 @@ shows the relationship between the macro and its expansion:
     (if condition-1 result-1
       (if condition-2 result-2))
 
+If only the condition is given in a branch, then the condition is also used as
+the result. The expansion of this single argument version is demonstrated
+below:
+
+.. code-block:: clj
+
+    (cond [condition-1]
+          [condition-2])
+
+    (if condition-1 condition-1
+      (if condition-2 condition-2))
+
 As shown below, only the first matching result block is executed.
 
 .. code-block:: clj

hy/core/macros.hy

@@ -56,10 +56,10 @@
        (if (not (= (type branch) HyList))
          (macro-error branch "cond branches need to be a list"))
        (if (< (len branch) 2)
-         (macro-error branch "cond branches need at least two items: a test and one or more code branches"))
-       (setv test (first branch))
-       (setv thebranch (cut branch 1))
-       `(if ~test (do ~@thebranch)))
+         (do
+           (setv g (gensym))
+           `(if (do (setv ~g ~(first branch)) ~g) ~g))
+         `(if ~(first branch) (do ~@(cut branch 1)))))
 
      (setv root (check-branch branch))
      (setv latest-branch root)

tests/native_tests/language.hy

@@ -291,7 +291,17 @@
   (cond
    [(= 1 2) (assert (is True False))]
    [(is None None) (setv x True) (assert x)])
-  (assert (= (cond) None)))
+  (assert (= (cond) None))
+
+  (assert (= (cond
+    [False]
+    [[]]
+    [8])) 8)
+
+  ;make sure test is only evaluated once
+  (setv x 0)
+  (cond [(do (+= x 1) True)])
+  (assert (= x 1)))
 
 
 (defn test-if []