Merge pull request #1349 from woodrush/fix-unquote-splice-none

Allow `unquote-splice` to accept any false value as empty

AUTHORS

@@ -78,3 +78,4 @@
 * John Patterson <john@johnppatterson.com>
 * Kai Lüke <kailueke@riseup.net>
 * Neil Lindquist <archer1mail@gmail.com
+* Hikaru Ikuta <woodrush924@gmail.com>

NEWS

@@ -1,6 +1,7 @@
 Changes from 0.13.0
 
    [ Language Changes ]
+   * The unquote-splice or ~@ form now accepts any false value as empty.
    * `yield-from` is no longer supported under Python 2
    * `apply` has been replaced with Python-style unpacking operators `#*` and
      `#**` (e.g., `(f #* args #** kwargs)`)

docs/language/api.rst

@@ -1604,19 +1604,31 @@ unquote-splice
 --------------
 
 ``unquote-splice`` forces the evaluation of a symbol within a quasiquoted form,
-much like ``unquote``. ``unquote-splice`` can only be used when the symbol
+much like ``unquote``. ``unquote-splice`` can be used when the symbol
 being unquoted contains an iterable value, as it "splices" that iterable into
-the quasiquoted form. ``unquote-splice`` is aliased to the ``~@`` symbol.
+the quasiquoted form. ``unquote-splice`` can also be used when the value
+evaluates to a false value such as ``None``, ``False``, or ``0``, in which
+case the value is treated as an empty list and thus does not splice anything
+into the form. ``unquote-splice`` is aliased to the ``~@`` syntax.
 
 .. code-block:: clj
 
     (def nums [1 2 3 4])
     (quasiquote (+ (unquote-splice nums)))
-    ;=> (u'+' 1L 2L 3L 4L)
+    ;=> ('+' 1 2 3 4)
 
     `(+ ~@nums)
-    ;=> (u'+' 1L 2L 3L 4L)
+    ;=> ('+' 1 2 3 4)
 
+    `[1 2 ~@(if (< (nth nums 0) 0) nums)]
+    ;=> ('+' 1 2)
+
+Here, the last example evaluates to ``('+' 1 2)``, since the condition
+``(< (nth nums 0) 0)`` is ``False``, which makes this ``if`` expression
+evaluate to ``None``, because the ``if`` expression here does not have an
+else clause. ``unquote-splice`` then evaluates this as an empty value,
+leaving no effects on the list it is enclosed in, therefore resulting in
+``('+' 1 2)``.
 
 when
 ----

hy/compiler.py

@@ -707,7 +707,9 @@ class HyASTCompiler(object):
                                                                          level)
                 imports.update(f_imports)
                 if splice:
-                    to_add = HyExpression([HySymbol("list"), f_contents])
+                    to_add = HyExpression([
+                        HySymbol("list"),
+                        HyExpression([HySymbol("or"), f_contents, HyList()])])
                 else:
                     to_add = HyList([f_contents])
 

tests/native_tests/quote.hy

@@ -74,17 +74,17 @@
 (defn test-unquote-splice []
   "NATIVE: test splicing unquotes"
   (setv q (quote (c d e)))
-  (setv qq (quasiquote (a b (unquote-splice q) f (unquote-splice q))))
+  (setv qq `(a b ~@q f ~@q ~@0 ~@False ~@None g ~@(when False 1) h))
-  (assert (= (len qq) 9))
+  (assert (= (len qq) 11))
-  (assert (= qq (quote (a b c d e f c d e)))))
+  (assert (= qq (quote (a b c d e f c d e g h)))))
 
 
 (defn test-nested-quasiquote []
   "NATIVE: test nested quasiquotes"
-  (setv qq (quasiquote (1 (quasiquote (unquote (+ 1 (unquote (+ 2 3))))) 4)))
+  (setv qq `(1 `~(+ 1 ~(+ 2 3) ~@None) 4))
-  (setv q (quote (1 (quasiquote (unquote (+ 1 5))) 4)))
+  (setv q (quote (1 `~(+ 1 5) 4)))
   (assert (= (len q) 3))
-  (assert (= (get qq 1) (quote (quasiquote (unquote (+ 1 5))))))
+  (assert (= (get qq 1) (quote `~(+ 1 5))))
   (assert (= q qq)))