Allow comprehensions with no looping parts

hy/compiler.py

@@ -1094,6 +1094,13 @@ class HyASTCompiler(object):
         # Compile the parts.
         if is_for:
             parts = parts[0]
+        if not parts:
+            return Result(expr=ast.parse({
+                asty.For: "None",
+                asty.ListComp: "[]",
+                asty.DictComp: "{}",
+                asty.SetComp: "{1}.__class__()",
+                asty.GeneratorExp: "(_ for _ in [])"}[node_class]).body[0].value)
         parts = [
             Tag(p.tag, self.compile(p.value) if p.tag in ["if", "do"] else [
                 self._storeize(p.value[0], self.compile(p.value[0])),

tests/native_tests/comprehensions.hy

@@ -98,6 +98,18 @@
     (assert (= (sorted result) answer) (str expr)))))
 
 
+(defn test-fors-no-loopers []
+
+  (setv l [])
+  (for [] (.append l 1))
+  (assert (= l []))
+
+  (assert (= (lfor 1) []))
+  (assert (= (sfor 1) #{}))
+  (assert (= (list (gfor 1)) []))
+  (assert (= (dfor [1 2]) {})))
+
+
 (defn test-raise-in-comp []
   (defclass E [Exception] [])
   (setv l [])