allow empty products and 0-argument properties

... while at the same time making the code more Pythonic.

TODO.md

@@ -1,8 +1,6 @@
 TODO for leancheck.py
 =====================
 
-* make `check()` work for 0-argument properties?
-
 * add a `leancheck.main()` similar to `unittest.main()`
 
 * simplify code

src/leancheck.py

@@ -60,12 +60,12 @@ def map(self, f):
         return Enumerator(lambda: mmap(f, self.tiers()))
 
     @classmethod
-    def product(cls, enumerator, *enumerators):
-        # TODO: FIXME: this seems quite unpythonic
+    def product(cls, *enumerators):
         if len(enumerators) == 0:
-            return enumerator.map(lambda x: (x,))
+            return Enumerator(lambda: (xs for xs in [[()]]))
         else:
-            return (enumerator * cls.product(*enumerators)).map(lambda t: (t[0],) + t[1])
+            e, *es = enumerators
+            return (e * cls.product(*es)).map(lambda t: (t[0],) + t[1])
 
     _enumerators = None
 

tests/test_leancheck.py

@@ -51,6 +51,11 @@ def test_tuple(self):
         self.assertEnum(tuple[int,int,int], [(0, 0, 0), (0, 0, 1), (0, 1, 0), (1, 0, 0), (0, 0, 2), (0, 1, 1)])
         self.assertEnum(tuple[bool,bool], [(False, False), (False, True), (True, False), (True, True)])
 
+    def test_empty_product(self):
+        e = Enumerator.product()
+        self.assertEqual(list(e), [()])
+        self.assertEqual(list(e.tiers()), [[()]])
+
 
 if __name__ == '__main__':
     unittest.main()