consistency: allow only rebinding in set() also in let, lispylet

Technologicat · Technologicat · commit 6e90e4da51e0 · 2018-07-25T04:53:26.000+03:00
diff --git a/unpythonic/let.py b/unpythonic/let.py
@@ -91,10 +91,12 @@ def __str__(self):
     def set(self, name, value):
         """Convenience method to allow assignment in expression contexts.
 
-        Like Scheme's set! function.
+        Like Scheme's set! function. Only rebinding is allowed.
 
         For convenience, returns the `value` argument.
         """
+        if not hasattr(self, name):  # allow only rebinding
+            raise AttributeError("name '{:s}' is not defined".format(name))
         setattr(self, name, value)
         return value  # for convenience
 
@@ -426,6 +428,14 @@ def result(*, env):
         return env.evenp(23)
     assert result is False
 
+    try:
+        let(x=0,
+            body=lambda e: e.set('y', 3))  # error, y is not defined
+    except AttributeError as err:
+        pass
+    else:
+        assert False
+
     print("All tests PASSED")
 
 if __name__ == '__main__':
diff --git a/unpythonic/lispylet.py b/unpythonic/lispylet.py
@@ -146,6 +146,7 @@ def bletrec(bindings):
     This chains ``@dletrec`` and ``@immediate``."""
     return _blet(bindings, mode="letrec")
 
+# TODO: switch to the same env implementation as let.py to avoid the k=set issue?
 class _env:
     """Environment.
 
@@ -154,11 +155,15 @@ class _env:
     def set(self, k, v):
         """Convenience method to allow assignment in expression contexts.
 
-        For extra convenience, return the assigned value.
+        Like Scheme's set! function. Only rebinding is allowed.
+
+        For convenience, returns the `value` argument.
 
         DANGER:
             avoid the name ``k="set"``; it will happily shadow this method,
             because instance attributes are seen before class attributes."""
+        if not hasattr(self, k):  # allow only rebinding
+            raise AttributeError("name '{:s}' is not defined".format(k))
         setattr(self, k, v)
         return v
 
@@ -272,6 +277,14 @@ def result(*, env):
         return env.evenp(42)
     assert result is True
 
+    try:
+        let((('x', 0),),
+            lambda e: e.set('y', 3))  # error, y is not defined
+    except AttributeError:
+        pass
+    else:
+        assert False
+
     print("All tests passed")
 
 if __name__ == '__main__':
