hdl.ast: recursively cast ValueCastable objects to values.

Author: whitequark

amaranth/hdl/ast.py

@@ -135,16 +135,22 @@ def cast(obj):
 
         Booleans and integers are wrapped into a :class:`Const`. Enumerations whose members are
         all integers are converted to a :class:`Const` with a shape that fits every member.
+        :class:`ValueCastable` objects are recursively cast to an Amaranth value.
         """
-        if isinstance(obj, Value):
-            return obj
-        if isinstance(obj, int):
-            return Const(obj)
-        if isinstance(obj, Enum):
-            return Const(obj.value, Shape.cast(type(obj)))
-        if isinstance(obj, ValueCastable):
-            return obj.as_value()
-        raise TypeError("Object {!r} cannot be converted to an Amaranth value".format(obj))
+        while True:
+            if isinstance(obj, Value):
+                return obj
+            elif isinstance(obj, int):
+                return Const(obj)
+            elif isinstance(obj, Enum):
+                return Const(obj.value, Shape.cast(type(obj)))
+            elif isinstance(obj, ValueCastable):
+                new_obj = obj.as_value()
+            else:
+                raise TypeError("Object {!r} cannot be converted to an Amaranth value".format(obj))
+            if new_obj is obj:
+                raise RecursionError("Value-castable object {!r} casts to itself".format(obj))
+            obj = new_obj
 
     def __init__(self, *, src_loc_at=0):
         super().__init__()
@@ -1276,7 +1282,7 @@ def _rhs_signals(self):
 
 
 class ValueCastable:
-    """Base class for classes which can be cast to Values.
+    """Interface of objects can be cast to :class:`Value`s.
 
     A ``ValueCastable`` can be cast to ``Value``, meaning its precise representation does not have
     to be immediately known. This is useful in certain metaprogramming scenarios. Instead of

docs/lang.rst

@@ -220,7 +220,7 @@ Value casting
 
 Like shapes, values may be *cast* from other objects, which are called *value-castable*. Casting allows objects that are not provided by Amaranth, such as integers or enumeration members, to be used in Amaranth expressions directly.
 
-.. TODO: link to UserValue
+.. TODO: link to ValueCastable
 
 Casting to a value can be done explicitly with ``Value.cast``, but is usually implicit, since value-castable objects are accepted anywhere values are.
 

tests/test_hdl_ast.py

@@ -1060,6 +1060,15 @@ def test_lower_to_user_value(self):
             self.assertEqual(uv.lower_count, 1)
 
 
+class MockValueCastable(ValueCastable):
+    def __init__(self, dest):
+        self.dest = dest
+
+    @ValueCastable.lowermethod
+    def as_value(self):
+        return self.dest
+
+
 class MockValueCastableChanges(ValueCastable):
     def __init__(self, width=0):
         self.width = width
@@ -1097,14 +1106,14 @@ def __getattr__(self, attr):
 class ValueCastableTestCase(FHDLTestCase):
     def test_not_decorated(self):
         with self.assertRaisesRegex(TypeError,
-                r"^Class 'MockValueCastableNotDecorated' deriving from `ValueCastable` must decorate the `as_value` "
-                r"method with the `ValueCastable.lowermethod` decorator$"):
+                r"^Class 'MockValueCastableNotDecorated' deriving from `ValueCastable` must "
+                r"decorate the `as_value` method with the `ValueCastable.lowermethod` decorator$"):
             vc = MockValueCastableNotDecorated()
 
     def test_no_override(self):
         with self.assertRaisesRegex(TypeError,
-                r"^Class 'MockValueCastableNoOverride' deriving from `ValueCastable` must override the `as_value` "
-                r"method$"):
+                r"^Class 'MockValueCastableNoOverride' deriving from `ValueCastable` must "
+                r"override the `as_value` method$"):
             vc = MockValueCastableNoOverride()
 
     def test_memoized(self):
@@ -1121,6 +1130,17 @@ def test_custom_getattr(self):
         vc = MockValueCastableCustomGetattr()
         vc.as_value() # shouldn't call __getattr__
 
+    def test_recurse_bad(self):
+        vc = MockValueCastable(None)
+        vc.dest = vc
+        with self.assertRaisesRegex(RecursionError,
+                r"^Value-castable object <.+> casts to itself$"):
+            Value.cast(vc)
+
+    def test_recurse(self):
+        vc = MockValueCastable(MockValueCastable(Signal()))
+        self.assertIsInstance(Value.cast(vc), Signal)
+
 
 class SampleTestCase(FHDLTestCase):
     def test_const(self):