hdl.ast: implement ShapeCastable (like ValueCastable).

Refs #693.

amaranth/hdl/ast.py

@@ -12,7 +12,7 @@ from .._unused import *
 
 
 __all__ = [
-    "Shape", "signed", "unsigned",
+    "Shape", "signed", "unsigned", "ShapeCastable",
     "Value", "Const", "C", "AnyConst", "AnySeq", "Operator", "Mux", "Part", "Slice", "Cat", "Repl",
     "Array", "ArrayProxy",
     "Signal", "ClockSignal", "ResetSignal",
@@ -32,6 +32,22 @@ class DUID:
         DUID.__next_uid += 1
 
 
+class ShapeCastable:
+    """Interface of user-defined objects that can be cast to :class:`Shape`s.
+
+    An object deriving from ``ShapeCastable`` is automatically converted to a ``Shape`` when it is
+    used in a context where a ``Shape`` is expected. Such objects can contain a richer description
+    of the shape than what is supported by the core Amaranth language, yet still be transparently
+    used with it.
+    """
+    def __new__(cls, *args, **kwargs):
+        self = super().__new__(cls)
+        if not hasattr(self, "as_shape"):
+            raise TypeError(f"Class '{cls.__name__}' deriving from `ShapeCastable` must override "
+                            f"the `as_shape` method")
+        return self
+
+
 class Shape:
     """Bit width and signedness of a value.
 
@@ -62,30 +78,33 @@ class Shape:
         self.width = width
         self.signed = signed
 
+    # TODO(nmigen-0.4): remove
     def __iter__(self):
         return iter((self.width, self.signed))
 
     @staticmethod
     def cast(obj, *, src_loc_at=0):
+        while True:
             if isinstance(obj, Shape):
                 return obj
-        if isinstance(obj, int):
+            elif isinstance(obj, int):
                 return Shape(obj)
-        if isinstance(obj, tuple):
+            # TODO(nmigen-0.4): remove
+            elif isinstance(obj, tuple):
                 width, signed = obj
                 warnings.warn("instead of `{tuple}`, use `{constructor}({width})`"
                               .format(constructor="signed" if signed else "unsigned", width=width,
                                       tuple=obj),
                               DeprecationWarning, stacklevel=2 + src_loc_at)
                 return Shape(width, signed)
-        if isinstance(obj, range):
+            elif isinstance(obj, range):
                 if len(obj) == 0:
                     return Shape(0, obj.start < 0)
                 signed = obj.start < 0 or (obj.stop - obj.step) < 0
                 width  = max(bits_for(obj.start, signed),
                              bits_for(obj.stop - obj.step, signed))
                 return Shape(width, signed)
-        if isinstance(obj, type) and issubclass(obj, Enum):
+            elif isinstance(obj, type) and issubclass(obj, Enum):
                 min_value = min(member.value for member in obj)
                 max_value = max(member.value for member in obj)
                 if not isinstance(min_value, int) or not isinstance(max_value, int):
@@ -94,7 +113,13 @@ class Shape:
                 signed = min_value < 0 or max_value < 0
                 width  = max(bits_for(min_value, signed), bits_for(max_value, signed))
                 return Shape(width, signed)
-        raise TypeError("Object {!r} cannot be used as value shape".format(obj))
+            elif isinstance(obj, ShapeCastable):
+                new_obj = obj.as_shape()
+            else:
+                raise TypeError("Object {!r} cannot be converted to an Amaranth shape".format(obj))
+            if new_obj is obj:
+                raise RecursionError("Shape-castable object {!r} casts to itself".format(obj))
+            obj = new_obj
 
     def __repr__(self):
         if self.signed:
@@ -103,6 +128,7 @@ class Shape:
             return "unsigned({})".format(self.width)
 
     def __eq__(self, other):
+        # TODO(nmigen-0.4): remove
         if isinstance(other, tuple) and len(other) == 2:
             width, signed = other
             if isinstance(width, int) and isinstance(signed, bool):
@@ -112,9 +138,11 @@ class Shape:
                         "not {!r}"
                         .format(other))
         if not isinstance(other, Shape):
-            raise TypeError("Shapes may be compared with other Shapes and (int, bool) tuples, "
+            try:
-                    "not {!r}"
+                other = self.__class__.cast(other)
-                    .format(other))
+            except TypeError as e:
+                raise TypeError("Shapes may be compared with shape-castable objects, not {!r}"
+                                .format(other)) from e
         return self.width == other.width and self.signed == other.signed
 
 
@@ -1282,12 +1310,14 @@ class UserValue(Value):
 
 
 class ValueCastable:
-    """Interface of objects can be cast to :class:`Value`s.
+    """Interface of user-defined objects that can be cast to :class:`Value`s.
 
-    A ``ValueCastable`` can be cast to ``Value``, meaning its precise representation does not have
+    An object deriving from ``ValueCastable`` is automatically converted to a ``Value`` when it is
-    to be immediately known. This is useful in certain metaprogramming scenarios. Instead of
+    used in a context where a ``Value`` is expected. Such objects can implement different or
-    providing fixed semantics upfront, it is kept abstract for as long as possible, only being
+    richer semantics than what is supported by the core Amaranth language, yet still be
-    cast to a concrete Amaranth value when required.
+    transparently used with it as long as the final underlying representation is a single Amaranth
+    ``Value``. These objects also need not commit to a specific representation until they are
+    converted to a concrete Amaranth value.
 
     Note that it is necessary to ensure that Amaranth's view of representation of all values stays
     internally consistent. The class deriving from ``ValueCastable`` must decorate the ``as_value``
@@ -1301,7 +1331,6 @@ class ValueCastable:
         if not hasattr(self, "as_value"):
             raise TypeError(f"Class '{cls.__name__}' deriving from `ValueCastable` must override "
                             "the `as_value` method")
-
         if not hasattr(self.as_value, "_ValueCastable__memoized"):
             raise TypeError(f"Class '{cls.__name__}' deriving from `ValueCastable` must decorate "
                             "the `as_value` method with the `ValueCastable.lowermethod` decorator")

tests/test_hdl_ast.py

@@ -42,7 +42,7 @@ class ShapeTestCase(FHDLTestCase):
 
     def test_compare_wrong(self):
         with self.assertRaisesRegex(TypeError,
-                r"^Shapes may be compared with other Shapes and \(int, bool\) tuples, not 'hi'$"):
+                r"^Shapes may be compared with shape-castable objects, not 'hi'$"):
             Shape(1, True) == 'hi'
 
     def test_compare_tuple_wrong(self):
@@ -141,10 +141,46 @@ class ShapeTestCase(FHDLTestCase):
 
     def test_cast_bad(self):
         with self.assertRaisesRegex(TypeError,
-                r"^Object 'foo' cannot be used as value shape$"):
+                r"^Object 'foo' cannot be converted to an Amaranth shape$"):
             Shape.cast("foo")
 
 
+class MockShapeCastable(ShapeCastable):
+    def __init__(self, dest):
+        self.dest = dest
+
+    def as_shape(self):
+        return self.dest
+
+
+class MockShapeCastableNoOverride(ShapeCastable):
+    def __init__(self):
+        pass
+
+
+class ShapeCastableTestCase(FHDLTestCase):
+    def test_no_override(self):
+        with self.assertRaisesRegex(TypeError,
+                r"^Class 'MockShapeCastableNoOverride' deriving from `ShapeCastable` must "
+                r"override the `as_shape` method$"):
+            sc = MockShapeCastableNoOverride()
+
+    def test_cast(self):
+        sc = MockShapeCastable(unsigned(2))
+        self.assertEqual(Shape.cast(sc), unsigned(2))
+
+    def test_recurse_bad(self):
+        sc = MockShapeCastable(None)
+        sc.dest = sc
+        with self.assertRaisesRegex(RecursionError,
+                r"^Shape-castable object <.+> casts to itself$"):
+            Shape.cast(sc)
+
+    def test_recurse(self):
+        sc = MockShapeCastable(MockShapeCastable(unsigned(1)))
+        self.assertEqual(Shape.cast(sc), unsigned(1))
+
+
 class ValueTestCase(FHDLTestCase):
     def test_cast(self):
         self.assertIsInstance(Value.cast(0), Const)