diff --git a/debugger_protocol/arg/_decl.py b/debugger_protocol/arg/_decl.py
index 4f0c2ff9..3b62e91c 100644
--- a/debugger_protocol/arg/_decl.py
+++ b/debugger_protocol/arg/_decl.py
@@ -9,6 +9,17 @@ REF = '<ref>'
 TYPE_REFERENCE = sentinel('TYPE_REFERENCE')
 
 
+def _is_simple(datatype):
+    if datatype is ANY:
+        return True
+    elif datatype in list(SIMPLE_TYPES):
+        return True
+    elif isinstance(datatype, Enum):
+        return True
+    else:
+        return False
+
+
 def _normalize_datatype(datatype):
     cls = type(datatype)
     if datatype == REF or datatype is TYPE_REFERENCE:
@@ -51,31 +62,31 @@ def _replace_ref(datatype, target):
         return datatype
 
 
-class Enum(namedtuple('Enum', 'datatype choices')):
+class Enum(namedtuple('Enum', 'datatype choice')):
     """A simple type with a limited set of allowed values."""
 
     @classmethod
-    def _check_choices(cls, datatype, choices, strict=True):
-        if callable(choices):
-            return choices
+    def _check_choice(cls, datatype, choice, strict=True):
+        if callable(choice):
+            return choice
 
-        if isinstance(choices, str):
-            msg = 'bad choices (expected {!r} values, got {!r})'
-            raise ValueError(msg.format(datatype, choices))
+        if isinstance(choice, str):
+            msg = 'bad choice (expected {!r} values, got {!r})'
+            raise ValueError(msg.format(datatype, choice))
 
-        choices = frozenset(choices)
-        if not choices:
-            raise TypeError('missing choices')
+        choice = frozenset(choice)
+        if not choice:
+            raise TypeError('missing choice')
         if not strict:
-            return choices
+            return choice
 
-        for value in choices:
+        for value in choice:
             if type(value) is not datatype:
-                msg = 'bad choices (expected {!r} values, got {!r})'
-                raise ValueError(msg.format(datatype, choices))
-        return choices
+                msg = 'bad choice (expected {!r} values, got {!r})'
+                raise ValueError(msg.format(datatype, choice))
+        return choice
 
-    def __new__(cls, datatype, choices, **kwargs):
+    def __new__(cls, datatype, choice, **kwargs):
         strict = kwargs.pop('strict', True)
         normalize = kwargs.pop('_normalize', True)
         (lambda: None)(**kwargs)  # Make sure there aren't any other kwargs.
@@ -88,18 +99,19 @@ class Enum(namedtuple('Enum', 'datatype choices')):
         if normalize:
             # There's no need to normalize datatype (it's a simple type).
             pass
-        choices = cls._check_choices(datatype, choices, strict=strict)
+        choice = cls._check_choice(datatype, choice, strict=strict)
 
-        self = super(Enum, cls).__new__(cls, datatype, choices)
+        self = super(Enum, cls).__new__(cls, datatype, choice)
         return self
 
 
-class Union(frozenset):
+class Union(tuple):
     """Declare a union of different types.
 
-    Sets and frozensets are treated equivalently in declarations.
+    The declared order is preserved and respected.
+
+    Sets and frozensets are treated Unions in declarations.
     """
-    __slots__ = ()
 
     @classmethod
     def _traverse(cls, datatypes, op):
@@ -122,11 +134,31 @@ class Union(frozenset):
                 datatypes,
                 lambda dt: _transform_datatype(dt, _normalize_datatype),
             )
-        return super(Union, cls).__new__(cls, datatypes)
+        self = super(Union, cls).__new__(cls, datatypes)
+        self._simple = all(_is_simple(dt) for dt in datatypes)
+        return self
 
     def __repr__(self):
         return '{}{}'.format(type(self).__name__, tuple(self))
 
+    def __hash__(self):
+        return super(Union, self).__hash__()
+
+    def __eq__(self, other):  # honors order
+        if not isinstance(other, Union):
+            return NotImplemented
+        if super(Union, self).__eq__(other):
+            return True
+        if set(self) != set(other):
+            return False
+        if self._simple and other._simple:
+            return True
+        return NotImplemented
+
+
+    def __ne__(self, other):
+        return not (self == other)
+
     @property
     def datatypes(self):
         return set(self)
diff --git a/tests/debugger_protocol/arg/test__decl.py b/tests/debugger_protocol/arg/test__decl.py
index ec296402..c3dfd2ee 100644
--- a/tests/debugger_protocol/arg/test__decl.py
+++ b/tests/debugger_protocol/arg/test__decl.py
@@ -37,8 +37,8 @@ class ModuleTests(unittest.TestCase):
             (ParameterImplBase(str), NOOP),
             (Arg(object(), object()), NOOP),
             (SimpleParameter(str), NOOP),
-            (UnionParameter(str), NOOP),
-            (ArrayParameter(str), NOOP),
+            (UnionParameter(Union(str)), NOOP),
+            (ArrayParameter(Array(str)), NOOP),
             (ComplexParameter(Fields()), NOOP),
             (NOT_SET, NOOP),
             (object(), NOOP),
@@ -73,8 +73,8 @@ class ModuleTests(unittest.TestCase):
             ParameterImplBase(str),
             Arg(object(), object()),
             SimpleParameter(str),
-            UnionParameter(str, int),
-            ArrayParameter(str),
+            UnionParameter(Union(str, int)),
+            ArrayParameter(Array(str)),
             ComplexParameter(Fields()),
             NOT_SET,
             object(),
@@ -204,8 +204,8 @@ class UnionTests(unittest.TestCase):
     def test_normalized(self):
         tests = [
             (REF, TYPE_REFERENCE),
-            ({str, int}, Union(str, int)),
-            (frozenset([str, int]), Union(str, int)),
+            ({str, int}, Union(*{str, int})),
+            (frozenset([str, int]), Union(*frozenset([str, int]))),
             ([str], Array(str)),
             ((str,), Array(str)),
             (None, None),