2 files changed, 45 insertions, 0 deletions
diff --git a/abdl/feature.py b/abdl/feature.py
new file mode 100644
index 0000000..9821bd2
--- /dev/null
+++ b/abdl/feature.py
@@ -0,0 +1,40 @@
+# This file is part of A Boneless Datastructure Language
+# Copyright (C) 2020  Soni L.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+"""Feature Deprecation Control.
+
+When features get deprecated, they show up here.
+
+Libraries must never use this module directly. Instead, it should be up to
+applications and test suites to use this module.
+"""
+
+class PredicateTypePredicate(DeprecationWarning):
+    """Indicates that a Predicate type is being used as a predicate.
+
+    This usually indicates a bug.
+
+    The following triggers this warning:
+
+        >>> import abdl
+        >>> abdl.match("->x:$x", "x", {"x": abdl.predicates.Predicate})
+
+    The following doesn't:
+
+        >>> import abdl
+        >>> predicate = abdl.predicates.IsInstance(abdl.predicates.Predicate)
+        >>> abdl.match("->x:$x", "x", {"x": predicate})
+    """
diff --git a/abdl/predicates.py b/abdl/predicates.py
index 4df9cdf..57fb976 100644
--- a/abdl/predicates.py
+++ b/abdl/predicates.py
@@ -21,6 +21,9 @@ See ``abdl.predicates.Predicate`` and the language reference for details.
 
 # pylint: disable=too-few-public-methods
 
+import warnings
+import abdl.feature
+
 class Predicate:
     """A predicate checks if an object is accepted in an ABDL expression.
     """
@@ -89,6 +92,8 @@ def _to_predicate(obj):
         return obj
     if isinstance(obj, tuple):
         return Union(obj)
+    if isinstance(obj, type) and issubclass(obj, Predicate):
+        warnings.warn("deprecated", abdl.feature.PredicateTypePredicate)
     # I don't know if anyone relies on the old behaviour of passing the thing directly to isinstance
     # but this lets the exceptions be raised almost exactly like before
     return IsInstance(obj)