Warn/error when using Predicate types as predicate

This only applies to Predicate types. Predicate instances still
function as expected, as do non-Predicate types.

1 files changed, 40 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})
+    """