From 7a4c963ee776082e682bc4b934e934701b12cda2 Mon Sep 17 00:00:00 2001
From: Brian Hicks <brian@brianthicks.com>
Date: Tue, 5 Jan 2021 20:27:31 -0600
Subject: [PATCH] drop anonymous variables when considering if negation is safe

---
 src/Datalog/Rule.elm        | 1 +
 tests/Datalog/RuleTests.elm | 8 ++++++++
 2 files changed, 9 insertions(+)

diff --git a/src/Datalog/Rule.elm b/src/Datalog/Rule.elm
index a9f9c14..731b17a 100644
--- a/src/Datalog/Rule.elm
+++ b/src/Datalog/Rule.elm
@@ -89,6 +89,7 @@ isNegationSafe (Rule _ body_) =
         |> (\{ positive, negative } ->
                 negative
                     |> Set.dropIf (Set.memberOf positive)
+                    |> Set.dropIf ((==) Term.Anonymous)
                     |> Set.isEmpty
            )
 
diff --git a/tests/Datalog/RuleTests.elm b/tests/Datalog/RuleTests.elm
index d000552..94ab1dd 100644
--- a/tests/Datalog/RuleTests.elm
+++ b/tests/Datalog/RuleTests.elm
@@ -38,6 +38,14 @@ ruleTest =
                     , negative (atom "reachable" [ variable "a", variable "b" ])
                     ]
                     |> Expect.ok
+        , test "negative terms which are introduced outside the head are still allowed if they appear in positive form" <|
+            \_ ->
+                rule
+                    (atom "peopleWithoutEmails" [ variable "Name" ])
+                    [ positive (atom "people" [ variable "Id", variable "Name" ])
+                    , negative (atom "peopleToEmails" [ variable "Id", anonymous ])
+                    ]
+                    |> Expect.ok
         , test "negative terms are not allowed if they don't also appear in a positive form" <|
             \_ ->
                 rule