creating a list of head variables for a rule

haskell-experiments/src/Datalog/NaiveDatabase.hs

@@ -195,11 +195,11 @@ withFactsAndRules facts rules =
         toConstraintElement :: RuleElement -> ConstraintElement
         toConstraintElement (RuleElementConstant constant) = ConstraintElementConstant constant
         toConstraintElement (RuleElementVariable varName) = ConstraintElementIndex (varIndex varName)
-        extractVarName :: RuleElement -> Text
+        extractVarName :: RuleElement -> Maybe Text
-        extractVarName (RuleElementVariable varName) = varName
+        extractVarName (RuleElementVariable varName) = Just varName
-        extractVarName (RuleElementConstant constant) = throw $ UnexpectedConstantException constant
+        extractVarName (RuleElementConstant constant) = Nothing
         newRule = RelationRule {
-          headVariables = extractVarName <$> variables',
+          headVariables = catMaybes $ extractVarName <$> variables',
           bodyElements = fromBodyConstraint <$> (_bodyConstraints context'')
         } where
           fromBodyConstraint :: BodyConstraint -> RuleBodyElement

haskell-experiments/test/Test/Datalog/NaiveDatabaseSpec.hs

@@ -22,8 +22,6 @@ import qualified Data.Map as Map
 spec :: Spec
 spec = do
   describe "NaiveDatabase operations" $ do
-    it "..." $ do
-      1 `shouldBe` (1 :: Int)
     it "can ingest facts into relations & a universe" $ do
       let db = NaiveDatabase.withFacts 
             [ "parent(\"alice\", \"bob\")."
@@ -79,6 +77,28 @@ spec = do
           ("parent", parentRelation )
         ]
 
+    it "can ingest facts and rules with constants" $ do
+      let db = NaiveDatabase.withFactsAndRules 
+            [] 
+            [ "ancestor(X,\"patriarch\") :- ." ]
+      let ancestorRule = RelationRule {
+          headVariables = [ "X" ], 
+          bodyElements = []
+        }
+      let ancestorRelation = Relation {
+          _arity = 2, 
+          _name = "ancestor",
+          _tuples = Set.empty,
+          _rules = [ ancestorRule ]
+        }
+      relations db `shouldBe`
+        Map.fromList [
+          ("ancestor", ancestorRelation)
+        ]
+
+      constants db `shouldBe` 
+        (Set.fromList $ Sym <$> ["patriarch"])
+
     it "can do basic queries" $ do
       let db = NaiveDatabase.withFacts 
             [ "parent(\"alice\", \"bob\")."