feat: allow unknown on the is operator

Also make the IS values strict at the Parser level since IS cannot be
parametrized.

src/PostgREST/Request/Parsers.hs

@@ -195,15 +195,22 @@ pOpExpr pSVal = try ( string "not" *> pDelimiter *> (OpExpr True <$> pOperation)
     pOperation =
           Op . toS <$> foldl1 (<|>) (try . ((<* pDelimiter) . string) . toS <$> M.keys ops) <*> pSVal
       <|> In <$> (try (string "in" *> pDelimiter) *> pListVal)
+      <|> Is <$> (try (string "is" *> pDelimiter) *> pTriVal)
       <|> pFts
       <?> "operator (eq, gt, ...)"
 
+    pTriVal = try (string "null"    $> TriNull)
+          <|> try (string "unknown" $> TriUnknown)
+          <|> try (string "true"    $> TriTrue)
+          <|> try (string "false"   $> TriFalse)
+          <?> "null or trilean value (unknown, true, false)"
+
     pFts = do
       op   <- foldl1 (<|>) (try . string . toS <$> ftsOps)
       lang <- optionMaybe $ try (between (char '(') (char ')') (many (letter <|> digit <|> oneOf "_")))
       pDelimiter >> Fts (toS op) (toS <$> lang) <$> pSVal
 
-    ops = M.filterWithKey (const . flip notElem ("in":ftsOps)) operators
+    ops = M.filterWithKey (const . flip notElem ("in":"is":ftsOps)) operators
     ftsOps = M.keys ftsOperators
 
 pSingleVal :: Parser SingleVal

src/PostgREST/Request/Types.hs

@@ -31,6 +31,7 @@ module PostgREST.Request.Types
   , ReadRequest
   , SelectItem
   , SingleVal
+  , TrileanVal(..)
   , fstFieldNames
   ) where
 
@@ -209,6 +210,7 @@ data OpExpr =
 data Operation
   = Op Operator SingleVal
   | In ListVal
+  | Is TrileanVal
   | Fts Operator (Maybe Language) SingleVal
   deriving (Eq)
 
@@ -220,3 +222,11 @@ type SingleVal = Text
 
 -- | Represents a list value in a filter, e.g. id=in.(val1,val2,val3)
 type ListVal = [Text]
+
+-- | Three-valued logic values
+data TrileanVal
+  = TriTrue
+  | TriFalse
+  | TriNull
+  | TriUnknown
+  deriving Eq