feat: allow unknown on the is operator

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

src/PostgREST/Query/SqlFragment.hs

@@ -61,7 +61,8 @@ import PostgREST.Request.Types           (Alias, Field, Filter (..),
                                           Operation (..),
                                           OrderDirection (..),
                                           OrderNulls (..),
-                                          OrderTerm (..), SelectItem)
+                                          OrderTerm (..), SelectItem,
+                                          TrileanVal (..))
 
 import Protolude hiding (cast)
 
@@ -234,9 +235,18 @@ pgFmtFilter table (Filter fld (OpExpr hasNot oper)) = notOp <> " " <> case oper
    Op op val  -> pgFmtFieldOp op <> " " <> case op of
      "like"  -> unknownLiteral (T.map star val)
      "ilike" -> unknownLiteral (T.map star val)
-     "is"    -> isAllowed val
      _       -> unknownLiteral val
 
+   -- IS cannot be prepared. `PREPARE boolplan AS SELECT * FROM projects where id IS $1` will give a syntax error.
+   -- The above can be fixed by using `PREPARE boolplan AS SELECT * FROM projects where id IS NOT DISTINCT FROM $1;`
+   -- However that would not accept the TRUE/FALSE/NULL/UNKNOWN keywords. See: https://stackoverflow.com/questions/6133525/proper-way-to-set-preparedstatement-parameter-to-null-under-postgres.
+   -- This is why `IS` operands are whitelisted at the Parsers.hs level
+   Is triVal -> pgFmtField table fld <> " IS " <> case triVal of
+     TriTrue    -> "TRUE"
+     TriFalse   -> "FALSE"
+     TriNull    -> "NULL"
+     TriUnknown -> "UNKNOWN"
+
    -- We don't use "IN", we use "= ANY". IN has the following disadvantages:
    -- + No way to use an empty value on IN: "col IN ()" is invalid syntax. With ANY we can do "= ANY('{}')"
    -- + Can invalidate prepared statements: multiple parameters on an IN($1, $2, $3) will lead to using different prepared statements and not take advantage of caching.
@@ -252,13 +262,6 @@ pgFmtFilter table (Filter fld (OpExpr hasNot oper)) = notOp <> " " <> case oper
    sqlOperator o = SQL.sql $ M.lookupDefault "=" o operators
    notOp = if hasNot then "NOT" else mempty
    star c = if c == '*' then '%' else c
-   -- IS cannot be prepared. `PREPARE boolplan AS SELECT * FROM projects where id IS $1` will give a syntax error.
-   -- The above can be fixed by using `PREPARE boolplan AS SELECT * FROM projects where id IS NOT DISTINCT FROM $1;`
-   -- However that would not accept the TRUE/FALSE/NULL keywords. See: https://stackoverflow.com/questions/6133525/proper-way-to-set-preparedstatement-parameter-to-null-under-postgres.
-   isAllowed :: Text -> SQL.Snippet
-   isAllowed v = SQL.sql $ maybe
-     (pgFmtLit v <> "::unknown") encodeUtf8
-     (find ((==) . T.toLower $ v) ["null","true","false"])
 
 pgFmtJoinCondition :: JoinCondition -> SQL.Snippet
 pgFmtJoinCondition (JoinCondition (qi1, col1) (qi2, col2)) =

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