Greater type safety for OrderTerm

src/PostgREST/Parsers.hs

@@ -101,8 +101,12 @@ pOrderTerm =
   try ( do
     c <- pFieldName
     _ <- pDelimiter
-    d <- string "asc" <|> string "desc"
-    nls <- optionMaybe (pDelimiter *> ( try(string "nullslast" *> pure ("nulls last"::String)) <|> try(string "nullsfirst" *> pure ("nulls first"::String))))
-    return $ OrderTerm (cs c) (cs d) (cs <$> nls)
+    d <- (string "asc" *> pure OrderAsc)
+         <|> (string "desc" *> pure OrderDesc)
+    nls <- optionMaybe (pDelimiter *> (
+                 try(string "nullslast" *> pure OrderNullsLast)
+             <|> try(string "nullsfirst" *> pure OrderNullsFirst)
+           ))
+    return $ OrderTerm c d nls
   )
-  <|> OrderTerm <$> (cs <$> pFieldName) <*> pure "asc" <*> pure Nothing
+  <|> OrderTerm <$> (cs <$> pFieldName) <*> pure OrderAsc <*> pure Nothing

src/PostgREST/QueryBuilder.hs

@@ -321,8 +321,20 @@ orderF ts =
     queryTerm :: OrderTerm -> Text
     queryTerm t = " "
            <> cs (pgFmtIdent $ otTerm t) <> " "
-           <> cs (otDirection t)         <> " "
-           <> maybe "" cs (otNullOrder t) <> " "
+           <> sqlOrderDirection (otDirection t) <> " "
+           <> maybe "" sqlOrderNulls (otNullOrder t) <> " "
+
+sqlOrderDirection :: OrderDirection -> SqlFragment
+sqlOrderDirection d =
+  case d of
+    OrderDesc -> "desc"
+    OrderAsc  -> "asc"
+
+sqlOrderNulls :: OrderNulls -> SqlFragment
+sqlOrderNulls d =
+  case d of
+    OrderNullsFirst -> "nulls first"
+    OrderNullsLast  -> "nulls last"
 
 insertableValue :: JSON.Value -> SqlFragment
 insertableValue JSON.Null = "null"

src/PostgREST/Types.hs

@@ -1,8 +1,7 @@
 module PostgREST.Types where
 import Data.Text
 import Data.Tree
-import qualified Data.ByteString.Char8 as BS
-import qualified Data.ByteString.Lazy  as BL
+import qualified Data.ByteString.Lazy as BL
 import Data.Aeson
 import Data.Map
 
@@ -51,10 +50,13 @@ data PrimaryKey = PrimaryKey {
   , pkName  :: Text
 } deriving (Show, Eq)
 
+data OrderDirection = OrderAsc | OrderDesc deriving (Show, Eq)
+data OrderNulls = OrderNullsFirst | OrderNullsLast deriving (Show, Eq)
+
 data OrderTerm = OrderTerm {
   otTerm      :: Text
-, otDirection :: BS.ByteString
-, otNullOrder :: Maybe BS.ByteString
+, otDirection :: OrderDirection
+, otNullOrder :: Maybe OrderNulls
 } deriving (Show, Eq)
 
 data QualifiedIdentifier = QualifiedIdentifier {