Replace ad-hoc isLeft by import from Data.Either

lib/Database/PostgreSQL/Opium.hs

@@ -67,8 +67,9 @@ execParams conn query params = do
         Nothing -> pure result
         Just message -> except $ Left $ ErrorInvalidResult status $ Encoding.decodeUtf8 message
 
+-- The order of the type parameters is important, because it is more common to use type applications for providing the row type.
 fetch
-  :: forall a b. (ToParamList a, FromRow b)
+  :: forall b a. (ToParamList a, FromRow b)
   => Connection
   -> Text
   -> a

test/Database/PostgreSQL/OpiumSpec.hs

@@ -7,6 +7,7 @@
 module Database.PostgreSQL.OpiumSpec (spec) where
 
 import Data.ByteString (ByteString)
+import Data.Either (isLeft)
 import Data.Functor.Identity (Identity (..))
 import Data.Proxy (Proxy (..))
 import Data.Text (Text)
@@ -53,10 +54,6 @@ newtype Only a = Only
 
 instance Opium.FromField a => Opium.FromRow (Only a) where
 
-isLeft :: Either a b -> Bool
-isLeft (Left _) = True
-isLeft _ = False
-
 shouldHaveColumns
   :: Opium.FromRow a
   => Proxy a