Replace ad-hoc isLeft by import from Data.Either

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