Add fetch and toParamList for passing parameters

test/Database/PostgreSQL/OpiumSpec.hs

@@ -7,6 +7,7 @@
 module Database.PostgreSQL.OpiumSpec (spec) where
 
 import Data.ByteString (ByteString)
+import Data.Functor.Identity (Identity (..))
 import Data.Proxy (Proxy (..))
 import Data.Text (Text)
 import Database.PostgreSQL.LibPQ (Connection)
@@ -46,6 +47,12 @@ data ScoreByAge = ScoreByAge
 
 instance Opium.FromRow ScoreByAge where
 
+data Only a = Only
+  { only :: a
+  } deriving (Eq, Generic, Show)
+
+instance Opium.FromField a => Opium.FromRow (Only a) where
+
 isLeft :: Either a b -> Bool
 isLeft (Left _) = True
 isLeft _ = False
@@ -117,6 +124,15 @@ spec = do
       row <- Opium.fromRow result columnTable 0
       row `shouldBe` Right (ManyFields "abc" 42 1.0 "test" True)
 
+  describe "fetch" $ do
+    it "Passes numbered parameters and retrieves a list of rows" $ \conn -> do
+      rows <- Opium.fetch conn "SELECT ($1 + $2) AS only" (17 :: Int, 25 :: Int)
+      rows `shouldBe` Right [Only (42 :: Int)]
+
+    it "Uses Identity to pass single parameters" $ \conn -> do
+      rows <- Opium.fetch conn "SELECT count(*) AS only FROM person WHERE name = $1" $ Identity ("paul" :: Text)
+      rows `shouldBe` Right [Only (1 :: Int)]
+
   describe "fetch_" $ do
     it "Retrieves a list of rows" $ \conn -> do
       rows <- Opium.fetch_ conn "SELECT * FROM person"