Implement date -> Day decoding

test/Database/PostgreSQL/Opium/FromFieldSpec.hs

@@ -4,6 +4,7 @@
 module Database.PostgreSQL.Opium.FromFieldSpec (spec) where
 
 import Data.ByteString (ByteString)
+import Data.Time (Day (..), fromGregorian)
 import Data.Text (Text)
 import Database.PostgreSQL.LibPQ (Connection)
 import Database.PostgreSQL.Opium (FromRow)
@@ -72,6 +73,12 @@ newtype ABool = ABool
 
 instance FromRow ABool where
 
+newtype ADay = ADay
+  { day :: Day
+  } deriving (Eq, Generic, Show)
+
+instance FromRow ADay where
+
 shouldFetch :: (Eq a, FromRow a, Show a) => Connection -> ByteString -> [a] -> IO ()
 shouldFetch conn query expectedRows = do
   actualRows <- Opium.fetch_ conn query
@@ -200,3 +207,10 @@ spec = do
       shouldFetch conn "SELECT 'no'::boolean AS bool" [ABool False]
       shouldFetch conn "SELECT 'off'::boolean AS bool" [ABool False]
       shouldFetch conn "SELECT 0::boolean AS bool" [ABool False]
+
+  describe "FromField Day" $ do
+    it "Decodes date" $ \conn -> do
+      shouldFetch conn "SELECT date '1970-01-01' AS day" [ADay $ fromGregorian 1970 1 1]
+      shouldFetch conn "SELECT date '2023-09-23' AS day" [ADay $ fromGregorian 2023 9 23]
+      -- Example from postgres doc page
+      shouldFetch conn "SELECT date 'J2451187' AS day" [ADay $ fromGregorian 1999 1 8]