Implement date -> Day decoding

README.md

@@ -63,6 +63,7 @@ getScoreByAge conn = do
 - [x] Implement `Float` and `Double` decoding
 - [x] Clean up and document column table stuff
 - [x] Decode `LibPQ.Binary`
+- [x] Implement `date -> Day` decoding
 - [ ] Implement `fetch` (`fetch_` but with parameter passing)
 - [ ] Implement `UTCTime` and zoned time decoding
 - [ ] Implement JSON decoding

flake.nix

@@ -20,6 +20,7 @@
             hspec
             postgresql-libpq
             text
+            time
             transformers
             vector
           ]))

lib/Database/PostgreSQL/Opium/FromField.hs

@@ -11,7 +11,9 @@ import Data.Attoparsec.ByteString (Parser)
 import Data.Bits (Bits (..))
 import Data.ByteString (ByteString)
 import Data.Functor (($>))
+import Data.Int (Int32)
 import Data.Proxy (Proxy (..))
+import Data.Time (Day (..))
 import Data.Text (Text)
 import Data.Word (Word32, Word64)
 import Database.PostgreSQL.LibPQ (Oid)
@@ -107,3 +109,11 @@ boolParser = AP.choice
 instance FromField Bool where
   validOid Proxy = Oid.boolean
   parseField = boolParser
+
+-- | See https://www.postgresql.org/docs/current/datatype-datetime.html.
+-- Relevant as well: https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/utils/adt/datetime.c;h=267dfd37b2e8b9bc63797c69b9ca2e45e6bfde61;hb=HEAD#l267.
+instance FromField Day where
+  validOid Proxy = Oid.date
+  parseField = fromJulianDay . fromIntegral <$> intParser @Int32
+    where
+      fromJulianDay x = ModifiedJulianDay $ x + 51544

lib/Database/PostgreSQL/Opium/Oid.hs

@@ -48,3 +48,7 @@ doublePrecision = eq $ Oid 701
 -- | Boolean
 boolean :: Oid -> Bool
 boolean = eq $ Oid 16
+
+-- | Single days/dates.
+date :: Oid -> Bool
+date = eq $ Oid 1082

opium.cabal

@@ -79,6 +79,7 @@ library
         containers,
         postgresql-libpq,
         text,
+        time,
         transformers,
         vector
 
@@ -122,4 +123,5 @@ test-suite opium-test
         bytestring,
         hspec,
         postgresql-libpq,
+        time,
         text

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]