Implement UTCTime decoding

Remove LocalTime decoding because it requires IO to convert from Postgres time.

README.md

@@ -67,6 +67,7 @@ getScoreByAge conn = do
 - [x] Implement `UTCTime`
 - [x] Implement `ByteString` decoding (`bytea`)
 - [x] Test negative integer decoding, especially for `Integer`
+- [ ] Implement time intervals
 - [ ] and zoned time decoding
 - [ ] Implement `fetch` (`fetch_` but with parameter passing)
 - [ ] Implement JSON decoding

lib/Database/PostgreSQL/Opium/FromField.hs

@@ -19,8 +19,8 @@ import Data.Proxy (Proxy (..))
 import Data.Time
   ( Day (..)
   , DiffTime
-  , LocalTime (..)
   , TimeOfDay
+  , UTCTime (..)
   , addDays
   , fromGregorian
   , picosecondsToDiffTime
@@ -171,23 +171,20 @@ instance FromField TimeOfDay where
   validOid Proxy = Oid.time
   parseField = timeToTimeOfDay <$> parseField @DiffTime
 
--- | See https://www.postgresql.org/docs/current/datatype-datetime.html.
--- | Accepts @timestamp@.
--- Note that Postgres uses the proleptic Gregorian calendar, whereas @Show Day@ and @fromGregorian@ use an astronomical calendar.
--- In short, Postgres treats 1 BC as a leap year and doesn't have a year zero.
--- This means that working with negative dates will be different in Postgres and your application code.
-instance FromField LocalTime where
-  validOid Proxy = Oid.timestamp
-  parseField = fromPostgresTimestamp <$> readBigEndian @Int <$> AP.takeByteString
+fromPostgresTimestamp :: Int -> (Day, DiffTime)
+fromPostgresTimestamp ts = (day, time)
   where
-      fromPostgresTimestamp :: Int -> LocalTime
-      fromPostgresTimestamp ts =
-        let
     (days, microseconds) = ts `divMod` (86400 * 1000000)
     day = fromPostgresJulian $ fromIntegral days
     time = picosecondsToDiffTime $ fromIntegral microseconds * 1000000
-        in
-          LocalTime day (timeToTimeOfDay time)
+
+-- | See https://www.postgresql.org/docs/current/datatype-datetime.html.
+-- Accepts @timestamp with timezone@.
+instance FromField UTCTime where
+  validOid Proxy = Oid.timestampWithTimezone
+  parseField = toUTCTime . fromPostgresTimestamp <$> readBigEndian @Int <$> AP.takeByteString
+    where
+      toUTCTime (day, time) = UTCTime day time
 
 newtype RawField a = RawField a
   deriving (Eq, Show)

lib/Database/PostgreSQL/Opium/Oid.hs

@@ -60,3 +60,7 @@ time = eq $ Oid 1083
 -- | A point in time.
 timestamp :: Oid -> Bool
 timestamp = eq $ Oid 1114
+
+-- | A point in time.
+timestampWithTimezone :: Oid -> Bool
+timestampWithTimezone = eq $ Oid 1184

test/Database/PostgreSQL/Opium/FromFieldSpec.hs

@@ -7,10 +7,11 @@ import Data.ByteString (ByteString)
 import Data.Time
   ( Day (..)
   , DiffTime
-  , LocalTime (..)
   , TimeOfDay (..)
+  , UTCTime (..)
   , fromGregorian
   , secondsToDiffTime
+  , timeOfDayToTime
   )
 import Data.Text (Text)
 import Database.PostgreSQL.LibPQ (Connection)
@@ -100,11 +101,11 @@ newtype ATimeOfDay = ATimeOfDay
 
 instance FromRow ATimeOfDay where
 
-newtype ALocalTime = ALocalTime
-  { localtime :: LocalTime
+newtype AUTCTime = AUTCTime
+  { utctime :: UTCTime
   } deriving (Eq, Generic, Show)
 
-instance FromRow ALocalTime where
+instance FromRow AUTCTime where
 
 newtype ARawField = ARawField
   { raw :: Opium.RawField ByteString
@@ -283,20 +284,20 @@ spec = do
       shouldFetch conn "SELECT time '00:01:00' AS timeofday" [ATimeOfDay $ TimeOfDay 0 1 0]
       shouldFetch conn "SELECT time '13:07:43' AS timeofday" [ATimeOfDay $ TimeOfDay 13 7 43]
 
-  describe "FromField LocalTime" $ do
-    it "Decodes timestamp" $ \conn -> do
-      let ts0 = LocalTime (fromGregorian 2023 10 2) (TimeOfDay 12 42 23)
-      shouldFetch conn "SELECT timestamp '2023-10-02 12:42:23' AS localtime" [ALocalTime ts0]
+  describe "FromField UTCTime" $ do
+    it "Decodes timestamp with timezone" $ \conn -> do
+      let ts0 = UTCTime (fromGregorian 2023 10 2) (timeOfDayToTime $ TimeOfDay 12 42 23)
+      shouldFetch conn "SELECT timestamp with time zone '2023-10-02 12:42:23' AS utctime" [AUTCTime ts0]
 
-      let ts1 = LocalTime (fromGregorian 294275 12 31) (TimeOfDay 23 59 59)
-      shouldFetch conn "SELECT timestamp '294275-12-31 23:59:59' AS localtime" [ALocalTime ts1]
+      let ts1 = UTCTime (fromGregorian 294275 12 31) (timeOfDayToTime $ TimeOfDay 23 59 59)
+      shouldFetch conn "SELECT timestamp with time zone '294275-12-31 23:59:59' AS utctime" [AUTCTime ts1]
 
-      let ts2 = LocalTime (fromGregorian 1 1 1) (TimeOfDay 0 0 0)
-      shouldFetch conn "SELECT timestamp '0001-01-01 00:00:00' AS localtime" [ALocalTime ts2]
+      let ts2 = UTCTime (fromGregorian 1 1 1) (timeOfDayToTime $ TimeOfDay 0 0 0)
+      shouldFetch conn "SELECT timestamp with time zone '0001-01-01 00:00:00' AS utctime" [AUTCTime ts2]
 
       -- See note at the FromField Day instance.
-      let ts3 = LocalTime (fromGregorian 0 2 29) (TimeOfDay 0 0 0)
-      shouldFetch conn "SELECT timestamp '0001-02-29 BC 00:00:00' AS localtime" [ALocalTime ts3]
+      let ts3 = UTCTime (fromGregorian 0 2 29) (timeOfDayToTime $ TimeOfDay 0 0 0)
+      shouldFetch conn "SELECT timestamp with time zone '0001-02-29 BC 00:00:00' AS utctime" [AUTCTime ts3]
 
   describe "FromField RawField" $ do
     it "Simply returns the bytestring without decoding it" $ \conn -> do