Add unit tests and reimplement FromField

lib/Database/PostgreSQL/Opium.hs

@@ -5,12 +5,19 @@
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeOperators #-}
 
-module Database.PostgreSQL.Opium where
+module Database.PostgreSQL.Opium
+  ( FromField (..)
+  , FromRow (..)
+  , fetch_
+  )
+  where
 
+import Control.Monad.Trans.Maybe (MaybeT (..))
 import Data.ByteString (ByteString)
 import Data.Proxy (Proxy (Proxy))
 import Database.PostgreSQL.LibPQ
-  (Result
+  ( Connection
+  , Result
   , Row
   )
 import GHC.Generics (C, D, Generic, K1 (..), M1 (..), Meta (..), Rec0, Rep, S, to, (:*:) (..))
@@ -21,6 +28,18 @@ import qualified Data.Text as Text
 import qualified Data.Text.Encoding as Encoding
 import qualified Database.PostgreSQL.LibPQ as LibPQ
 
+import Database.PostgreSQL.Opium.FromField (FromField (..))
+
+fetch_ :: FromRow a => Connection -> ByteString -> IO (Maybe [a])
+fetch_ conn query = runMaybeT $ do
+  result <- MaybeT $ LibPQ.execParams conn query [] LibPQ.Text
+  MaybeT $ fetchResult result
+
+fetchResult :: FromRow a => Result -> IO (Maybe [a])
+fetchResult result = do
+  nRows <- LibPQ.ntuples result
+  runMaybeT $ mapM (MaybeT . flip fromRow result) [0..nRows - 1]
+
 class FromRow a where
   fromRow :: Row -> Result -> IO (Maybe a)
   default fromRow :: (Generic a, FromRow' (Rep a)) => Row -> Result -> IO (Maybe a)
@@ -48,16 +67,13 @@ instance (KnownSymbol nameSym, FromField t) => FromRow' (M1 S ('MetaSel ('Just n
       Nothing -> pure Nothing
       Just column -> do
         mbField <- LibPQ.getvalue result row column
-        printf "%s: %s" (show name) (show mbField)
-        pure $ M1 . K1 <$> fromField mbField
+        ty <- LibPQ.ftype result column
+        case fromField ty . Encoding.decodeUtf8 =<< mbField of
+          Nothing -> do
+            format <- LibPQ.fformat result column
+            printf "field %s: %s (oid: %s, format: %s)\n" (show name) (show mbField) (show ty) (show format)
+            pure Nothing
+          Just value ->
+            pure $ Just $ M1 $ K1 value
     where
       name = Encoding.encodeUtf8 $ Text.pack $ symbolVal (Proxy :: Proxy nameSym)
-
-class FromField a where
-  fromField :: Maybe ByteString -> Maybe a
-
-instance FromField String where
-  fromField = fmap (Text.unpack . Encoding.decodeUtf8)
-
-instance FromField Int where
-  fromField = fmap (read . Text.unpack . Encoding.decodeUtf8)

lib/Database/PostgreSQL/Opium/FromField.hs

@@ -0,0 +1,52 @@
+{-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE FlexibleInstances #-}
+
+module Database.PostgreSQL.Opium.FromField (FromField (..)) where
+
+import Data.Attoparsec.Text
+  ( Parser
+  , decimal
+  , parseOnly
+  , signed
+  )
+import Data.Text (Text)
+import Database.PostgreSQL.LibPQ (Oid)
+
+import qualified Data.Text as Text
+import qualified Database.PostgreSQL.Opium.Oid as Oid
+
+(\/) :: (a -> Bool) -> (a -> Bool) -> a -> Bool
+p \/ q = \x -> p x || q x
+
+eitherToMaybe :: Either b a -> Maybe a
+eitherToMaybe = \case
+  Left _ -> Nothing
+  Right x -> Just x
+
+fromParser
+  :: (Oid -> Bool)
+  -> Parser a
+  -> Oid
+  -> Text
+  -> Maybe a
+fromParser validOid parser oid value
+  | validOid oid = eitherToMaybe $ parseOnly parser value
+  | otherwise = Nothing
+
+class FromField a where
+  fromField :: Oid -> Text -> Maybe a
+
+instance FromField Int where
+  fromField = fromParser
+    (Oid.smallint \/ Oid.integer \/ Oid.bigint)
+    (signed decimal)
+
+instance FromField Text where
+  fromField oid text =
+    if Oid.text oid || Oid.character oid || Oid.characterVarying oid then
+      Just text
+    else
+      Nothing
+
+instance FromField String where
+  fromField oid text = Text.unpack <$> fromField oid text

lib/Database/PostgreSQL/Opium/Oid.hs

@@ -0,0 +1,31 @@
+module Database.PostgreSQL.Opium.Oid where
+
+import Database.PostgreSQL.LibPQ (Oid (..))
+
+eq :: Eq a => a -> a -> Bool
+eq = (==)
+
+-- integer types
+
+-- | 16-bit integer
+smallint :: Oid -> Bool
+smallint = eq $ Oid 21
+
+-- | 32-bit integer
+integer :: Oid -> Bool
+integer = eq $ Oid 23
+
+-- | 64-bit integer
+bigint :: Oid -> Bool
+bigint = eq $ Oid 20
+
+-- string types
+
+text :: Oid -> Bool
+text = eq $ Oid 25
+
+character :: Oid -> Bool
+character = eq $ Oid 1042
+
+characterVarying :: Oid -> Bool
+characterVarying = eq $ Oid 1043