Add unit tests and reimplement FromField

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