Add errors and optional fields

lib/Database/PostgreSQL/Opium/Error.hs

@@ -0,0 +1,14 @@
+module Database.PostgreSQL.Opium.Error (Error (..)) where
+
+import Data.Text (Text)
+import Database.PostgreSQL.LibPQ (ExecStatus, Row)
+
+import Database.PostgreSQL.Opium.FromField (FieldError)
+
+data Error
+  = ErrorDecode Row Text FieldError
+  | ErrorNoResult
+  | ErrorInvalidResult ExecStatus Text
+  | ErrorMissingColumn Text
+  | ErrorUnexpectedNull Row Text
+  deriving (Eq, Show)

lib/Database/PostgreSQL/Opium/FromField.hs

@@ -1,13 +1,17 @@
-{-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE UndecidableInstances #-}
 
-module Database.PostgreSQL.Opium.FromField (FromField (..)) where
+module Database.PostgreSQL.Opium.FromField
+  ( FieldError (..)
+  , FromField (..)
+  ) where
 
 import Data.Attoparsec.Text
   ( Parser
   , decimal
   , parseOnly
   , signed
+  , takeText
   )
 import Data.Text (Text)
 import Database.PostgreSQL.LibPQ (Oid)
@@ -18,23 +22,30 @@ 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
+data FieldError
+  = FieldErrorUnexpectedNull
+  | FieldErrorInvalidOid Oid
+  | FieldErrorInvalidField Oid Text String
+  deriving (Eq, Show)
+
+mapLeft :: (b -> c) -> Either b a -> Either c a
+mapLeft f (Left l) = Left $ f l
+mapLeft _ (Right r) = Right r
 
 fromParser
   :: (Oid -> Bool)
   -> Parser a
   -> Oid
   -> Text
-  -> Maybe a
-fromParser validOid parser oid value
-  | validOid oid = eitherToMaybe $ parseOnly parser value
-  | otherwise = Nothing
+  -> Either FieldError a
+fromParser validOid parser oid field
+    | validOid oid =
+      mapLeft (FieldErrorInvalidField oid field) $ parseOnly parser field
+    | otherwise =
+      Left $ FieldErrorInvalidOid oid
 
 class FromField a where
-  fromField :: Oid -> Text -> Maybe a
+  fromField :: Oid -> Text -> Either FieldError a
 
 instance FromField Int where
   fromField = fromParser
@@ -42,11 +53,9 @@ instance FromField Int where
     (signed decimal)
 
 instance FromField Text where
-  fromField oid text =
-    if Oid.text oid || Oid.character oid || Oid.characterVarying oid then
-      Just text
-    else
-      Nothing
+  fromField = fromParser
+    (Oid.text \/ Oid.character \/ Oid.characterVarying)
+    takeText
 
 instance FromField String where
   fromField oid text = Text.unpack <$> fromField oid text