Make getColumnTable report missing columns

lib/Database/PostgreSQL/Opium.hs

@@ -62,16 +62,16 @@ fetchResult result = do
   runExceptT $ mapM (ExceptT . flip fromRow result) [0..nRows - 1]
 
 class FromRow a where
-  getColumnTable :: Proxy a -> Result -> IO [LibPQ.Column]
-  default getColumnTable :: (Generic a, GetColumnTable' (Rep a)) => Proxy a -> Result -> IO [LibPQ.Column]
-  getColumnTable Proxy = getColumnTable' @(Rep a) Proxy
+  getColumnTable :: Proxy a -> Result -> IO (Either Error [Column])
+  default getColumnTable :: (Generic a, GetColumnTable' (Rep a)) => Proxy a -> Result -> IO (Either Error [Column])
+  getColumnTable Proxy = runExceptT . getColumnTable' @(Rep a) Proxy
 
   fromRow :: Row -> Result -> IO (Either Error a)
   default fromRow :: (Generic a, FromRow' (Rep a)) => Row -> Result -> IO (Either Error a)
   fromRow row result = fmap to <$> fromRow' row result
 
 class GetColumnTable' f where
-  getColumnTable' :: Proxy (f p) -> Result -> IO [LibPQ.Column]
+  getColumnTable' :: Proxy (f p) -> Result -> ExceptT Error IO [Column]
 
 instance GetColumnTable' f => GetColumnTable' (M1 D c f) where
   getColumnTable' Proxy = getColumnTable' @f Proxy
@@ -83,17 +83,15 @@ instance (GetColumnTable' f, GetColumnTable' g) => GetColumnTable' (f :*: g) whe
   getColumnTable' Proxy result =
     (++) <$> getColumnTable' @f Proxy result <*> getColumnTable' @g Proxy result
 
-checkColumn :: FromField f => Proxy f -> String -> Result -> IO [Column]
-checkColumn Proxy nameStr result = LibPQ.fnumber result name >>= \case
-    Just column -> do
-      -- TODO: Rewrite FromField to check whether oid works for decoding t
-      _oid <- LibPQ.ftype result column
-      pure [column]
-    Nothing -> do
-      -- TODO: Return ErrorMissingColumn
-      undefined
+checkColumn :: FromField f => Proxy f -> String -> Result -> ExceptT Error IO [Column]
+checkColumn Proxy nameStr result = do
+    column <- ExceptT $ maybe (Left $ ErrorMissingColumn nameText) Right <$> LibPQ.fnumber result name
+    -- TODO: Rewrite FromField to check whether oid works for decoding t
+    _oid <- ExceptT $ Right <$> LibPQ.ftype result column
+    pure [column]
     where
-      name = Encoding.encodeUtf8 $ Text.pack nameStr
+      nameText = Text.pack nameStr
+      name = Encoding.encodeUtf8 nameText
 
 instance {-# OVERLAPPABLE #-} (FromField t, KnownSymbol nameSym) => GetColumnTable' (M1 S ('MetaSel ('Just nameSym) nu ns dl) (Rec0 t)) where
   getColumnTable' Proxy = checkColumn @t Proxy $ symbolVal @nameSym Proxy

lib/Database/PostgreSQL/Opium/FromField.hs

@@ -29,6 +29,7 @@ p \/ q = \x -> p x || q x
 
 data FieldError
   = FieldErrorUnexpectedNull
+  -- TODO: Move this to the normal Error
   | FieldErrorInvalidOid Oid
   | FieldErrorInvalidField Oid Text String
   deriving (Eq, Show)