Error handling

Errors

data A
data A'

data B
data B'

data C
data C'

data ExtraX
data ExtraY

data D = D A B C
data D' = D' A' B' C' ExtraX ExtraY

data AError
data BError
data CError 

data ExtraXError 
data ExtraYError

data DSubError = DSubError (Maybe AError) (Maybe BError) (Maybe CError)
instance Monoid DSubError

data DError = DError (Maybe DSubError) (Maybe ExtraXError) (Maybe ExtraYError)
instance Monoid DError

tA :: Monad m -> m (Either AError A')
tB :: Monad m -> m (Either BError B')
tC :: Monad m -> m (Either CError C')

getExtraX :: A' -> C' -> Either ExtraXError ExtraX
getExtraY :: A' -> B' -> Either ExtraYError ExtraY

tD :: Monad m -> m (Either DError D')
tD a b c = do
  a <- tA a
  b <- tB b
  c <- tC c
  x <- case (a, c) of
        (Just a', Just c') -> Just $ getExtraX a' c'
        _ -> Nothing
  y <- case (a', b') of
        (Just a', Just b') -> Just $ getExtraY a' b'
        _ -> Nothing
  case (a, b, c, x, y) of
    (Just a', Just b', Just c', Just (Right x'), Just (Right y')) -> pure $ Right (D' a' b' c' x' y')
    _ -> pure $ Left ...