either.hs

combine :: Monoid e
   => (a -> Either e r1)
   -> (b -> Either e r2)
   -> (a,b)
   -> Either (Either e1 (e1,e2)) (r1,r2)
combine fa fb (a,b) =
  let faa = fa a
      fbb = fb b
  in case faa of
    Left e1 -> Left (Left e1)
    Right r1 -> case fbb of
      Left e2 ->