throwError

throwError :: (Monad m, Error e) => e -> ErrorT e m a
transformers Control.Monad.Trans.Error
Signal an error value e. *  (throwError e) = return > (Left *  e >>= m = throwError
throwError :: MonadError e m => e -> m a
mtl Control.Monad.Error.Class, mtl Control.Monad.Error