Dear list, I'm unhappy about the current state of Control.Monad.Error; the module is deprecated, stating: Module ‘Control.Monad.Error’ is deprecated: Use "Control.Monad.Except" instead However, Control.Monad.Except implements 'fail' as an error, inheriting the behavior of Identity. To my mind the whole point of Control.Monad.Error is that it replicates the old non-error behavior of `fail` that the Either type and EitherT used to have. To wit:
fail "123" :: ErrorT String Identity () ErrorT (Identity (Left "123"))
fail "123" :: Except String () ExceptT (Identity *** Exception: 123
In this light I find this deprecation message disingenuous. The situation is similar with the MonadFail class; there is are instance (Monad m, Error e) => MonadFail (ErrorT e m) and instance MonadFail m => MonadFail (ExceptT e m) which means that Except doesn't have a MonadFail instance, because Identity has none, whereas ErrorT implements a non-error version of `fail`. Can we undeprecate the Control.Monad.Error module? Cheers, Bertram