Re: [Haskell-cafe] What is MonadPlus good for?

On Mon, 14 Feb 2005 10:07:41 -0500, Jacques Carette wrote:

Josef Svenningsson wrote:

...

You claimed that monad transformers break the mzero-is-right-identity-for-bind law because they can be applied to IO. I say, it's not the monad transformers fault. They cannot possibly be expected to repair the law if they are given a faulty monad.

Doesn't that argue for allowing proven and unproven Monads in Haskell?

I believe you misunderstand me. The point that I was trying to make was about monad transformers. I was saying that the best they can do with respect to a law is to preserve it, which I believe all monad transformers should do. Otherwise they don't deserve the name. Turning to monad they should certainly fulfil the laws we associate with a monad. Otherwise they wouldn't be monads! I am not proposing or encouraging that one should declare a data type an instance of monad if they do not obey the monad laws. Cheers, /Josef