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

G'day all. Quoting Josef Svenningsson :

I think it's unfair to the monad transformers to simply say that they don't obey the law. The interesting thing is whether they *preserve* the law. A monad transformer T preserves a law if given a monad M which obeys the law holds then the monad T M obeys the law.

The law in question is that mzero is a right-zero for bind. How can an underlying monad be said to "obey" this law if it doesn't support mzero? Cheers, Andrew Bromage