[Haskell-cafe] What is MonadPlus good for?

[ajb at spamcop.net]

G'day all.

Quoting Josef Svenningsson <josef.svenningsson at gmail.com>:

> 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