I proposed MonadZero originally, so it feels somehow appropriate that I’m going to be the one to argue for its demise.

Anyway, as noted in #51, the MonadZero annihilation law empty >>= f = empty is automatically satisfied for any law-abiding monad, due to parametricity: while evaluating empty >>= f, the function f can’t ever be called, since that would require empty to produce a value, which means that empty >>= f must be the same as empty >>= pure, which by the monad laws is just empty.

As far as I’m aware, the only place we’re using MonadZero is in the definition of guard. However, I’d argue that the Alternative annihilation law empty <*> f = empty already gives us enough to ensure that guard behaves sensibly. Haskell seems to agree, with its guard only requiring Alternative.

(Note: The Alternative annihilation law isn’t redundant: for a type which isn’t a monad, the effects of the second argument of <*> could be performed before the first).

Therefore, I propose we do the following:

• Add a deprecation notice to the MonadZero class
• Change guard to only require Alternative
• Change MonadPlus to have Monad and Alternative superclasses instead of MonadZero
• In a subsequent breaking release, remove MonadZero.