Re: MonadPlus instance for ContT

On Mon, 24 Jun 2013, John Wiegley wrote:

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Ross Paterson writes:

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Alistair Lynn has proposed the following instance:

instance (Monoid r, Monad m) => MonadPlus (ContT r m) where mzero = ContT $ const $ return mempty m `mplus` n = ContT $ \ c -> liftM2 mappend (runContT m c) (runContT n c)

but this would also be possible:

instance (MonadPlus m) => MonadPlus (ContT r m) where mzero = ContT $ const mzero m `mplus` n = ContT $ \ c -> runContT m c `mplus` runContT n c

Is one of them better?

I would think that allowing ContT to transform any Monad over any Monoid has more utility than only transforming another MonadPlus. But I have no real world data to back this up, just a hunch.

I guess that if someone has a MonadPlus monad, then he wants to continue to use it when the monad stack grows by a ContT. This would give preference to the second instance.