MonadPlus instance for ContT

[Ashley Yakeley]

Wolfgang Jeltsch wrote:
> Hello,
> 
> I wonder why there isn’t a MonadPlus instance for ContT. In my opinion, this 
> one would be natural:
> 
>> instance (MonadPlus monad) => MonadPlus (ContT result monad) where
>>
>>     mzero = ContT $ return mzero
>>
>>     ContT cont1 `mplus` ContT cont2 = ContT $ liftM2 mplus cont1 cont2
> 
> return and liftM2 are the ones of (->) (o -> monad result).
> 
> What do you think?

As long as it satisfies the MonadPlus rules.

All of these:

   mplus mzero a = a

   mplus a mzero = a

   mplus (mplus a b) c = mplus a (mplus b c)

   mzero >>= k = mzero

and one of these:

   mplus a b >>= k = mplus (a >>= k) (b >>= k)

   mplus (return a) b = return a

-- 
Ashley Yakeley