Re: MonadPlus instance for ContT

11 Jan 2009

      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