Am Sonntag, 11. Januar 2009 12:13 schrieben Sie:

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

Should all be satisfied.

and one of these:

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

Should be satisfied.

mplus (return a) b = return a

Obviously not satisfied in general. What has to be done to get such an instance into the libs? Best wishes, Wolfgang