[Haskell-cafe] Re: monadplus as monoid, safe?

Of course I find it two minutes after I send the question. Sorry for the two noop emails. Nick On 8/30/06, Nicolas Frisby wrote:

I'm using the following code in some of my projects:

...

newtype MonadPlusAsMonoid a = MPM { unMPM :: a } instance MonadPlus m => Monoid (MonadPlusAsMonoid (m a)) where mempty = MPM mzero MPM l `mappend` MPM r = MPM (l `mplus` r)

Is there some sort of pitfall I'm not considering?

It seems fine to me, but I ask because I couldn't find this instance in the darcs repo for the next base library package. I expected to find it there because it has similar instances for numbers as monoids (e.g. Sum and Product from Data.Monoid).

Thanks, Nick