Am Freitag, 17. Juli 2009 21:57 schrieb Edward Kmett:
The main concern with generalizing (++) is that it was once generalized -- completely differently! -- for mplus in MonadPlus. So whether Monoid's mappend is the natural generalization of (++) or MonadPlus's mplus is, is not entirely clear. Neither one can completely subsume all of the use-cases of the other.
I hope that in the long run, we will be able to drop Alternative and MonadPlus. This will be possible once we allow universal quantification in contexts. Instead of writing (MonadPlus m), we can write (Monad m, forall a. Monoid (m a)) then. This makes it rather clear that Monoid (mappend) would be the better generalization of (++). MonadPlus (mplus) is a bit of a hack. Best wishes, Wolfgang