Re: MonadPlus instance for ContT

Because there are many monads for which you do not want this construction! It isn't what you mean when you use mappend on [a] for sure! It is also different from the behavior for Monad, and it rules out those and many many other perfectly valid uses by overlap. -Edward On Mon, Jun 24, 2013 at 3:20 PM, John Wiegley wrote:

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Edward Kmett writes:

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The third construction is the 'universal' lifting that works for every Applicative, which would look like:

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instance (Monad m, Monoid a) => Monoid (ContT r m a) where mempty = return mempty mappend = liftM2 mappend

Why not then this? That is, once AMP is out...

instance (Applicative m, Monoid a) => Monoid (m a) where mempty = pure mempty mappend = liftA2 mappend

-- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net

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