Instance overlap would break a lot of Monad instances (notably [a]), IIRC. This instance comes up on the mailing list every two or three years, it looks like. 2007: https://mail.haskell.org/pipermail/libraries/2007-March/006997.html 2011: https://mail.haskell.org/pipermail/libraries/2011-December/017369.html 2013: http://haskell.1045720.n5.nabble.com/MonadPlus-instance-for-ContT-tp5732028p... There's a newtype-wrapped version of this in Control.Compose [1] [1]: https://hackage.haskell.org/package/TypeCompose-0.9.11/docs/Control-Compose.... Brian Hurt writes:

This may be a stupid question, but I'm wondering why the following instance isn't in prelude:

instance (Applicative m, Monoid a) => Monoid (m a) where mempty = pure mempty mappend a b = mappend <$> a <*> b

?

I ask, because being able to use a WriterT (IO ()) STM a to debug complex STM expressions would be really nice. But it depends upon IO () being a monoid.

Brian _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

-- Jack

I believe this is the App newtype in the monoids package [0]. Erik [0] https://hackage.haskell.org/package/monoids-0.3.2/docs/ Data-Monoid-Applicative.html#t:App On 5 September 2016 at 20:54, Brian Hurt wrote:

This may be a stupid question, but I'm wondering why the following instance isn't in prelude:

instance (Applicative m, Monoid a) => Monoid (m a) where mempty = pure mempty mappend a b = mappend <$> a <*> b

?

I ask, because being able to use a WriterT (IO ()) STM a to debug complex STM expressions would be really nice. But it depends upon IO () being a monoid.

Brian

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