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
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