Re: [Haskell-cafe] Applicative Monoid instance

Probably something to do with not making overlapping instances. Not sure. Easiest/best approach is probably to make a newtype wrapper around IO or IO (). Then you can define the monoid instance you want. Will

On Sep 5, 2016, at 13:54, 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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