IO () as Monoid

[Conal Elliott]

Is there a Monoid instance defined for IO () anywhere?  I'm using the
following,

instance Monoid (IO ()) where { mempty = pure mempty; mappend = (*>) }

This definition follows a general form mentioned in
http://haskell.org/haskellwiki/TypeComposition,

instance (Applicative f, Monoid a) => Monoid (f a) where
  mempty = pure mempty
  mappend = (*>)

without the conflicts (overlapping instances) introduced by this definition.

   - Conal
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