[Haskell-cafe] Re: The danger of Monad ((->) r)

[Isaac Dupree]

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Conal Elliott wrote:
> -- Standard instance: applicative functor applied to monoid
> instance Monoid a => Monoid (IO a) where
>  mempty  = pure mempty
>  mappend = (*>)
> 
> On second thought, I don't really like (*>), since it's easy to
> accidentally
> discard a useful value.  (I dislike (>>) for the same reason.) 

Exactly; because they don't make monoids, because (x `mappend` mempty)
isn't (x), so mempty isn't a right-identity of mappend, so the instance
doesn't follow the monoid laws.

Isaac

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