[Haskell-cafe] Applicative Monoid instance

[Erik Hesselink]

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 <bhurt at spnz.org> 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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