[UPDATED] Add missing Monad instances to tuples

[Fumiaki Kinoshita]

They are writer monads for sure, but WriterT does not subsume them. If a
typeclass method has type Writer w a, it does not allow
GeneralizedNewtypeDeriving on it due to the type roles.

I don't really have any idea how instance warnings should look like.

2019年4月7日(日) 17:58 Henning Thielemann <lemming at henning-thielemann.de>:

>
> On Sun, 7 Apr 2019, Fumiaki Kinoshita wrote:
>
> > I removed Foldable/Traversable instances from the patch with reluctance.
> I'd still like to add these instances:
> >
> > Functor ((,,) a b)
> > (Monoid a, Monoid b) => Applicative ((,,) a b)
> > (Monoid a, Monoid b) => Monad ((,,) a b)
> > Functor ((,,,) a b c)
> > (Monoid a, Monoid b, Monoid c) => Applicative ((,,,) a b c)
> > (Monoid a, Monoid b, Monoid c) => Monad ((,,,) a b c)
>
> Still duplicates to the Writer monad.
>
> How about extending the patch by the long needed instance warnings? :-)
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