Add missing Monad/Traversable instances to tuples

[Fumiaki Kinoshita]

Fine, let's forget about Foldable instances. What about Monad and
Traversable?

2019年4月2日(火) 13:28 Fumiaki Kinoshita <fumiexcel at gmail.com>:

> I submitted https://gitlab.haskell.org/ghc/ghc/merge_requests/644 but it
> might be still controversial. I'd like to get more opinions again.
>
> Here's the list of proposed instances:
>
> Foldable ((,,) a b)
> Foldable ((,,,) a b c)
> Traversable ((,,) a b)
> Traversable ((,,,) a b c)
> 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)
>
> The absence of Traversable instances for tuples feels quite inconsistent
> given that there are Bitraversable instances already.
>
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