Proposal: Functor and friends for the wrappers in Data.Monoid

[David Feuer]

I would suggest that the coerce-based instances be stolen from
Data.Identity.
On Feb 21, 2015 10:34 AM, "Oleg Grenrus" <oleg.grenrus at iki.fi> wrote:

> I propose to add Functor, Applicative, Monad, Foldable, and Traversable
> and maybe even MonadFix instances to wrapper newtypes in the Data.Monoid
> module.
> The same way as in the semigroups package, e.g.
> https://hackage.haskell.org/package/semigroups-0.16.1/docs/Data-Semigroup.html#t:Min
> <
> https://hackage.haskell.org/package/semigroups-0.16.1/docs/Data-Semigroup.html#t:Min
> >
>
> Basically:
>
> instance Functor Sum where
>   fmap f (Sum x) = Sum (f x)
>
> instance Foldable Sum where
>   foldMap f (Sum a) = f a
>
> instance Traversable Sum where
>   traverse f (Sum a) = Sum <$> f a
>
> instance Applicative Sum where
>   pure = Sum
>   a <* _ = a
>   _ *> a = a
>   Sum f <*> Sum x = Sum (f x)
>
> instance Monad Sum where
>   return = Sum
>   _ >> a = a
>   Sum a >>= f = f a
>
> instance MonadFix Sum where
>   mfix f = fix (f . getSum)
>
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