Proposal: Functor and friends for the wrappers in Data.Monoid
Edward Kmett
ekmett at gmail.com
Sun Feb 22 07:56:37 UTC 2015
Many instances for data types in Data.Monoid have already been added in
7.10.
Are these still missing?
-Edward
On Sat, Feb 21, 2015 at 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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