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

Oleg Grenrus oleg.grenrus at iki.fi
Sun Feb 22 12:40:13 UTC 2015

```And there are the ticket and the patch:

> On 22 Feb 2015, at 12:07, Oleg Grenrus <oleg.grenrus at iki.fi> wrote:
>
> To clarify the discussion:
>
> Let's add the following instances:
>
> A: Functor, Foldable, Traversable to Sum, Product, Dual, First, and Last
> B: Data to everything in Data.Monoid (except Endo): Sum, Product, Dual, First and Last, All, Any, Alt
> C: Applicative, Monad, MonadFix to Sum, Product, Dual, First and Last
> D: Data to Identity
>
> All seem to agree on A and B, C is "not sure", and D is new item.
>
> - Oleg
>
>> On 21 Feb 2015, at 17:34, 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)
>>
>> return = Sum
>> _ >> a = a
>> Sum a >>= f = f a
>>
>> mfix f = fix (f . getSum)
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