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

[Edward Kmett]

Fair enough, it should just be a matter of writing a patch then.

The general resolution to adopt these sorts of "obvious"
only-one-way-to-do-it instances is already in place.

https://ghc.haskell.org/trac/ghc/ticket/8797
https://mail.haskell.org/pipermail/libraries/2014-June/023229.html
https://ghc.haskell.org/trac/ghc/ticket/9043#comment:23

etc.

At some point during 7.12 we'll need to go through and do an audit to see
which ones are still missing.

-Edward

On Sun, Feb 22, 2015 at 3:04 AM, Oleg Grenrus <oleg.grenrus at iki.fi> wrote:

> Based on
> http://git.haskell.org/ghc.git/blob/refs/heads/ghc-7.10:/libraries/base/Data/Monoid.hs
> only First and Last have Functor, Applicative and Monad; Foldable,
> Traversable and Data are missing.
>
>
> On 22 Feb 2015, at 09:56, Edward Kmett <ekmett at gmail.com> wrote:
>
> 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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>>
>>
>
>
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