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

[Oleg Grenrus]

As Sean pointed out there is already a thread about this issue.
I agree with Erik’s opinion, https://mail.haskell.org/pipermail/libraries/2014-December/024509.html :

> I'm +1 on the obviously useful instances (Data, Functor) and wouldn’t mind the others either, since I don't see any downsides.

Simple use case for Functor is e.g. fromIntegral <$> s

—

Sean, is there a trac ticket for this issue? I could make a patch early next week, as this seems to be discussed previously.

- Oleg

> On 21 Feb 2015, at 20:21, Joachim Breitner <mail at joachim-breitner.de> wrote:
> 
> Hi,
> 
> Am Samstag, den 21.02.2015, 17:34 +0200 schrieb Oleg Grenrus:
>> I propose to add Functor, Applicative, Monad, Foldable, and
>> Traversable and maybe even MonadFix instances to wrapper newtypes in
>> the Data.Monoid module.
> 
> can you explain when these are useful?
> 
> Especially, the Monad instance might be rather confusing: When I see
> "Monad Sum", I might somehow expect that >>= would somehow, well, sum
> things up. But
> 
>  foo = getSum $ do
>     return 1
>     return 2
>     return 3
>     return 4
> 
> does not return 10
> 
> (At least you are not proposing a MonadPlus instance; that would be then
> really confusing.)
> 
> Greetings,
> Joachim
> 
> 
> 
> 
> --
> Joachim “nomeata” Breitner
>  mail at joachim-breitner.de • http://www.joachim-breitner.de/
>  Jabber: nomeata at joachim-breitner.de  • GPG-Key: 0xF0FBF51F
>  Debian Developer: nomeata at debian.org
> 
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