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

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 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@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata@joachim-breitner.de • GPG-Key: 0xF0FBF51F Debian Developer: nomeata@debian.org

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries