Proposal: newtype for applicatives of monoids

[David Casperson]

On 2015-08-03, Mario Blažević wrote:

> Date: Mon, 3 Aug 2015 10:22:56
> From: Mario Blažević <blamario at ciktel.net>
> To: libraries at haskell.org
> Subject: Re: Proposal: newtype for applicatives of monoids
> 
> On 07/26/2015 11:34 PM, David Feuer wrote:
>> 
>> Sorry for the terrible title, but I think we should add something like the 
>> following to Data.Monoid (I already mentioned this in the discussion of a 
>> Monoid instance for ZipList).
>> 
>> newtype Appl f m = Appl (f m)
>> 
>> instance (Applicative f, Monoid m) => Monoid (Appl f m) where
>>   mempty = Appl $ pure mempty
>>   mplus (Appl x) (Appl y) = Appl $ liftA2 mplus x y
>> 
>
> The method is called mappend, not mplus. Otherwise +1.
>
>
>> It may be possible to use coercions to improve mplus; I haven't tried it.
>> 
>> This type is the same as WrappedApplicative from semigroupoids, do it might 
>> be best just to give it that name.

+1, with mappend.

(I just tripped over the Monad instance of the same thing. It's
  very nice to be able to use (<>) on something like Monad m =>
  Control.Monad.Trans.Exception m () .)

Cheers,
David
-- 
David Casperson, PhD, R.P.,                  |  David.Casperson at unbc.ca
Associate Professor and Chair,               |  (250)   960-6672 Fax 960-5544
Computer Science                             |  3333 University Way
University of Northern British Columbia      |  Prince George, BC   V2N 4Z9
                                              |  CANADA