[Haskell-beginners] Re: Design Question: fmap with two lists

[Brent Yorgey]

On Fri, Dec 25, 2009 at 01:48:25AM +0100, Maciej Piechotka wrote:
> On Thu, 2009-12-24 at 22:21 +0000, jonathanGfischoff at gmail.com wrote:
> > I have a class that is basically:
> > 
> 
> Hmm. Class in haskell means something similar to interface in OOP. Yes
> it is confusing.
> 
> > data TwoLists = TL {doubleList :: [Double], intList :: [Int]}
> > 
> > Almost all of the operations of this type map, fold, etc on one of the
> > lists or the other, and then return a new instance of the type.
> 
> It may be my level of English bit I cannot understend this sentence.
> 
> > So I am implementing to functions dFmap, and iFmap.
> 
> What is dFmap and iFmap?
> 
> > I actually would like all of the list functions, but I don't want to
> > reimplement them.
> > 
> 
> Hmm. If you need to combine the lists I'd suggest zip/zipWith. If simply
> map/fold over one then just write (or similar):
> 
> foldr f i . doubleList 
> 
> If you mean if there is class:
> class BiFunctor f where
>   fmap2 :: (a -> c) -> (b -> d) -> f a b -> f c d
>   fmap2 f g = fmap2b g . fmap2a f
>   fmap2a :: (a -> c) -> f a b -> f c b
>   fmap2a f = fmap2 f id
>   fmap2b :: (b -> d) -> f a b -> f a d
>   fmap2b = fmap2 id

There is such a class in the category-extras package, although it is
so general that it is probably not worth the effort of trying to
understand it unless you really need the extra abstraction.

-Brent