[Haskell-cafe] Variable-arity zipWith (re)invented.

[José Pedro Magalhães]

Hi Takayuki,

Just thought I'd mention another approach to a variadic zipWith, this one
using type families:

http://typesandkinds.wordpress.com/2012/11/26/variable-arity-zipwith/

The current lack of overlap in type families makes things a bit more
complicated, but it can
be solved using the upcoming overlapping type families.


Cheers,
Pedro

On Sat, Dec 8, 2012 at 3:27 PM, Takayuki Muranushi <muranushi at gmail.com>wrote:

> Continued discussion from
>
> https://groups.google.com/d/topic/haskell-cafe/-e-xaCEbd-w/discussion
> https://groups.google.com/d/topic/haskell-cafe/kM_-NvXAcx8/discussion
>
> Thank you for all the answeres and thinkings;
>
>
> Here's zipWithN for general Zip functors: [1] . This, together with
> [2] may constitute a small hackage. A modification from Wren's idea to
> [1] is the use of fmap instead of repeat.
>
> I'm wondering if there are any laws for Zip functors. I first thought
> that there are similarity between Zips and Applicatives, as [3] states
>
> >    instance Applicative f => Zip f where
> >        zip = liftA2 (,)
>
> However, my intuition is that zipping two arrays should result in an
> array of size of the same order as two, giving rise to a Zip functor
> law candidate:
>
> zipWith const xs $ zipWith const xs ys == zipWith const xs ys
>
> which is violated by the above statement "zip = liftA2 (,)" .
>
>
>
>
> [1]
> https://github.com/nushio3/practice/blob/master/variable-arity/ZipWithN-2.hs
> [2]
> https://github.com/nushio3/practice/blob/master/free-objects/zipf-12.hs
> [3]
> http://hackage.haskell.org/packages/archive/TypeCompose/0.9.7/doc/html/Data-Zip.html
>
>
>
>
> --
> Takayuki MURANUSHI
> The Hakubi Center for Advanced Research, Kyoto University
> http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html
>
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