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
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-...
-- 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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