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

[Takayuki Muranushi]

Repeated thanks to you, Adam! Your code is brilliantly simple.

Sadly, I cannot reproduce the behaviors in your comments on my ghci
(7.6.1)  .....
Can we guess why? The version of packages we are using?

Mines are here.

https://github.com/nushio3/practice/tree/master/variable-arity/adam


>>>  :t forZ [1,2,3] (+)
forZ [1,2,3] (+)
  :: (Num t, Num a, TypeCast br HFalse,
      HBuild2' br (HCons [t] HNil) (a -> a -> a) r) =>
     r
>>>  forZ [1,2,3] [10] (+)

<interactive>:13:1:
    Couldn't match type `[y]' with `(a0 -> a0 -> a0) -> t0'
    When using functional dependencies to combine
      Apply ApplyZap (a, b) [y],
        arising from the dependency `f a -> r'
        in the instance declaration at Part1.lhs:193:12
      Apply ApplyZap ([[t2]], [t1]) ((a0 -> a0 -> a0) -> t0),
        arising from a use of `forZ' at <interactive>:13:1-4
    In the expression: forZ [1, 2, 3] [10] (+)
    In an equation for `it': it = forZ [1, 2, 3] [10] (+)
>>> forZ [1,2,3] "hi there" (,)

<interactive>:14:1:
    Couldn't match type `[y]' with `(a0 -> b0 -> (a0, b0)) -> t0'
    When using functional dependencies to combine
      Apply ApplyZap (a, b) [y],
        arising from the dependency `f a -> r'
        in the instance declaration at Part1.lhs:193:12
      Apply ApplyZap ([[Char]], [t1]) ((a0 -> b0 -> (a0, b0)) -> t0),
        arising from a use of `forZ' at <interactive>:14:1-4
    In the expression: forZ [1, 2, 3] "hi there" (,)
    In an equation for `it': it = forZ [1, 2, 3] "hi there" (,)



Best,

Takayuki

2012/12/11 adam vogt <vogt.adam at gmail.com>:
> On Sat, Dec 8, 2012 at 10:27 AM, 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
>
> Hi again, Takayuki
>
> While the forZN in zipf-12 is able to infer the result type given
> arguments, it doesn't give any useful information about types for
> arguments unlike an example here:
>
> http://code.haskell.org/~aavogt/flip_zipWithN/P4.hs
>
> which imports a slight modification of Paczesiowa's code:
> http://code.haskell.org/~aavogt/flip_zipWithN/Part1.lhs
>
> But maybe it isn't possible to infer much about earlier arguments
> given later ones since there is an instance   Zip ((->) a), that forZN
> apparently can work with.
>
> Adam



-- 
Takayuki MURANUSHI
The Hakubi Center for Advanced Research, Kyoto University
http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html