[Haskell-cafe] Understanding GHC's instance inference.

[Takayuki Muranushi]

Thank you, Adam,

for I didn't know about paczesiowa's article. That will be useful for me.

What I was trying to make is a zipWithN that takes the zipper function
as its "last" argument, not the first. This is because in my
applications the zipper functions tend to be complicated lambdas, as
illustrated in [1] . Since we live in curried world where all
functions are superficially unary, to define the "last" argument, and
to implement forZN, needs extra work than to implement zipWithN, I
believe [2] . I'm interested how much we can make these two share
their internal mechanisms.


[1] https://github.com/nushio3/practice/blob/master/free-objects/zipf-05.hs
[2] https://groups.google.com/forum/?fromgroups=#!topic/haskell-cafe/-e-xaCEbd-w

2012/12/6 adam vogt <vogt.adam at gmail.com>:
> On Wed, Dec 5, 2012 at 12:12 AM, Takayuki Muranushi <muranushi at gmail.com> wrote:
>> Dear everyone,
>>
>> I have a code
>> https://github.com/nushio3/practice/blob/master/instance-inference/zipf-11-1.hs
>>
>> that produces a type-error when I remove a type signature.
>> https://github.com/nushio3/practice/blob/master/instance-inference/zipf-11.hs
>
> Hi Takayuki,
>
> The ghc manual sections about the extensions are a good place to
> start. Also check out http://okmij.org/ftp/Haskell/
>
> I think you are expecting forZN to be able to use the number of -> in
> the function(s) supplied to decide how many lists to take, as done
> here: http://paczesiowa.blogspot.ca/2010/03/generalized-zipwithn.html
>
> Replacing the [] container used in the above zipWithN with a
> `Data.Key.Zip v => v' that is the same for all of the arguments might
> be straightforward. But there are a lot of type signatures that have
> to add that parameter, and maybe that will interfere with the
> incoherent instance business going on.
>
> Adam
>
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-- 
Takayuki MURANUSHI
The Hakubi Center for Advanced Research, Kyoto University
http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html