Proposal: Add conspicuously missing Functor instances for tuples

[amindfv at gmail.com]

> El 19 ene 2016, a las 10:24, Alois Cochard <alois.cochard at gmail.com> escribió:
> 
> +1
> 
> Agree for consistency, I can also see those instances as being useful in some specific context, even if I agree with Andreas that in general they should be discouraged (especially for newcomers).
> 

Can you give us an example where using e.g. the Functor instance for a 5-tuple would be the correct/best design decision?

Tom


> 
>> On 19 January 2016 at 09:20, Herbert Valerio Riedel <hvriedel at gmail.com> wrote:
>> On 2016-01-18 at 21:10:07 +0100, David Feuer wrote:
>> > For some reason I really can't imagine, it seems the only tuple type
>> > with a Functor instance is (,) a. I was astonished to find that
>> >
>> > fmap (+1) (1,2,3)
>> >
>> > doesn't work. Since this is *useful*, and there is *only one way to do
>> > it*, I propose we add the following:
>> >
>> > instance Functor ((,,) a b) where
>> >   fmap f (a,b,c) = (a,b,f c)
>> > instance Functor ((,,,) a b c) where
>> >   fmap f (a,b,c,d) = (a,b,c,f d)
>> > etc.
>> 
>> As stated elsewhere in this thread already, there is the issue about
>> consistency. Here's a relevant section from the Haskell 2010 report[1]:
>> 
>> > 6.1.4 Tuples
>> >
>> > ...
>> >
>> > However, every Haskell implementation must support tuples up to size
>> > 15, together with the instances for Eq, Ord, Bounded, Read, and Show.
>> 
>> IMO, we either have no `Functor` instances for tuples at all, or we have
>> them for all tuples up to size 15. The current situations of having them
>> defined only for 2-tuples is inconsistent.
>> 
>> 
>> Cheers,
>>   hvr
>> 
>>  [1]: https://www.haskell.org/onlinereport/haskell2010/haskellch6.html#x13-1210006.1.4
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> 
> 
> 
> -- 
> Λ\ois
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