Proposal: Add conspicuously missing Functor instances for tuples
alois.cochard at gmail.com
Tue Jan 19 20:54:38 UTC 2016
I don't have any use case like that. I'm in favor of this proposal for
That last part of my comment about usefulness/discouraging usage was about
using Functor instance on tuple in general, no matter the arity.
On 19 January 2016 at 21:32, <amindfv at gmail.com> wrote:
> El 19 ene 2016, a las 10:24, Alois Cochard <alois.cochard at gmail.com>
> 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?
> On 19 January 2016 at 09:20, Herbert Valerio Riedel <hvriedel at gmail.com>
>> 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:
>> > 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.
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