Re: Proposal: Add conspicuously missing Functor instances for tuples

19 Jan 2016

      Of course, the proper/correct "implementation" for "Functor" with tuples should be 

instance Functor (,) where
  fmap (f,g) (a,b) = (f a, g b)

instance Functor (,,) where  
  fmap (f,g,h) (a,b) = (f a, g b, h c)

instance Functor (,,,) where
  fmap (f,g,h,k) (a,b) = (f a, g b, h c, k d)

and so on... (where fmap's type sig alters in a "tuple-structured" way with each instance...)

The current instance for (,)a  is my fmap(id, f)

Or should we have 15 classes ?

class Functor...
class BiFunctor...
class TriFunctor....
...
class QuincadecaFunctor /PentadecaFunctor - depending if you prefer Latin/Greek  ;-)

This probably doesn't clear anything up but does explain why heavy tuple use is deprecated
as per an earlier mail in this thread.....

Regards, Andrew
...
On 19 Jan 2016, at 12:30, Carter Schonwald  wrote:
Agreed. +1
On Tuesday, January 19, 2016, Herbert Valerio Riedel mailto:hvriedel@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... https://www.haskell.org/onlinereport/haskell2010/haskellch6.html#x13-1210006...
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Andrew Butterfield
School of Computer Science & Statistics
Trinity College
Dublin 2, Ireland