Re: Proposal: Add conspicuously missing Functor instances for tuples

El 19 ene 2016, a las 16:22, Christopher Allen escribió:

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Why I'm not quite convinced about large tuples being entirely silly:

They happen to be a useful dumb type for things like rows you parse from a csv file or similar as well. Pretty sure the various *-simple database clients use this as well.

In these cases large tuples are useful, but a Functor instance that operates on the last element?

They are, admittedly, sort of a paradigm case for why `lens` is often more useful than Functors, but Functor isn't going anywhere and there's only one valid instance for these types so they might as well live with the types or the class.

My disagreement is only in "might as well": there is a downside to these, so I'd like there to be an upside (that's worth it!). Tom

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On Tue, Jan 19, 2016 at 3:20 PM, David Feuer wrote: The problem, I think, is that

1. Many of us thing the instance for pairs is useful. Some of us think the instance for triples is occasionally useful. Most of us doubt the instances for large tuples are often useful (but I'm not quite convinced). 2. Most of us value consistency very highly. Even if we think the instance for pairs is great and the instance for septuples is silly, we want to have the same answer for all tuple sizes.

Thus we have the usual "The axiom of choice is obviously true, the well-ordering principle is obviously false, and who can say about Zorn's lemma."

Why I'm not quite convinced about large tuples being entirely silly: people do weird things with Template Haskell. When they do so, they want to be able to use functions as uniformly as possible. A function (fmap) that always hits the last component of a tuple, even if it has 15 components, may be just what they need.

On Tue, Jan 19, 2016 at 4:04 PM, wrote:

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If the only uses we can imagine for these instances are - Cases where it's not a great design decision - Cases where it's used accidentally and results in a silent runtime failure instead of a compile-time error

then I'm a strong -1

Tom

El 19 ene 2016, a las 15:54, Alois Cochard escribió:

I don't have any use case like that. I'm in favor of this proposal for consistency sake.

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, wrote:

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El 19 ene 2016, a las 10:24, Alois Cochard 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 wrote:

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On 2016-01-18 at 21:10:07 +0100, David Feuer wrote:

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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]:

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6.1.4 Tuples

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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... _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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