Re: Proposal: Add conspicuously missing Functor instances for tuples

A surprising amount has already been written on the topic of Functor/Foldable/Traversable instances (both for and against) for tuples, so I feel it's fair to link them here: * FTP dangers: http://nattermorphisms.blogspot.com/2015/02/ftp-dangers.html * Either and (,) in Haskell are not arbitrary: http://bitemyapp.com/posts/2015-10-19-either-is-not-arbitrary.html * Foldable for non-Haskellers: Haskell's controversial FTP proposal: http://tojans.me/blog/2015/10/13/foldable-for-non-haskellers-haskells-contro... * The Not-A-Wat in Haskell: https://www.youtube.com/watch?v=87re_yIQMDw&feature=youtu.be Ryan S. On Mon, Jan 18, 2016 at 3:44 PM, Christopher Allen wrote:

I've addressed this here:

http://bitemyapp.com/posts/2015-10-19-either-is-not-arbitrary.html

The thousand-papercuts opposition to typeclass instances on the premise that a Functor for (a, b, c) maps over the final type not making sense is a rejection of how higher kinded types and typeclasses work together. This is natural and predictable if one bothers to explain it.

On Mon, Jan 18, 2016 at 2:42 PM, Henning Thielemann wrote:

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On Mon, 18 Jan 2016, David Feuer wrote:

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Actually, I currently get a much worse error message:

Prelude> fmap (+1) (1,2,3)

<interactive>:2:1: Non type-variable argument in the constraint: Functor ((,,) t t) (Use FlexibleContexts to permit this) When checking that ‘it’ has the inferred type it :: forall b t t1. (Functor ((,,) t t1), Num b, Num t, Num t1) => (t, t1, b)

That there is a *lousy* error message.

indeed

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I understand that there are newcomers who struggle to understand the difference between tuples and lists; I don't think that's a good enough reason to omit a good and valid instance.

I would not qualify myself as a newcomer. For me these instances are a problem, not a help. _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

-- Chris Allen Currently working on http://haskellbook.com