Re: [Haskell-cafe] Textbook example of instance Foldable ((,) a)

>>>>> "TE" == Tom Ellis writes: TE> On Mon, Nov 23, 2020 at 09:54:26PM +0100, Francesco Ariis wrote: TE> Which of the following would they give up? TE> A. Foldable as superclass of Traversable TE> B. (a,) as Traversable For me it is B. Even the Foldable hackage description states "Functors representing data structures that can be traversed from left to right." The tuple is however not traversed from left to right, but solely on the right (as it's half filled). For me it doesn't make sense to implement it, just because it's implementable. I think the current implementation does not add more value than it harms. El lun., 23 nov. 2020 22:23, Tom Ellis < tom-lists-haskell-cafe-2017@jaguarpaw.co.uk> escribió:

On Mon, Nov 23, 2020 at 09:54:26PM +0100, Francesco Ariis wrote:

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Il 23 novembre 2020 alle 14:40 Zemyla ha scritto:

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So what should maximum (2, "potato") be?

For people who did not agree with `instance Foldable (,) a`, a type error!

Which of the following would they give up?

A. Foldable as superclass of Traversable

B. (a,) as Traversable

Personally I think in retrospect I think I'd give up both and use explicit Traversals (i.e. optics instead). That way one would have to write

allOf each (\ i -> contents p i == contents q i) ...

By contrast

allOf _2 (\ i -> contents p i == contents q i) ...

obviously looks wrong.

Tom _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.