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

[Bryan Richter]

In homage to the ironic phrase "It's technically true, which is the best
kind of true," perhaps we need a superclass TechnicallyFoldable. :D Then
(,) a could be an instance of TechnicallyFoldable but not Foldable ,
tlength could be defined as tlength = length for (Foldable a =>
TechnicallyFoldable a), and anybody who really wants the maximally general
option could always use tlength.

The parallel with fmap speaks for itself. :)

Den tis 24 nov. 2020 11:56Manuel Schneckenreither <manuel.schnecki at gmail.com>
skrev:

>     >>>>> "TE" == Tom Ellis <tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk>
> 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 at jaguarpaw.co.uk> escribió:
>
>> On Mon, Nov 23, 2020 at 09:54:26PM +0100, Francesco Ariis wrote:
>> > Il 23 novembre 2020 alle 14:40 Zemyla ha scritto:
>> > > 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
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