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

[Zemyla]

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

On Mon, Nov 23, 2020, 14:19 Sven Panne <svenpanne at gmail.com> wrote:

> Am Mo., 23. Nov. 2020 um 16:32 Uhr schrieb Sebastiaan Joosten <
> sjcjoosten+haskell at gmail.com>:
>
>> [...] We obtain: length (a,b) = length [b] = 1
>>
>
> Quoting my former self (
> https://mail.haskell.org/pipermail/libraries/2017-April/027905.html), you
> get even more "fun" stuff with lots of potential for late-night debugging
> hours:
>
>    maximum (3,2)   =>   2
>    minimum (4,5)   => 5
>    sum (6,7)   =>   7
>    product (8,9)   =>   9
>
> Yes, you can think of (X, Y) as "Y with context X" or "a one-element container with Y in it", but is this really what comes to your mind first? I still highly doubt that. The Foldable-Traversable-in-Prelude change was largely a good thing, but very surprising (and not really needed) changes coming stealthily with it were only communicated when it was already too late... :-/
>
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