Re: Correct behaviour of Bounded (Down a)

I would consider removing Enum a => Enum (Down a), and fixing Bounded (Down a). There isn't really a good equivalent of `enumFromThenTo` for Down and performing negation in `toEnum` is not obvious to me. On Sat, Sep 19, 2020 at 3:00 AM Kim-Ee Yeoh wrote:

On Sat, Sep 19, 2020 at 1:34 AM Asad Saeeduddin wrote:

...

This is something I've never understood about `Enum`. How can a

type be in bijection with the integers, but simultaneously have a

minimum and maximum bound?

It's not a bijection with the integers.

In the method fromEnum :: a -> Int, the Int stands in for the disjoint sum of all possible types with an Enum instance.

And the way Int finesses that impersonation is by conflating together the parts of the disjoint sum.

...

On 9/18/20 10:40 AM, Oleg Grenrus

wrote:

Is it (a bug)?

What about [minBound .. maxBound]. If you change `Bounded`, you

need to change `Enum` too.

From

https://hackage.haskell.org/package/base-4.14.0.0/docs/GHC-Enum.html#t:Enum

- The calls succ maxBound and pred minBound should result in a

runtime error.

- enumFrom and enumFromThen should be defined with an implicit

bound

The `min` and `max` in `minBound/maxBound` are related to

`succ`, `pred`, not to `compare`. If you argue against, then we

should add `Ord` super-class to `Enum` (and `Bounded`).

- Oleg

On 18.9.2020 16.54, Carter Schonwald

wrote:

Def a bug! Plz at myself and the core libraries

handle for code review plz.

This sounds kinda related to an MR that has

languished too long regarding the behavior of Down on Ord1,

which I think is related??? (I tried to arbitrate / layout

possible answers for the related issue in the associated mr,

but none of the clc members have engaged in the design space

challenge)

On Fri, Sep 18, 2020 at

7:44 AM David Beacham

wrote:

...

Hi all,

The current instance for `Bounded a => Bounded

(Down a)` derives the `minBound` and `maxBound`

exactly as they are for the underlying type `a` where

I think they should be flipped to respect the flipped

ordering of `Down a`?

I have a feeling this has been come up in some

other context/list before but I couldn't find a

reference to it when searching - so sorry if this is a

duplicate.

I've got a corresponding MR here: https://gitlab.haskell.org/ghc/ghc/-/merge_requests/4081

Thanks, David.

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