Correct behaviour of Bounded (Down a)

[David Beacham]

That is a good spot - I had wondered if the minBound/maxBound changes would
affect other classes but had missed Enum.

I wouldn't be surprised that `Down a` had a reversed enumeration despite
the `Ord`/`Enum` separation.

Looking at the `Bounded` docs

> The Bounded class is used to name the upper and lower limits of a type.
Ord is not a superclass of Bounded since types that are not totally ordered
may also have upper and lower bounds.

does suggest that flipping minBound and maxBound is correct? That would
then require `Enum a => Enum (Down a)` should also be flipped given the way
it interacts with `Bounded`?

I think the instance is achievable by

1. flipping pred/succ
2. carefully using "negated Int"s when looking at `fromEnumThenTo` etc.

I think you get the choice of keeping the same `Int` conversions, `toEnum =
Down . toEnum`, or negating it, `toEnum = Down . toEnum . negate`, so e.g.
0 -> False, 1 -> True becomes 0 -> Down False, -1 -> Down True.

I can add potential implementations to the MR so that there is something
concrete to look at (and check it all works) or do it here.

But I think it all needs a bit more discussion than I'd initially thought!

David.

On Fri, Sep 18, 2020 at 3:41 PM Oleg Grenrus <oleg.grenrus at iki.fi> 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 <mail at dbeacham.co.uk> 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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