Correct behaviour of Bounded (Down a)

[Ganesh Sittampalam]

Is the fundamental question whether we require that types that implement
both Ord and Enum have these laws?

    succ x > x

    x > y == fromEnum x > fromEnum y

If we require it then this change is mandatory, if we don't require that
then it's optional and whatever argument we have against the laws might
be an argument against this change.


On 18/09/2020 17:19, David Beacham wrote:
> 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
> <mailto: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 <mailto: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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