Correct behaviour of Bounded (Down a)

[David Feuer]

That strikes me as quite reasonable.

On Mon, Sep 21, 2020, 1:09 PM Krzysztof Gogolewski <krz.gogolewski at gmail.com>
wrote:

> 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 <ky3 at atamo.com> wrote:
> >
> >
> >
> > On Sat, Sep 19, 2020 at 1:34 AM Asad Saeeduddin <masaeedu at gmail.com>
> 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 <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.
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>> _______________________________________________
> >>>
> >>>
> >>>
> >>>
> >>>
> >>> Libraries mailing list
> >>>
> >>>
> >>>
> >>>
> >>>
> >>> Libraries at haskell.org
> >>>
> >>>
> >>>
> >>>
> >>>
> >>> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
> >>>
> >>>
> >>>
> >>>
> >>>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >> _______________________________________________
> >>
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> >>
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> >>
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> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
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> >>
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> >>
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> >>
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> >>
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> >>
> >>
> >>
> >>
> >>
> >>
> >>
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> >>
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> >>
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> >>
> > --
> > -- Kim-Ee
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