Correct behaviour of Bounded (Down a)

[Carter Schonwald]

Good questions!

1) enum unfortunately uses int

2) there’s no reason for there not to be lots of bijections! The barrier or
obstacle I think you’re envisioning is if you want the bijection to
preserve ordering. Right?

On Fri, Sep 18, 2020 at 2:34 PM Asad Saeeduddin <masaeedu at gmail.com> wrote:

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> This is something I've never understood about `Enum`. How can a
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> type be in bijection with the integers, but simultaneously have a
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> minimum and maximum bound?
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> On 9/18/20 10:40 AM, Oleg Grenrus
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> Is it (a bug)?
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> What about [minBound .. maxBound]. If you change `Bounded`, you
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> need to change `Enum` too.
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> From
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> https://hackage.haskell.org/package/base-4.14.0.0/docs/GHC-Enum.html#t:Enum
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> - The calls succ maxBound and pred minBound should result in a
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> runtime error.
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> - enumFrom and enumFromThen should be defined with an implicit
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> bound
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> The `min` and `max` in `minBound/maxBound` are related to
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> `succ`, `pred`, not to `compare`. If you argue against, then we
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> should add `Ord` super-class to `Enum` (and `Bounded`).
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> - Oleg
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> On 18.9.2020 16.54, Carter Schonwald
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> Def a bug! Plz at myself and the core libraries
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> handle for code review plz.
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> This sounds kinda related to an MR that has
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> languished too long regarding the behavior of Down on Ord1,
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> which I think is related??? (I tried to arbitrate / layout
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> possible answers for the related issue in the associated mr,
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> but none of the clc members have engaged in the design space
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> challenge)
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> On Fri, Sep 18, 2020 at
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> 7:44 AM David Beacham <mail at dbeacham.co.uk>
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> wrote:
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>> Hi all,
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>> The current instance for `Bounded a => Bounded
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>> (Down a)` derives the `minBound` and `maxBound`
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>> exactly as they are for the underlying type `a` where
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>> I think they should be flipped to respect the flipped
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>> ordering of `Down a`?
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>> I have a feeling this has been come up in some
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>> other context/list before but I couldn't find a
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>> reference to it when searching - so sorry if this is a
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>> duplicate.
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>> I've got a corresponding MR here:
>> https://gitlab.haskell.org/ghc/ghc/-/merge_requests/4081
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>> Thanks, David.
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