Correct behaviour of Bounded (Down a)

Tue Sep 22 17:56:08 UTC 2020

The lack of a coherent notion for Enum seems to indicate it fails the
uniquely/conventionally defined meaning for me. In particular enumFrom has
to make some awkward choices to flip sense; it can't probe for pred without
introducing spurious bottoms. It seems to me the only sensible action there
is to rip out the instance.

-Edward

On Mon, Sep 21, 2020 at 11:29 AM Carter Schonwald <
carter.schonwald at gmail.com> wrote:

> i mean, if we straightup cant have a coherent definition for it, by
> all means! (i think one lint for stuff being in base is "is there
> reasonably a unique most general usable defiinition/ semantics/meaning",
> and perhaps we dont have one here?)
>
> On Mon, Sep 21, 2020 at 1:47 PM David Beacham <mail at dbeacham.co.uk> wrote:
>
>> I think that would be my preferred solution
>>
>> You can keep the numbering the same in the Enum instance if you wish and
>> hide the negation in the implementation, but I'm not convince the Enum
>> instance is of general use.
>>
>>
>>
>> On Mon, Sep 21, 2020 at 6:21 PM David Feuer <david.feuer at gmail.com>
>> wrote:
>>
>>> 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.
>>>> >>>
>>>> >>>
>>>> >>>
>>>> >>>
>>>> >>>
>>>> >>>
>>>> >>>
>>>> >>>
>>>> >>>
>>>> >>>
>>>> >>>
>>>> >>>
>>>> >>> _______________________________________________
>>>> >>>
>>>> >>>
>>>> >>>
>>>> >>>
>>>> >>>
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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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