[Haskell-cafe] No Enum for (,), no Enum or Bounded for Either

[Brandon Allbery]

The BoundedEnum thing is already something of a sunk cost, since there's
already magic behavior of Enum when a type is also Bounded. (Go look at how
Enum deriving is specified.)

On Fri, Jun 1, 2018 at 3:28 PM MarLinn <monkleyon at gmail.com> wrote:

> Counterargument: overlapping instances
>
>     instance (Bounded b, Enum b) => Enum (Either a b)
>     instance (Bounded b) => Bounded (Either a b)
>     instance (Applicative f, Bounded a) => Bounded (f a)
>     instance (Bounded a, Enum a) => Enum (Either a b)
>     instance (Bounded a) => Bounded (Either a b)
>     instance (Bounded a, Enum a, Monoid b) => Enum (a, b)
>     instance (Bounded b, Enum b, Monoid a) => Enum (a, b)
>
> Also note that what you're talking about is a special type of objects,
> namely
>
>     type BoundedEnum a = (Bounded a, Enum a) -- using ConstraintKinds
>
> (I'm sure the mathematicians have a better name for this)
>
> So IF someone where to add these somewhere, might I suggest also adding
> essentials like
>
>     enumAll :: (Bounded a, Enum a) => [a]
>     -- i.e. enumAll :: (BoundedEnum a) => [a]
>
>
> Lastly, because it's its own type of objects, I'm sure there's a library
> out there doing just that. (Plus maybe other stuff like EnumMap's).
>
> On 2018-06-01 20:32, Tom Ellis wrote:
>
> True.  I think I would propose
>
>     instance (Bounded a, Bounded b, Enum a, Enum b) => Enum (Either a b)
>     instance (Bounded a, Bounded b) => Enum (Bounded a b)
>     instance (Bounded a, Bounded b, Enum a, Enum b) => Enum (a, b)
>
> On Fri, Jun 01, 2018 at 02:23:58PM -0400, Li-yao Xia wrote:
>
> One issue is that (Int, Int) is too big to define toEnum/fromEnum.
>
> On 06/01/2018 02:10 PM, Tom Ellis wrote:
>
> I'm a bit surprised that whilst `Either` and `(,)` have instances for `Ord`
>
> * `(,)` has no instance for `Enum`
> * `Either` has no instance for `Enum` or `Bounded`
>
> Is there a particular reason for that?  It might be tricky to implement
>
>      toEnum :: Int -> a
>      fromEnum :: a -> Int
>
> but in the presence of `Bounded` that should be possible.
>
>
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-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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