[Haskell-cafe] No Enum for (,), no Enum or Bounded for Either
Tom Ellis
tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk
Fri Jun 1 19:33:18 UTC 2018
Sorry, could you explain further? I don't understand what the
implementation of any of those proposed instances is supposed to be.
On Fri, Jun 01, 2018 at 09:28:07PM +0200, MarLinn 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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