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

[Viktor Dukhovni]

> On Jun 1, 2018, at 3:20 PM, Tom Ellis <tom-lists-haskell-cafe-2013 at jaguarpaw.co.uk> wrote:
> 
> instance forall a b. (Bounded a, Bounded b, Enum a, Enum b) => Enum (E a b) where
>  fromEnum = \case
>    L a -> fromEnum a
>    R b -> fromEnum (maxBound :: a) + fromEnum b + 1

This appears to assume that (fromEnum b) is never negative.
(effectively that (minBound :: b) >= 0).

Ignoring overflow issues, this should perhaps be:

   R b -> fromEnum (maxBound :: a) + (fromEnum b - fromEnum (minBound :: b)) + 1

This will of course overflow when ranges of a and/or b are large enough.

-- 
	Viktor.