Proposal: `Enum` class overhaul

[David Feuer]

I'm moderately opposed, on the basis that the Enum class is too
fundamentally broken/meaningless to be worth fiddling with. It attempts to
serve multiple barely-related purposes at once, and serves none of them
terribly well.

On Dec 13, 2017 11:17 PM, "M Farkas-Dyck" <strake888 at gmail.com> wrote:

> I propose to add some new methods of `Enum`:
>
> class Enum a where
>     ...
>
>     predMay, succMay :: a -> Maybe a
>     toEnum' :: Integer -> Maybe a
>     fromEnum' a -> Integer
>
> Rationale for `fromEnum'` and `toEnum'`:
>
> The docs for `Enum` now say the minimal complete definition is `toEnum`
> and `fromEnum`, but this is not enough to have sane default instances of
> the other methods, for example:
>
> data ABC = A | B | C deriving Show
> instance Enum ABC where
>     toEnum 0 = A
>     toEnum 1 = B
>     toEnum 2 = C
>     fromEnum A = 0
>     fromEnum B = 1
>     fromEnum C = 2
>
> main = print [A ..] -- [A,B,C,*** Exception: Non-exhaustive patterns in
> function toEnum
>
> In this case one could merely derive `Enum`, but not in some other cases,
> e.g. numeric types or GADTs. It is not possible to do better defining
> `toEnum` and `fromEnum` alone.
>
> If we default-define `toEnum'` and `fromEnum'` and their evil (i.e.
> partial) syblings in terms of each other, the user need merely define the
> total methods.
>
> Using `Integer` rather than `Int` allows these methods to not fail for
> types larger than an `Int`, which are not uncommon on 32-bit systems.
>
> Rationale for `predMay` and `succMay`:
>
> I include these partly for completeness, but `predMay` can not now be
> defined in general, and `succMay` only cumbersomely in terms of `enumFrom`.
>
> Note: All rationales imply "unless one uses `unsafePerformIO`". I'd rather
> not, myself.
>
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