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

On Sat, Jun 02, 2018 at 10:36:56AM +0100, Jon Fairbairn wrote:

Tom Ellis writes:

...

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.

You don’t need Bounded to do that. For example, you could start at (0,0) and go out in diamond shaped rings. A more pressing reason is that there are too many possible enumerations, and if we picked one it would probably be the wrong one for most applications.

This is all true, but I was implicitly assuming that Bounded and Enum ought to agree with Ord. We've already picked Ords for (,) and Either so why not also pick the Enums and Boundeds that agree with them?