Remove eq and show from num class

[Anthony Clayden]

> On Fri Sep 8 15:58:10 UTC 2017, Carter Schonwald wrote:
>
> I mostly wanted to confirm that we in fact will actually
say yes
> before doing the formal writtingup :)

Seriously -- and please stop using smileys: 
you're right to be cautious.
You need to rewrite the whole of Section 6.4
(nearly 5 pages), starting with changing the title.
And I think it'll be a struggle to clarify what applies
to genuine numbers vs what applies to 'other stuff'.
As Bardur says:
> far from trivial to spec without reference to
implementation details

I think there's another way: leave Sec 6.4 largely
unchanged.
Add a short note that implementations might extend
class `Num` to include non-numbers.

GHC 'morally complies' to section 6.4 for Numbers.
(i.e. all number types are members of `Num, Eq, Show`.)

GHC has (or allows) other types in `Num` which are not
numbers,
so section 6.4 doesn't apply.

I'm puzzled by Bardur's other comments:
> On Fri Sep 8 16:16:54 UTC 2017, Bardur Arantsson wrote:
>
> There aren't really any widely used Haskell compilers
> other than GHC ...

That's true and sad and a weakness for Haskell
in general. On this particular topic there are
other compilers: GHC prior v7.4, Hugs.

> and speccing for things that aren't actually used in
practice ...

But there's already a spec! namely the existing report.
And `Eq`, `Show` for numbers are used heavily.

I think this proposal is not to remove `Eq, Show`
from number types that already have them(?)
But Carter's subject line does seem to be saying that,
which is what alarmed me at first reading.


AntC