[core libraries] Data.Display - Textual representation of Haskell values (was Re: PROPOSAL: Add displayException to Exception typeclass)

[Simon Hengel]

On Sat, Nov 15, 2014 at 09:38:14AM +0800, Simon Hengel wrote:
> > *  I think usage of MINIMAL pragmas could help a lot for your concerns
> > about which of `display` or `displayBuilder` the user implements.
> 
> I'm not sure this will work.  We would need to specify something like
> 
>     {} | {displayBuilder}
> 
> as minimal complete definition (where {} denotes the empty set), but I'm
> not aware of a way to specify the empty set with MINIMAL.
> 
> Unless somebody knows a solution, I would probably open a GHC ticket and
> propose to extend the MINIMAL pragma to allow for empty sets.

Please ignore what I said above.  It does not make sense in multiple
regards.

The minimal complete definition for Display is the empty set.  We have
the invariant that if you define `display` you also have to define
`displayBuilder`.  If we would have a way to specify all sets of methods
that gives us a complete+consistent definition, then that would be:

  {{}, {displayBuilder}, {displayBuilder, display}}

I don know a way how to do that with MINIMAL, nor do I know a straight
forward way to extend MINIMAL to accommodate this use case.

Cheers.