Deprecate Foldable for Either

[Henning Thielemann]

On Sat, 18 Mar 2017, Lana Black wrote:

> On 18/03/17 19:49, Henning Thielemann wrote:
>> 
>> On Sat, 18 Mar 2017, Carter Schonwald wrote:
>> 
>>> for what?
>> 
>> A warning if someone e.g. calls 'length (a,b)', or more generally, if
>> certain instances are used.
>
> Please no. Many of us like our code Wall-clean while still being able to
> write polymorphic functions.

Whether the warning is enabled by Wall is a different question.
Though I think it should.

https://ghc.haskell.org/trac/ghc/ticket/11796