Deprecate Foldable for Either

[Lana Black]

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.
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Please no. Many of us like our code Wall-clean while still being able to
write polymorphic functions. Adding more warnings that are often
triggered by correct code (redundant constraints, anyone?) only leads to
more headache.

You could make that an hlint rule on the other hand.