Deprecate Foldable for Either

[amindfv at gmail.com]

Is there a clear way to implement this instance warning? I.e. given:

f x = 2 * length x

Can we guarantee at compile time that "f" will never be passed a 2-tuple?

Tom

> El 18 mar 2017, a las 20:04, Adam Bergmark <adam at bergmark.nl> escribió:
> 
> I'm on the fence about the instance existing. I'm +1 for a warning, and thus would be +1 on keeping the instance. +1 on making the warning opt-in and +1 keeping it out of -Wall.
> 
> 
> 
>> On Sun, 19 Mar 2017 at 00:51 <amindfv at gmail.com> wrote:
>> 
>> 
>> > El 18 mar 2017, a las 16:01, Lana Black <lanablack at amok.cc> escribió:
>> >
>> >> 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.
>> 
>> Can it be a hlint rule? It seems quite difficult to predict that "length" will not ever be passed e.g. a 2-tuple in the general case, within hlint.
>> 
>> I would also favor a warning, and happily have -Wall not include it (though I'd prefer inclusion).
>> 
>> Tom
>> 
>> 
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