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@gmail.com> wrote:


> El 18 mar 2017, a las 16:01, Lana Black <lanablack@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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