Re: suppress warning "Defined but not used: type variable ‘x’" in GHC-8.0

[Edward Kmett]

No, the type instance must match the class heading.

I *can* use

instance Foo [_a] where
  type Bar [_a] = Int
  whatever = ... where
    bar :: _a -> Int
    bar = ...

but that is a needlessly messy thing to request of every package everywhere.

The arguments being pattern matched in a class associated type aren't
really just bindings, they reference the surrounding context and so
shouldn't be treated the same as the basic type family case.

It isn't just the class associated type being mangled, it is every type
variable that comes from the instance head in the entire body of every
instance that happens to have a class associated type in it.

Note that in the example above I added another ScopedTypeVariables
reference to the same parameter, but it _also_ must be mangled despite
having absolutely nothing to do with the class associated type.

The existing convention has worked since 6.10 or so, when all of this stuff
was invented in the first place, and the new state of affairs is clearly
worse.

-Edward

On Sun, Jan 17, 2016 at 3:16 AM, Andrew Farmer <xichekolas at gmail.com> wrote:

> Can't you just:
>
> instance Foo [a] where
>   type Bar [_a] = Int
>
> (At least I think I did that somewhere...)
> On Jan 16, 2016 9:24 PM, "Edward Kmett" <ekmett at gmail.com> wrote:
>
>> As a data point I now get thousands of occurrences of this warning across
>> my packages.
>>
>> It is quite annoying.
>>
>> class Foo a where
>>   type Bar a
>>
>> instance Foo [a] where
>>   type Bar [a] = Int
>>
>> is enough to trigger it.
>>
>> And you can't turn it off by using _ as
>>
>> instance Foo [_] where
>>   type Bar [_] = Int
>>
>> isn't legal.
>>
>> I've been avoiding it for now by using
>>
>>   if impl(ghc >= 8)
>>
>>     ghc-options: -fno-warn-unused-matches
>>
>> but this is a pretty awful addition to this warning as it stands.
>> -Edward
>>
>> On Mon, Jan 11, 2016 at 2:12 PM, Henning Thielemann <
>> lemming at henning-thielemann.de> wrote:
>>
>>>
>>> On Mon, 11 Jan 2016, Richard Eisenberg wrote:
>>>
>>> On Jan 9, 2016, at 6:44 PM, Henning Thielemann <
>>>> lemming at henning-thielemann.de> wrote:
>>>>
>>>>>
>>>>> instance (Natural n) => Num.Integer (Un n) where
>>>>>    type Repr (Un _n) = Unary
>>>>>
>>>>>
>>>>> GHC-7.6.3 and GHC-7.4.2 complain:
>>>>>    Type indexes must match class instance head
>>>>>    Found `Un _n' but expected `Un n'
>>>>>    In the type synonym instance declaration for `Num.Repr'
>>>>>    In the instance declaration for `Num.Integer (Un n)'
>>>>>
>>>>>
>>>>> GHC-7.8.4, GHC-7.10.3 and GHC-8.0 are happy with the difference.
>>>>>
>>>>
>>>> I'm surprised this is accepted at all. Looks like hogwash to me. I
>>>> think you should post a bug report.
>>>>
>>>
>>> Ok, but then GHC must not warn about the unused argument of Repr.
>>>
>>> _______________________________________________
>>> Glasgow-haskell-users mailing list
>>> Glasgow-haskell-users at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users
>>>
>>
>>
>> _______________________________________________
>> Glasgow-haskell-users mailing list
>> Glasgow-haskell-users at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users
>>
>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/glasgow-haskell-users/attachments/20160117/0b769f0d/attachment.html>