Proposal: add -XPolyKinds to Data.Monoid

[Carl Howells]

Everything about this case is different in GHC 7.6.3 anyway.
Data.Proxy isn't in base, its Monoid instance isn't defined in
Data.Monoid.

On Mon, Jul 14, 2014 at 12:51 PM, adam vogt <vogt.adam at gmail.com> wrote:
> The example works with ghc-7.6.3. It looks like
> https://github.com/ghc/ghc/commit/1d1ff77aaa09efaddc8cfe0dcf92d6763297cf11
> is to blame.
>
> On Mon, Jul 14, 2014 at 12:37 PM, Carl Howells <chowells79 at gmail.com> wrote:
>> At the moment, the Monoid instance for Data.Proxy.Proxy is only
>> defined for types with kind *.
>>
>>     Prelude Data.Proxy Data.Monoid> (Proxy :: Proxy String) <> Proxy
>>     Proxy
>>     Prelude Data.Proxy Data.Monoid> :set -XDataKinds
>>     Prelude Data.Proxy Data.Monoid> (Proxy :: Proxy 5) <> Proxy
>>
>>     <interactive>:8:20:
>>         No instance for (Monoid (Proxy 5)) arising from a use of ‘<>’
>>         In the expression: (Proxy :: Proxy 5) <> Proxy
>>         In an equation for ‘it’: it = (Proxy :: Proxy 5) <> Proxy
>>
>> Enabling -XPolyKinds while compiling the Data.Monoid module is the
>> only change necessary to make the Monoid instance for Proxy polykinded
>> like Proxy itself is. I don't believe adding a language pragma would
>> have any other effect on that module, and I believe the only effect of
>> changing that instance would be allowing more correct programs to
>> compile.
>>
>> This is my first proposal to this list, so I'm starting with something
>> that I hope isn't controversial. Let me know if I should do anything
>> different procedurally in future proposals.
>>
>> Thanks.
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