Proposal: add -XPolyKinds to Data.Monoid

Carl Howells chowells79 at
Mon Jul 14 18:21:33 UTC 2014

I don't actually have a use case. I was just looking at Data.Monoid in
depth for some other changes I'd like to propose, and noticed that
instance was needlessly restrictive. This proposal was made for the
same reason you're in favor - it might make something better for
someone someday, and shouldn't make anything worse.

On Mon, Jul 14, 2014 at 11:46 AM, Niklas Haas <haskell at> wrote:
> On Mon, 14 Jul 2014 10:37:55 -0600, Carl Howells <chowells79 at> 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.
> +1 out of principle due to it being a harmless change that should be
> invisible to most users.
> It's a bit unclear what the purpose of this would be, but it's similarly
> unclear what the purpose of the Monoid instance on Proxy in general is.
> I wonder what you actually need this for.
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