On Mon, 14 Jul 2014 10:37:55 -0600, Carl Howells
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.