Re: Proposal: add -XPolyKinds to Data.Monoid

14 Jul 2014


      +1 for me. The Monoid instance for Proxy was polykinded in its argument in
tagged, it just lost the polymorphism in the move into base.

-Edward


On Mon, Jul 14, 2014 at 2:21 PM, Carl Howells  wrote:
...
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 
wrote:
...
On Mon, 14 Jul 2014 10:37:55 -0600, Carl Howells 
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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Re: Proposal: add -XPolyKinds to Data.Monoid

Edward Kmett