Kindness of strangers (or strangeness of Kinds)

AntC anthony_clayden at
Fri Jun 8 01:36:55 CEST 2012

José Pedro Magalhães <jpm <at>> writes:

> On Thu, Jun 7, 2012 at 2:46 AM, AntC <anthony_clayden <at>> 
> What does the `ArgKind' message mean?
> `ArgKind` and `OpenKind` is what previously was called `?` and `??` (or the 
otherway around; I can't remember).

Thanks Pedro, I'm trying to understand what's changing and why. (And I'd 
better repeat that I'm looking at 7.4.1, not HEAD; and SPJ's being perfectly 
clear that the promoted Kind stuff is not yet officially approved for prime 

GHC 7.2.1> :k (->) :: ?? -> ? -> *

GHC 7.4.1> :k (->) :: * -> * -> *

At first sight (->) is becoming less polyKinded. Is the eventual aim to be:

GHC 7.6+> :k (->) :: AnyKind1 -> AnyKind2 -> *

>You might also want to have a look at Richard and Stephanie's latest paper 
draft, about singletons, which is related to what you are trying in your 

That's what I'm doing, and trying to understand the machinery behind it. The 
naieve approach I started with was how to get one-way from type to its single 
value -- I wasn't aiming for the whole singleton gig.


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