[Haskell-cafe] A question on datakinds syntax

[Brandon Allbery]

Haddock displays kind annotations oddly. At a guess (which could well be
off base; I'm still rather weak on this stuff), Spock has PolyKinds
enabled, so an explicit (x :: *) kind annotation must be preserved as an
override to the default of allowing any kind; if PolyKinds is not enabled,
then * is the default kind and wouldn't be annotated. (Reader beware: * is
not a wildcard/"match any", it is specifically the kind of inhabited boxed
types!)

On Fri, Jan 13, 2017 at 3:31 AM, Haiwei Zhou <highfly22 at gmail.com> wrote:

> Hi,
>
>   I am reading the document of spock-0.11, and found two strange
> definitions in the Web.Spock.
>
> ````
> type Var a = Path ((:) * a ([] *)) Open
>
> var :: (Typeable * a, PathPiece a) => Path ((:) * a ([] *)) Open
>
> ````
>
> What's the meaning of the star between Typeable and a ?
>
> The kind of Typeable is * -> *. I cannot understand why the Typeable in
> the var prototype takes two kinds.
>
> In the Web.Routing.Combinators of reroute-0.4, those are defined as
> following:
>
> ````
> type Var a = Path (a ': '[]) Open
>
> var :: (Typeable a, PathPiece a) => Path (a ': '[]) Open
>
> ````
>
> Why there are two difference definition about the same thing?
>
> Thanks,
> Haiwei
>
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-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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