[Haskell-cafe] A question on datakinds syntax

[Richard Eisenberg]

Haddock struggles sometimes to render types with -XPolyKinds enabled. The problem is that GHC generally does not require kind arguments to be written and it does not print them out (unless you say -fprint-explicit-kinds). But Haddock, I believe, prints out kinds whenever -XPolyKinds is on. So the two different definitions are really the same: it's just that one module has -XPolyKinds and the other doesn't.

The * is the kind of ordinary types. So Int has kind * (we write Int :: *) while Maybe has kind * -> *. Typeable actually has kind `forall k. k -> Constraint`, meaning that it's polykinded. In the first snippet below, the * argument to Typeable instantiates k with *, because type variable a has kind *.

This all tends to be quite confusing, and I wish I could point you to a tutorial or some such on it all... but I don't know if one exists!

I hope this is helpful,
Richard

> On Jan 13, 2017, at 1:31 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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