Re: [Haskell-cafe] A question on datakinds syntax

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 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 _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.