[Haskell-cafe] Rank N Kinds
roma at ro-che.info
Wed Jul 31 21:45:10 CEST 2013
That's because types that belong to most non-star kinds cannot have
data Foo (a :: k) = Foo
data Foo (a :: k) = Foo a
is bad because there cannot be a field of type a :: k.
So no, no useful examples exist, because you wouldn't be able to use
such a data constructor even if you could declare it.
* Wvv <vitea3v at rambler.ru> [2013-07-31 11:40:17-0700]
> I still have 7.6.3. It has old Typeable.
> I misunderstood PolyKinds a bit. It looks like k /= **, k ~ *******...
> But we cannot use "CloseKinds" like
> data Foo (a::k) = Foo a -- catch an error "Expected kind `OpenKind', but `a' has
> kind `k'"
> with RankNKinds we could write:
> data Foo (a::**) = Foo a
> data Bar (a::***) = Bar a
> So, now the task is more easy:
> I'm asking for useful examples with "CloseKinds" with ** and higher, and any
> useful examples for *** and higher
> cheers, Wvv
> 29 Jul 2013 at 14:44:50, José Pedro Magalhães [via Haskell]
> (ml-node+s1045720n5733561h30 at n5.nabble.com) wrote:
> On Fri, Jul 26, 2013 at 10:42 PM, Wvv <[hidden email]> wrote:
> First useful use is in Typeable.
> In GHC 7.8
> class Typeable (a::k) where ... <<==>> class Typeable (a ::**) where ...
> But we can't write
> data Foo (a::k)->(a::k)->* ... deriving Typeable
> Why not? This works fine in 7.7, as far as I know.
> View this message in context: http://haskell.1045720.n5.nabble.com/Rank-N-Kinds-tp5733482p5733667.html
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