That's because types that belong to most non-star kinds cannot have
values.
data Foo (a :: k) = Foo
is okay,
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.
Roman
* Wvv
OMG! 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@n5.nabble.com) wrote:
Hi,
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.
Cheers, Pedro
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