[Haskell-cafe] type level function with different return kinds

[Tom Ellis]

On Sun, Apr 06, 2025 at 12:27:04AM +0200, Zoran Bošnjak wrote:
> is it possible to write a type/kind level function that returns types of
> different kinds for different inputs. Either by using open/closed type
> families, a typeclass, extensions or any other type level magic.

The way you do this at the type level is with Dynamic:

https://www.stackage.org/haddock/lts-23.17/base-4.19.2.0/Data-Dynamic.html#t:Dynamic

Or equivalently, something like

    data SomeTypeable where
      MkSomeTypeable :: Typeable a => a -> SomeTypeable

So if you can work out how to promote one of those to the kind level
then yes.  If not then probably no!

Tom