Dear haskellers,
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.
Something like this:
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
import GHC.TypeLits
data A = A1 | A2
data B = B1 | B2
type family F (n :: Nat) where
F 0 = 'A1
F 1 = 'A2
F 2 = 'B1
F 3 = 'B2
The problem with this approach is that (F 0) and (F 1) are ok, but not
in combination with (F 2). The error says:
• Expected kind ‘A’, but ‘'B1’ has kind ‘B’
If I instead write:
type family F (n :: Nat) :: k where
... I get compile error already on the (F 0) line
• Expected kind ‘k’, but ‘'A1’ has kind ‘A’
Any ideas how to remove the "same kind" restriction or how to workaround
it?
I am using ghc 9.6.6.
regards,
Zoran
