On Sun, 6 Apr 2025, Zoran Bošnjak wrote:
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’
...
Any ideas how to remove the "same kind" restriction or how to workaround it?
That's the behavior I would expect. Why do you want something different? You could define a type C that has two constructors for A and for B.