I played around a little, and it seems that GHC has some support for dependent quantification here: {-# LANGUAGE DataKinds #-} {-# LANGUAGE TypeFamilies #-} module M where import GHC.TypeLits data A = A1 | A2 data B = B1 | B2 type family K (n :: Nat) where K 0 = A K 1 = A K 2 = B K 3 = B type F :: forall (n :: Nat) -> K n type family F n where F 0 = 'A1 F 1 = 'A2 F 2 = 'B1 F 3 = 'B2 I put it online in a playground paste here: https://play.haskell.org/saved/Pcx912j0 It typechecks, but I didn't test if it indeed works in practice. On 06-04-2025 00:27, 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’
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 _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.