Glad you've found your answer :) By the way this is a technique I've used in the package fcf-family: https://hackage.haskell.org/package/fcf-family The idea is that given a Name which uniquely identifies a type family, it should be possible to map it to, well, the type family it names. You will then also need a type family to map to its type, so you get essentially this: type family TypeOf (n :: Name) :: Type type family Eval (n :: Name) :: TypeOf n I've used fcf-family to extend kind-generics to enable Generic instances for data types that involve type families. Cheers, Li-yao On 2025-04-06 2:43 PM, Tom Smeding wrote:
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.
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