{-# LANGUAGE DataKinds, TypeFamilies #-}
module M where

data Tag = A | B | C

data STag tag where
  SA :: STag A
  SB :: STag B
  SC :: STag C

type family Fam tag where
  Fam A = Int
  Fam _ = Bool

foo :: STag tag -> Fam tag
foo SA = 42 :: Int
foo _ = True :: Bool