Sing.hs

data IdType = One | Two | Three
  deriving (Eq, Ord, Enum, Bounded)

genSingletons [''IdType]

type ID = Int

-- | Simplest newtype implementaion almost everithing can be derived
-- without defining handwritten instances. Can be stored in heterogeneous
-- collections
newtype CompoundID = CompoundID (IdType,ID)
  deriving (Eq, Ord)

instance Show IntfName where
    shows (CompoundID (t, iD)) = show t ++ '/':show iD

-- | Typed implementation usefull for prcocessing in id type specific 
-- functions. This is newtype and newtype deriving is still possible
newtype TypedID (t::IdType) = TypedID ID
  deriving (Eq, Ord)

instance forall (t::IdType). (SingI t) => Show (IntfName' t) where
  show (TypedID v) = show (fromSing $ sing @t) ++ '/':show v

-- | Existential wrapper for TypedID. newtype is not compatible with existential
-- types. So many biolerplate and probably worth perfomance comparing to CompoundID.
data ExistID = forall (t::IdType) . (SingI t) => ExistID (TypedID t)

getIDType :: ExistID -> IdType
getIDType (ExistID tv) = getIDType' sing tv
  where
    getIDType' :: forall (t::IdType). Sing t -> TypedID t -> IdType
    getIDType' s _ = fromSing s

getID :: ExistID -> ID
getID (ExistID (TypedID v)) = v

instance Eq ExistID where
  (==) :: ExistID -> ExistID -> Bool
  (==) l r = getIDType l == getIDType r && getID l == getID r

instance Show ExistID where
  show (ExistID tv) = show tv