RowToList-sorted.hs

type family RowToList (r :: * -> *) :: [(Symbol, *)] where
  RowToList (l :*: r)
    = Merge (RowToList l) (RowToList r)
  RowToList (S1 ('MetaSel ('Just name) _ _ _) (Rec0 a))
    = '[ '(name, a)]
  RowToList (M1 _ m a)
    = RowToList a
  RowToList _
    = '[]

class Ord' t where
  type Cmp (a :: t) (b :: t) :: Ordering

instance Ord' Symbol where
  type Cmp a b = CmpSymbol a b

instance Ord' k => Ord' (k, v) where
  type Cmp '(k1, _) '(k2, _) = Cmp k1 k2

type family Merge (xs :: [t]) (ys :: [t]) :: [t] where
  Merge xs '[] = xs
  Merge '[] ys = ys
  Merge (x ': xs) (y ': ys) = Merge' (Cmp x y) x y xs ys

type family Merge' (ord :: Ordering) (x :: t) (y :: t) (xs :: [t]) (ys :: [t]) :: [t] where
  Merge' 'LT x y xs ys = x ': Merge xs (y ': ys)
  Merge' _ x y xs ys   = y ': Merge (x ': xs) ys