Eq, Show, ... instances

[Icelandjack]

In #5 you discuss Show instances, they can be implemented using ForallT as in this comment

instance ForallT Show graph => Show (TList graph a b) where

should I add a PR for those instances?

[Icelandjack]

Classes newly added to base like Show2, Eq2, ... won't cut it, you need something of the form forall a b. Show (c a b) which is exactly what ForallT Show c is.

[Icelandjack]

I won't submit an Eq instance since I haven't read the discussion in #5 but I want to point out that syntactic has higher-kinded equality where the elements have different types:

class Equality e
  where
    -- | Higher-kinded equality
    --
    -- Comparing elements of different types is often needed when dealing with expressions with
    -- existentially quantified sub-terms.
    equal :: e a -> e b -> Bool

What we basically need is:

class Equality2 e where
  equal2 :: e a b -> e a' b' -> Bool

data Node = Unit | N
data NGraph :: Cat Node where
  O :: NGraph Unit N
  S :: NGraph N    N

instance Equality2 NGraph where
  O `equal2` O = True
  S `equal2` S = True
  _ `equal2` _ = False

instance Equality2 graph => Eq (ConsList graph a b) where
  CNil      `equal2` CNil      = True
  Cons x xs `equal2` Cons y ys = equal2 x y && equal2 xs ys
  _         `equal2` _         = False