Minor bugs in CH5 GADTs

[JonathanLorimer]

Require proper Eq instance for Ord instance to work

For exercise 5.3-iii you need to have the Eq instance for HList defined in terms of All. This is because the tail of HList needs an Eq instance as well as the whole thing when defining the Ord instance:

this will work

instance All Eq ts => Eq (HList ts) where
  HNil == HNil = True
  (a :# as) == (b :# bs) = a == b && as == bs

instance (All Eq ts, All Ord ts) => Ord (HList ts) where
  compare HNil HNil = EQ
  compare (a :# as) (b :# bs) = compare a b <> compare as bs

this will not work

instance Eq (HList '[]) where
  HNil == HNil = True

instance (Eq t, Eq (HList ts)) => Eq (HList (t ': ts)) where
  (a :# as) == (b :# bs) = a == b && as == bs

instance (All Eq ts, All Ord ts) => Ord (HList ts) where
  compare HNil HNil = EQ
  compare (a :# as) (b :# bs) = compare a b <> compare as bs

Typo in solutions

Solution for exercise 5.3-ii on pg 249:

currently is

instance (Show t, Show (HList ts)) => Show (HList (t ': ts)) where
  show (x :# xs) = show x <> ":#" show xs

should be

instance (Show t, Show (HList ts)) => Show (HList (t ': ts)) where
  show (x :# xs) = show x <> ":#" <> show xs