also parse class/instance declarations as "HEAD <= CONTEXT where"
When looking at a list of instance declarations, it can be difficult to visually locate the instance heads. In particular, the varying size of the contexts means that it's difficult to align the instance heads without introducing extraneous white-space.
I suggest allowing the head to come first (like all other declarations), so that the varying context lengths no longer prevent easy identification of the class/instance heads.
For example, I think it's easier to see which instances are being declared in this following list of (,) instances
instance Bounded (a, b) <= (Bounded a, Bounded b) where … instance Eq (a, b) <= (Eq a, Eq b) where … instance Functor ((,) a) where … instance Ord (a, b) <= (Ord a, Ord b) where … instance Read (a, b) <= (Read a, Read b) where … instance Show (a, b) <= (Show a, Show b) where …
than it is with the default syntax
instance (Bounded a, Bounded b) => Bounded (a, b) where … instance (Eq a, Eq b) => Eq (a, b) where … instance Functor ((,) a) where … instance (Ord a, Ord b) => Ord (a, b) where … instance (Read a, Read b) => Read (a, b) where … instance (Show a, Show b) => Show (a, b) where …
A more compelling example would involve multiparameter type classes and juxtaposition of instances for different combinations of types.
(Bonus: the proposed ordering might even help beginner grasp that the instance context is not used for instance selection.)