* Henning Thielemann
On Sat, 17 Nov 2012, Edward Kmett wrote:
Honestly, the main issue is that even if you have the ability to describe default definitions for how to implement superclasses, it isn't really all that useful. :( e.g. Every monad transformer still needs each of its instances crafted by hand. This even applies to simpler types:
Given
instance (Traversable f, Traversable g) => Traversable (Compose f g)
you don't want the derived instances for Functor and Foldable,
instance (Traversable f, Traversable g) => Functor (Compose f g) instance (Traversable f, Traversable g) => Foldable (Compose f g)
you want the more permissive ones you can roll by hand.
instance (Functor f, Functor g) => Functor (Compose f g) instance (Functor f, Functor g) => Foldable (Compose f g)
Good example. Frequently I think that we should replace all these type class instance extensions by a generic way to program instances.
What do you mean? Don't we already have TH and plenty of generic programming libraries? Roman