Refactoring Semigroup/Monoid (was: instance Applicative Data.Map)

[Roman Cheplyaka]

* Henning Thielemann <lemming at henning-thielemann.de> [2012-11-20 00:26:17+0100]
> 
> 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