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

[Roman Cheplyaka]

* Henning Thielemann <lemming at henning-thielemann.de> [2013-03-17 21:11:19+0100]
> 
> On Sun, 17 Mar 2013, Roman Cheplyaka wrote:
> 
> >* 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?
> 
> That was last year, how shall I know today what I meant? :-)

Oops! Sorry, I was searching for something in my mailbox, found your
message and didn't look at the timestamp.

> The
> various meta-programming utilites can be used to _implement_ type
> class instances. But I think there should be a way to unify all the
> UndecidableInstances, OverlappingInstances, FlexibleInstances,
> FlexibleContexts, TypeSynonymInstances, that is there might a
> programmable way to _select_ a class dictionary for some given
> concrete types.

Ok, I see what you mean, thanks.

Roman