Proposal: Functor and friends for the wrappers in Data.Monoid

[Henning Thielemann]

Am 22.02.2015 um 09:25 schrieb Edward Kmett:

> I generally feel having these only-one-way-to-do-it instances just
> defined is better than forcing orphans upon those who _do_ want them.
>
> Inevitably those users seem to wind up with separate orphans that tend
> to collide, or just give up and define their own type. Both of those
> outcomes are rather suboptimal.

I am generally opposed to adding instances just because there is one and 
only one way to write them. For me instances must also have some 
reasonable use. Every defined instance reduces the chance to find a 
silly error early. The question is: Is using the instance Monad Sum more 
likely to be useful or more likely to be an accident? I guess it is the 
second one. Then I would propose to use the known technique to write an 
instance that cannot be defined fully:
    https://ghc.haskell.org/trac/ghc/ticket/9334#comment:9

This way, programmers can achieve more confidence in their code and 
orphan instances are ruled out. I also think that it is easier to switch 
from an unimplementable instance to an implemented instance if someone 
finds a use, than to alter or remove an intentionally implemented instance.