Easily import all instances contained in GHC distributed libraries.
This package simply has a number of modules which import all the modules from each package that is distributed with GHC.
There's also a module Control.Instances.GHC_Packages, that imports all of the above modules, essentially importing every module distributed with GHC.
Why would you want to do this?
Lets say I've got a data type "D a". And a class "C".
Further, lets say I can define "C (D a)" if "a" is of class "C1". I can also "C (D a)" if "a" is of class "C2"
Lets try:
instance C1 a => C (D a) where ...
instance C2 a => C (D a) where ...
These are going to be overlapping. We can't do this.
But instead, we import Control.Instances.GHC_Packages. And then we can use reify from Template Haskell to bring every "C1" and "C2" instance in to scope that's defined anywhere in "base" or any GHC module.
We can then use Template Haskell to define all our instances. We can explicitly define what happens when both "C1" and "C2" instances are defined, perhaps prefering one over the other.
As we've got the full power of Haskell in Template Haskell, we can do these manipulations.
The package static-closure is an example of using these instances to generate it's own instances for it's data type.
This package will need to be updated with each release of GHC but currently the cabal file and modules should contain appropriate pre-processor directives that this package should work from least from GHC 7.8 to GHC 8.2.