concrete functor and monad transformers
#55Migrate Contravariant instances
The Data.Functor.Contravariant module (originally from the contravariant library) will be added to base in version 4.12 (GHC 8.6). This module defined Contravariant instances for data types from transformers, so we should migrate those instances to transformers itself.
I've implemented this change in https://hub.darcs.net/RyanGlScott/transformers/patch/b00658faa4578b0eb578cf158c75185c1c0ec697 on my transformers fork.
