I'm working on extending this ([1]) to contravariant functors while making the whole thing polytypic and typesafe (so I can abstract over type products and coproducts, among other things). My stuff will be open-sourced next year sometime at the latest. I think it's a combination of feature interactions and frequent refactoring during development that is a source for a lot of stuff that is not nice to the compiler.

edit: The extension to [1] is actually pretty benign and the compiler does well with it. The part that puts a big load on the compiler is that it is used as an interpreter for a popular, yet complicated, visualization DSL. The compiler, especially the typechecker, has to do a huge amount of work.

[1] https://github.com/jdegoes/scalaworld-2015/tree/master/src/m...