Handling ImplicitParams in Instance Declaration
The GHC documentation states that
ImplicitParam's may not be used in class or instance declaration constraints. see for example http://www.haskell.org/ghc/docs/7.6.1/html/users_guide/other-type-extensions.html In practice though, at least on the Mac, all recent version of GHC have permitted such instances. At the very least the documentation should make clear that the compiler might permit such instances, but they have undefined behaviour. Ideally, these would simply be permitted.