Per the GHC manual section on instance overlap: If an instance declaration is compiled without -XOverlappingInstances, then that instance can never be overlapped. This could perhaps be inconvenient. It is inconvenient for certain things. If I want to declare a special `instance Show [MyType] where...` I am out of luck. However, the solution need not involve a change to the way instances are handled -- if all the `GHC.*` instances were compiled with overlapping instances, I'd be able to declare my instance. Is there a substantial barrier, either theoretical or practical, to recompiling GHC this way? -- _jsn |...GHC manual section...| http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extension...