I have just pushed an initial release of ban-instance[1] to Hackage. ban-instance uses template haskell to generate compile errors for typeclass instances that should not exist: -- Declare that Foo should never have a ToJSON instance $(banInstance [t|ToJSON Foo|] "why ToJSON Foo should never be defined") The custom errors then say why an instance is banned: • Attempt to use banned instance (ToJSON Foo) Reason for banning: why ToJSON Foo should never be defined Instance banned at [moduleName] filePath:lineNumber We have found this useful to prevent definition of ToJSON/FromJSON instances on core data types, as this forces programmers to instead place the serialisation instances on newtypes at the serialisation boundary. GitHub issues and PRs are welcome[2]. In particular, there are currently a few limitations: 1. There is currently no support for classes with associated types or associated data types; 2. It would be great to generate haddocks for banned instances, marking them as such; and 3. Type quotations [t|...|] do not support free variables (GHC#5616). Nevertheless, I hope that it is useful. Best, -- Jack [1]: http://hackage.haskell.org/package/ban-instance-0.1.0.1 [2]: https://github.com/qfpl/ban-instance/