As I said before, it still doesn't solve the problem I'm trying to solve. Look at a package like criterion, for example. criterion depends on aeson. Why? Because statistics depends on it. Why? Because statistics wants a couple types it defines to be instances of classes defined in aeson. John Lato's proposal would require the pragma to appear in the relevant aeson module, and would prevent *anyone* else from defining instances of those classes. With my proposal, statistics would be able to declare

{-# InstanceIn Statistics.AesonInstances AesonModule.AesonClass StatisticsType #-}

Then it would split the Statistics.AesonInstances module off into a statistics-aeson package and accomplish its objective without stepping on anyone else. We'd get a lot more (mostly tiny) packages, but in exchange the dependencies would get much thinner.