Aha! You are right! The relevant ticket is #2182. One thing unsaid
#13102: orphan family instances can leak through the EPS in --make mode -------------------------------------+------------------------------------- Reporter: rwbarton | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by rwbarton): Replying to [comment:3 simonpj]: there is that non-orphan instances cannot be a problem because the module defining a non-orphan instance must be in the transitive closure of modules below the one being compiled. It's just orphan modules that we need to take care about. So the logic here is: a non-orphan (class or type family) instance is one which is either an instance of a class/family defined in the same module or an instance that mentions a type constructor defined in the same module. Either way, in order to use the instance both the class/family and the type constructor must be in scope, so we must have actually imported them (rather than obtained them through leakage from compiling another module). The last step is not entirely true since the type constructor might also be in scope by virtue of being a wired-in type that corresponds to Haskell syntax, like a tuple. Then I don't actually have to import anything to be able to write the type. If the module `W` defining a wired-in thing defines an instance of a class defined in another module for that thing, a user program could see the instance without having to import `W`. I don't know whether this currently occurs in base (it seems to at least not happen for type families), but it's what I wanted to add in #13072. Not saying this should block adoption of the cunning plan, just that it is another wrinkle to be aware of. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13102#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler