#10270: inconsistent semantics of type class instance visibility outside recursive modules -------------------------------------+------------------------------------- Reporter: skilpat | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: | Keywords: Operating System: MacOS X | Architecture: x86_64 Type of failure: GHC rejects | (amd64) valid program | Test Case: Blocked By: | Blocking: Related Tickets: #9562 | Differential Revisions: -------------------------------------+------------------------------------- Comment (by ezyang): Actually, the situation is a bit better here, because hs-boot operates strictly locally. Here is a fix proposal: For non-orphan instances, we can't easily tell if an instance is supposed to be in scope unless we know whether or not the defining module is transitively reachable from the imports of a module. Fortunately, in `--make`, we do know this information from the import graph. So, for each module we compile, calculate a set of local hs/hs-boot modules which should be visible, and pass that along to the type-checker. Then, like for orphans, we only treat an instance as visible if it is in this set. (Note: we can't do this strategy for external instances, because we don't have the full dependency graph anymore.) I thought of this technique when I noticed that `--make` behavior was inconsistent, depending on whether or not a module was compiled before or after we retypecheck an hs-boot loop; if you always make sure modules which transitively have a `{-# SOURCE #-}` import on a booted module are type-checked BEFORE you typecheck the real implementation (even if there isn't a dependency), you will ensure that only the correct interfaces are in scope. But this is awfully delicate and wouldn't work well in the parallel make case, so just calculating the reachable set of nodes seems better. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10270#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler