#13092: family instance consistency checks are too pessimistic -------------------------------------+------------------------------------- Reporter: rwbarton | Owner: rwbarton Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Compile-time Unknown/Multiple | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- FamInst has this logic for checking type family instance consistency among imports: {{{ Note [Checking family instance consistency] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ For any two family instance modules that we import directly or indirectly, we check whether the instances in the two modules are consistent, *unless* we can be certain that the instances of the two modules have already been checked for consistency during the compilation of modules that we import. [...] How do we know which pairs of modules have already been checked? Any pair of modules where both modules occur in the `HscTypes.dep_finsts' set (of the `HscTypes.Dependencies') of one of our directly imported modules must have already been checked. Everything else, we check now. (So that we can be certain that the modules in our `HscTypes.dep_finsts' are consistent.) }}} However, suppose one of the modules `A` we import directly is itself a type family instance module. Then it too has been checked for consistency with its dependencies `B`, `C`, etc., so we should skip checking the pairs `A` & `B`, `A` & `C`, etc. The current behavior means that whenever we directly import a type family instance module, we still have to load the interface files for all its type family module dependencies, which largely defeats the purpose of the optimization in this case. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13092 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler