#9562: Type families + hs-boot files = unsafeCoerce -------------------------------------+------------------------------------- Reporter: goldfire | Owner: ezyang Type: bug | Status: new Priority: high | Milestone: Component: Compiler | Version: 7.8.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC accepts | Unknown/Multiple invalid program | Test Case: Blocked By: | Blocking: Related Tickets: #10270 | Differential Revisions: -------------------------------------+------------------------------------- Comment (by ezyang): Richard and I had a discussion, and we decided that solution 2 (when GHC is preparing the list of modules to send to the linker, perform an overlap check on type functions) is the only solution that works for linking together LIBRARY files (e.g. libfoo.a). Solution 1 does not work in this case, since when you link a library there isn't actually an executable to overlap check. This has a consequence that (at least for safe usage) you can't just use `ar` to put `.o` files together: you must call GHC to do the overlap check. This means we need a new major mode for GHC, to do the overlap check for a set of modules (or even to just do the linking). The overlap check should be implied when `--make` is used. It's harmless for `--make` to report an overlap early (as is the case currently), but we must always do overlap check at the end, in case `D` is compiled before `B` (as was the case in 7.8). The overlap check can be skipped if there are no boot files. Aside: There is an alternate strategy we can use which avoids the need for an overlap check at the very end. However, it requires that one-shot compilation be done in a ''specific'' order, so we don't think we should actually use it. Here's what you do: every `A.hs`/`A.hs-boot` pair induces a cycle of imports, which must be compiled before `A.hs` can be compiled. We ''always'' compile this cycle before we compile any other modules which depend on `A.hs-boot`. When compiling a module which transitively imports a boot file, we check if the real module has already been compiled; if so, we load it and add its instances to our environment. An instance which conflicts with the instance in `A.hs` will either be in a critical cycle, or not. If it is in the critical cycle, we will report overlap when `A.hs` is typechecked. Otherwise, we will load `A.hi` when typechecking the module and report overlap. One silly way to enforce this ordering is, when compiling a module which transitively depends on a boot file, to produce a pre-hi file; only when the real module has been compiled, only then can you re-process the hi file to produce the real hi file. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9562#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler