#1012: ghc panic with mutually recursive modules and template haskell -------------------------------------+------------------------------------- Reporter: guest | Owner: Type: bug | Status: new Priority: lowest | Milestone: Component: Template Haskell | Version: 6.8.2 Resolution: | Keywords: hs-boot Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: | TH_import_loop Blocked By: | Blocking: Related Tickets: #9032 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by ezyang): This bug still exists in GHC 8.0, although it can be a bit tricky to trigger if the topsort picks an ordering of modules which masks the error. I found this especially surprising because in the parallel upsweep case we seem to try to block, c.f. this comment in GhcMake {{{ 2. A module that depends on a module in an external loop can't proceed until the entire loop is re-typechecked. }}} and this code: {{{ -- If this module depends on a module within a loop then it must wait for -- that loop to get re-typechecked, i.e. it must wait on the module that -- finishes that loop. These extra dependencies are this module's -- "external" loop dependencies, because this module is outside of the -- loop(s) in question. let ext_loop_deps = Set.fromList [ head loop | loop <- comp_graph_loops , any (`Set.member` textual_deps) loop , this_build_mod `notElem` loop ] }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/1012#comment:42 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler