Following Bryan's suggestion I've improved GHC's error message when there's a module cycle: Module imports form a cycle: module `Foo4' imports `Foo' which imports `Foo2' which imports `Foo3' which imports `Foo4' Simon | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users- | bounces@haskell.org] On Behalf Of Bryan Richter | Sent: 14 April 2011 18:24 | To: glasgow-haskell-users@haskell.org | Subject: Re: ghc cyclic import error confusing | | On Wed, Apr 13, 2011 at 11:44:33AM +0100, Simon Marlow wrote: | > On 09/04/2011 04:32, Evan Laforge wrote: | > >I've found ghc's cyclic import error to be rather confusing, and I | > >finally took the time to understand why. Here's an example: | > > | > >Module imports form a cycle for modules: | > > Cmd.Cmd (./Cmd/Cmd.hs) | > > imports: Perform.Midi.Instrument Instrument.MidiDb Instrument.Db | > > Perform.Midi.Instrument (./Perform/Midi/Instrument.hs) | > > imports: Cmd.Cmd | > > Instrument.MidiDb (./Instrument/MidiDb.hs) | > > imports: Perform.Midi.Instrument | > > Instrument.Db (./Instrument/Db.hs) | > > imports: Instrument.Search Instrument.MidiDb | > > Perform.Midi.Instrument | > > Instrument.Search (./Instrument/Search.hs) | > > imports: Instrument.MidiDb Perform.Midi.Instrument | > > | > >It seems to be in a strange order and mentions extraneous modules. I | > >would find this much easier to read: | > > | > >Perform.Midi.Instrument -> Cmd.Cmd -> Instrument.MidiDb -> | > >Perform.Midi.Instrument | > | > So the algorithm that GHC uses is this: | > | > - do a strongly connected component analysis | > - build until we hit a cycle | > - then, report the error for the cycle | > | > Now, the modules in the cycle are a strongly connected component: | > every module is reachable from every other module by following | > imports. We report all the modules in the strongly connected | > component and their imports, but omit imports of modules outside the | > cycle. | > | > >Instead, the order goes Cmd.Cmd -> Instrument.MidiDb, and then goes | > >backwards to Perform.Midi.Instrument -> Cmd.Cmd. Then it goes | > >forward again to Instrument.MidiDb -> Perform.Midi.Instrument. So | > >the order makes you jump around if you want to trace the import | > >chain. The duplicated module that joins the cycle is not visually highlighted. | > >Whats more, it further confuses the eye by merging in multiple loops. | > >I suppose it could be useful to include additional loops, but I would | > >find it easier to read if they were included on their own line, such | > >as: | > > | > >Cmd.Cmd -> Instrument.Db -> Instrument.Search -> | > >Perform.Midi.Instrument -> Cmd.Cmd | > > | > >However, I think probably the shortest loop is the most interesting | > >one, and if there are multiple shortest loops, simply picking one | > >randomly is likely to be sufficient. | > | > Picking the shortest one sounds reasonable. However, there could be a | > single edge which if removed will break all the loops, and they might | > want to know which it is. | > | > If you want to play with this, the code is very localised: it is a few | > lines in GhcMake.cyclicModuleError | > | > http://hackage.haskell.org/trac/ghc/browser/compiler/main/GhcMake.hs#L | > 1446 | | Hi Simon and Evan, | | Thanks for bringing up a problem, and providing useful information about it, in a way | that is understandable enough for a newcomer to have an opinion about it! | | So, here is my newcomer's opinion: The error displays a strongly connected graph, | with one or more cycles, but labels it a "cycle". As you both point out, having all | the information about the strongly connected modules is very useful. Labeling it a | cycle, however, gives the reader an expectation that is bound to be confounded. | | Perhaps the following change would be sufficient? | | | | --- tmp/GhcMake.hs~ 2011-04-14 09:46:02.177298318 -0700 | +++ tmp/GhcMake.hs 2011-04-14 09:52:25.121290827 -0700 | @@ -1460,7 +1460,8 @@ | | cyclicModuleErr :: [ModSummary] -> SDoc cyclicModuleErr ms | - = hang (ptext (sLit "Module imports form a cycle for modules:")) | + = hang (ptext (sLit "Module imports form a strongly connected graph, | + with one or more cycles, for these modules:")) | 2 (vcat (map show_one ms)) | where | mods_in_cycle = map ms_mod_name ms | | | | | | -Bryan | | P.S. I'm not sure what the accepted method for formatting/wrapping string literals | is, so I left it as an exercise. :)