ghc cyclic import error confusing

[Simon Peyton-Jones]

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 at haskell.org [mailto:glasgow-haskell-users-
| bounces at haskell.org] On Behalf Of Bryan Richter
| Sent: 14 April 2011 18:24
| To: glasgow-haskell-users at 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. :)