ghc: panic! (the 'impossible' happened): Missing alternative

[calvins]

When I include the -auto-all option (in addition to -prof) on the entry for Main in my Cabal file, I get the ghc panic message below. Full compile output attached.

[10 of 10] Compiling Main             ( Rdf4hParseMain.hs, dist/build/rdf4h/rdf4h-tmp/Main.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 6.10.1 for x86_64-unknown-linux):
	Missing alternative
    main:Main.OutputFormat{d r16LZ}
    (main:Main.Help{d r16M9}, [], lvl_s17ar{v} [lid])
    (main:Main.Debug{d r16M7}, [], lvl_s17as{v} [lid])
    (main:Main.Version{d r16M5}, [], lvl_s17at{v} [lid])

To reproduce the bug, check out a tagged version of the code using:

darcs get -t 'ghc bug report'  http://protempore.net/rdf4h

And then edit the cabal file by adding "-prof -auto-all" to the library and main entries, and you should see what I see above if it's reproducible. It compiles fine if the options are only added to the library block, and it compiles fine if '-prof' but not '-auto-all' is added to the executable block, but fails when '-prof' and '-auto-all' are both added to the executable block.

[calvins]

Compiler output (-v3)

[simonpj]

Very odd. This happens in the situation where GHC's optimiser encounters

   case (OutputFormat ...) of 
      Help ->
      Debug ->
      Version ->

with no other alternatives, not even a default. That can't happen in source code, since GHC adds an invisible default, but it can happen internally if the optimiser proves that no other alternatives can happen. But here another alternative does! That's a bug. Needs investigation.

Simon

[simonpj]

I don't know whether this is anything to do with your problem, but I can't get started:

bash-3.2$ cabal install
Resolving dependencies...
cabal: dependencies conflict: HTTP-4000.0.4 requires parsec ==2.1.0.1 however
parsec-2.1.0.1 was excluded because rdf4h-0.7 requires parsec >=3 && >=3
bash-3.2$ 

Seems that your package dependencies aren't right.

Simon

[calvins]

Hmmm, that's odd. The  cabal file for HTTP-4004.0.4 doesn't list a version for parsec, and my package manager (on Gentoo) does not list a required version for parsec either.

I will try uninstalling things and reinstalling using cabal-install rather than my package manager, and will report back.

[calvins]

I'm unable to install 'cabal install' on a clean Ubuntu install because it doesn't seem to work with Parsec 3, which my library required.

Are you able to compile with plain cabal? I checked the cabal conf files for the dependencies and verified that none of them specify Parsec 2.1.0.1. They all compile fine for me against Parsec-3.0.0 as long as I use 'runhaskell Setup ...' to compile and install rather than 'cabal install'.

[simonpj]

Aha. We managed to reproduce it. Fixed by this

Thu Apr  2 16:28:34 BST 2009  simonpj@microsoft.com
  * Fix Trac #3118: missing alternative
  
  This patch fixes a rather obscure bug, whereby it's possible
  for (case C a b of <alts>) to have altenatives that do not inclue
  (C a b)!  See Note [Unreachable code] in CoreUtils.
  

    M ./compiler/basicTypes/MkId.lhs -2 +6
    M ./compiler/coreSyn/CoreUtils.lhs -11 +42
    M ./compiler/simplCore/Simplify.lhs -27 +28
    M ./compiler/specialise/SpecConstr.lhs -1 +3

Simon