[Haskell-cafe] Packages all screwed up

[Daniel Fischer]

On Monday 31 January 2011 23:59:57, Arnaud Bailly wrote:
> Hello,
> I recently tried to upgrade some package (eg. yesod) and it seems
> that, in the process, I screwed up my Haskell packages setup.

Big time.

>
> When I am trying to do a simple:
> > ghc --make Crete1941
>
> It fails with message:
>
> Loader\Communication.hs:14:7:
>     Could not find module `System.Process':
>       Use -v to see a list of the files searched for.

Scary. Does 'ghc-pkg list process' list
- no package process at all
- one package process in the global db
- more than one process (one in the global db)
- process in user db but not global ?

The first would mean your GHC is really borked and you'd need to reinstall.
The second and third would mean there's hope, four looks like a reinstall 
again.
Unfortunately, the error message lets me fear the first.

>
> which is quite annoying !
>
> Is there a way to reconstruct a sane baseline ?

1. Check whether ghc itself is affected, rename (or move, or delete if you 
think it's not worth saving) the directory your user db is in (ghc-pkg list 
process should tell you where if you don't know), so ghc doesn't see it. 
ghc-pkg check. If nothing is reported as broken, try compiling a programme 
or two using only the core libs, if it works, you probably don't need to 
reinstall ghc, if not, you have to start from zero.

2. If ghc itself is okay, you can decide whether you want to start fresh, 
then delete the old directory and cabal install what you need/want. If you 
think trying to rescue what you have and is not broken is worth the effort, 
ghc-pkg unregister the broken packages and cabal install them again.