I was trying to compile John Macheam's "GetOptions" with ghc-6.6. It failed saying

ben@sarun: .../repos/GetOptions > runhaskell Setup.hs build    
Preprocessing library GetOptions-1.0...
Building GetOptions-1.0...

GetOptions.hs:31:7:
    Could not find module `IO':
      it is a member of package haskell98, which is hidden

I exposed haskell98:

ben@sarun: .../repos/GetOptions > sudo ghc-pkg expose haskell98
Saving old package config file... done.
Writing new package config file... done.

But still no luck... Anyway then I said to myself "forget it, just use the hierarchical library stuff", hid haskell98 package again, and changed the import stuff a bit (see below). Result:

ben@sarun: .../repos/GetOptions > runhaskell Setup.hs build

<interactive>:1:84:
    Failed to load interface for `IO':
      it is a member of package haskell98, which is hidden
ghc-6.6: panic! (the 'impossible' happened)
  (GHC version 6.6 for x86_64-unknown-linux):
        interactiveUI:flush

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

This is the "fix" I applied:

ben@sarun: .../repos/GetOptions > darcs whatsnew
{
hunk ./GetOptions.hs 27
-import List
+import Data.List
hunk ./GetOptions.hs 30
-import System
-import IO
+import System.IO
+--import IO
}

Here is the repo I pulled from, so maybe you can reproduce the bug:

ben@sarun: .../repos/GetOptions > cat _darcs/prefs/defaultrepo 
http://repetae.net/john/repos/GetOptions

I could be that my ghc installation is completely screwed up but I wouldn't know how or why.

Ben