Thank you for reply Daniel . I installed the ghc-7.2.1 but now running  cabal install keys  --dry-run -v gives error. One more thing i would like to know. Before installing any package , i should check its dependency and if it depends on any global namespace package [/usr/local/lib/ghc-7.2.1/package.conf.d ] so i should not install the global namespace package using cabal install <global-package> otherwise it will hide the global package. I should only install those packages which are not in global namespace. Am i correct ?  Any idea how to remove this error . 

ntro@ntro-HP-dx2480-MT-NA121PA:~/Mukesh/Haskell$ ghc
ghc-7.2.1      ghci-7.2.1     ghc-pkg-7.2.1  
ntro@ntro-HP-dx2480-MT-NA121PA:~/Mukesh/Haskell$ cabal install keys --dry-run -v
cabal: The program ghc version >=6.4 is required but it could not be found
ntro@ntro-HP-dx2480-MT-NA121PA:~/Mukesh/Haskell$ cabal --v
cabal-install version 0.10.2
using version 1.10.2.0 of the Cabal library
ntro@ntro-HP-dx2480-MT-NA121PA:~/Mukesh/Haskell$ ghc-pkg-7.2.1 list
/usr/local/lib/ghc-7.2.1/package.conf.d
   Cabal-1.12.0
   array-0.3.0.3
   base-4.4.0.0
   bin-package-db-0.0.0.0
   binary-0.5.0.2
   bytestring-0.9.2.0
   containers-0.4.1.0
   directory-1.1.0.1
   extensible-exceptions-0.1.1.3
   ffi-1.0
   filepath-1.2.0.1
   ghc-7.2.1
   ghc-prim-0.2.0.0
   haskell2010-1.1.0.0
   haskell98-2.0.0.0
   hoopl-3.8.7.1
   hpc-0.5.1.0
   integer-gmp-0.3.0.0
   old-locale-1.0.0.3
   old-time-1.0.0.7
   pretty-1.1.0.0
   process-1.1.0.0
   rts-1.0
   template-haskell-2.6.0.0
   time-1.2.0.5
   unix-2.5.0.0
/home/ntro/.ghc/i386-linux-7.2.1/package.conf.d
   mtl-2.0.1.0
   parsec-3.1.1
   transformers-0.2.2.0
ntro@ntro-HP-dx2480-MT-NA121PA:~/Mukesh/Haskell$ which  ghc-7.2.1
/usr/local/bin/ghc-7.2.1
ntro@ntro-HP-dx2480-MT-NA121PA:~/Mukesh/Haskell$ echo $PATH
/home/ntro/.cabal/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games
ntro@ntro-HP-dx2480-MT-NA121PA:~/Mukesh/Haskell$ which cabal
/home/ntro/.cabal/bin/cabal

On Fri, Sep 16, 2011 at 4:18 AM, Daniel Fischer <daniel.is.fischer@googlemail.com> wrote:
On Thursday 15 September 2011, 21:37:29, mukesh tiwari wrote:
> I tried to resolve this issue on #haskell and i got suggestion that it
> was due to conflict in
>
> global and local namespace [ see here for more detail
> http://hpaste.org/51376 ].

Yes.The containers in the global db is shadowed by the user containers.

> One idea is i should wipe ~/.ghc and install all the libraries
> individually.

That'll work. If there are only few packages broken, unregistering only
those could be less work.

$ ghc-pkg check

should indicate whether there's a chance that surgical removal of
individual packages might be worthwhile. If the breakage is recent and only
few packages are affected, it is, otherwise wiping out the entire user db
would likely be simpler.

> Could some one please suggest if there is another idea to
> resolve this issue.

There is no fundamentally different way, the only method to fix broken
packages is to remove them. The only question is whether it's better to get
completely rid of the entire user db [and if you have the bad luck of
having breakage within the global db by doing global installs, you'd
probably need an entire new ghc installation] or only of individual
packages.

> Currently i have ghc-6.12.3  and  installing
> ghc-7.0 will resolve the issue ?

In a certain sense, yes. With a new version of ghc, you start with a clean
slate without broken packages. However, you could pretty much introduce the
same kind of breakage with that.

Generally, it's a bad idea to reinstall any library that came with the ghc
installation (there are some exceptions, e.g. installing a newer version of
Cabal has a fair chance of not causing havoc).
As rules of thumb,
- don't mess with the global db, user installs only
- don't install any library which already has a version in the global db[*]
- be careful when upgrading any library, it could break everything
depending on that.

Of course, if you know what you're doing, there can be good reasons to
break any of these rules, but if you don't know why it's right, it's
probably wrong.
Although it's tedious, checking all cabal install with a --dry-run first
helps avoiding breakage.

[*] and if you do, the more packages you have installed, the more likely it
will break some of those.