
To install GHC 7, have you completely de-installed GHC 6.12.3 and all related libraries ?
I am not sure if uninstalling GHC 6.12.3 is mandatory, but I did it to have a clean Haskell installation. Then I did what anonymous has summarized below:
Just download the Haskell Platform and GHC 7:
http://lambda.galois.com/hp-tmp/2011.2.0.1/haskell-platform-2011.2.0.1.tar.g... [5]
http://haskell.org/ghc/dist/7.0.3/ghc-7.0.3-i386-unknown-linux.tar.bz2 [6]
Extract ghc, change into the directory, run configure, then make install as root. After that, extract the haskell platform, change into the directory, configure, make, make install as root. Then run cabal update.
The haskell platform is broken on Natty.
https://bugs.launchpad.net/ubuntu/+source/haskell-platform/+bug/742052 [7]
Adrien