I am afraid you are out of options for the moment besides compiling it yourself (but editing the package seems alot less problematic).
Either you edit
var/cache/apt/pkgcache.bin as shown at comment #12 in the bug or you will have to wait until all the problems have been ironed out for using the Haskell platform on Ubuntu 11.04
I would go with editing the package. It is not that difficult and I expect you to be able to do that without any problem.
edit: now I am on my own machine.
sudo cp pkgcache.bin pkgcache.backup.bin
sudo cp pkgcache.bin pkgcache.test.bin
sudo vi pkgcache.test.bin
This will result in:
See the 6.12.1 at the bottom?
Navigate with the arrow keys to the 1 of
When on the 1 hit
That's the practise run. Remove the
.test from the
sudo vi pkgcache.test.bin to change the real file. After that install it in the normal way.