I've downloaded the binary distribution for GHC 7.8-RC1. It did a nice job installing separate ghc/ghci/ghc-pkg binaries for 7.6 and 7.8. I can launch ghci and do wonderful things, and I can compile simple hello world files, but if I try to do anything that uses any libraries, etc it doesn't find them (unsurprisingly).
I've done some googling and it doesn't seem obvious how I would go about messing with larger projects compiled with 7.8, and even more strangely I don't find others asking the question. Maybe I've missed something obvious? I'm a Haskell noob, but there are some things that require 7.8 that I'm keen to experiment with.
I'm running OS X Mavericks with Xcode 5 which I understand causes some trouble, but I've worked around the things so far by installing gcc 4.8 and referencing it as necessary.