I was lucky to avoid this myself, but I keep seeing it crop up: ghc-mod doesn't work with GHC < 7.10 and Cabal-1.22. If you are still on GHC 7.8.3 or 7.8.4, you may want to hold off on updating cabal or cabal-install. This isn't really my news to report, but folks need to know. See this issue for more information: https://github.com/kazu-yamamoto/ghc-mod/issues/417#issuecomment-68819938 Anthony