stack install ghc-mod fails with dependencies conflicts on OSX 10.13.4

The issue is that ghc-mod does not support newer versions of GHC, I believe the last supported version is GHC 8.0. You can try changing your Stack resolver to lts-8.24 or nightly-2017-06-28 (see the snapshots listed on ghc-mod's Stackage page).

Even easier, this worked for me: stack install ghc-mod --resolver lts-8.24