[Haskell-beginners] Install mess

I'm on Ubuntu 18.10 and I've made a real mess of my Haskell install. In the past I believe I installed from source. Then I must have tried to install through Ubuntu's repo, then I installed through the Haskell tool stack (curl -sSL https://get.haskellstack.org/ | sh). Now I can type ghci at the command line and I get 8.4.2, through stack ghci I get version 8.0.1, while starting a ghci REPL in Emacs goes with the 8.4.2. The Ubuntu repo version seems to be 8.2.2, BTW. What can I do/what should I do to straighten this out? What is the definitive install of Haskell, stack or Haskell platform, and what role should cabal play? LB