For anyone using Debian or a derivative and who wants to build from source and/or install without using the package manager, here's one way using Debian update-alternatives tool:

https://github.com/byrongibson/scripts/tree/master/install/h...

TLDR: Install GHC (binary) to a custom location like /opt/haskell/ghc, then add it to the system with the Debian update-alternatives tool, repeat with Haskell Platform. Only difference is you build Platform from source. update-alternatives scripts included for both.