I've installed the Haskell platform which came with the 7.10.2 version of ghc and the 1.22.6.0 version of cabal. My favorite editor is Atom, so I've installed the ide-haskell and haskell-ghc-mod. Since the ghc-mod has some problems with cabal>=1.22, I need to downgrade cabal. Haskel platform came with an activate-hs script to switch versions but I didn't understand how it works.
The ./activate-hs script only switches between different already installed platform versions. So if you don't have a prior version of the platform installed it won't help. More generally, you should not probably not downgrade cabal versions, as often compilers are coupled to particular releases of cabal.
But if you did want to, the correct way would just be to create a sandbox and then run cabal-install cabal-install-1.20.2.0 or the like. You can then take the resultant binary and move it into your path directly.