brew install ghc cabal-install
If you prefer homebrew.
Note: (Edited to add) Homebrew may have an older (or newer) version of ghc than the Haskell Platform download for OSX, and there might be other differences. You can check the release notes here for the version included in Haskell Platform, as well as other information of interest. brew info ghc cabal-install will give you up to date information about Homebrew’s versions.
Note: The last time I did this, I just installed haskell-stack instead, as described in Filippo’s answer.