I have installed the Haskell Platform through MacPorts. Although many Haskell libraries are available as MacPorts packages, some are not. What is the proper way to install them? (without inconsistencies or duplications)
Use cabal
cabal install <packagename>
One nice thing about cabal is, by default, it puts stuff in ~/.cabal/, so you won't have to worry about it conflicting with your system files.