Officially Arch Linux doesn't follow the platform. The Haskell platform tries to pick the most mature and well tested versions. Arch Linux, on the other hand, picks the upstream bleeding edge for all its packages, not only Haskell ones. Right now, the policy seems to pick the latest package versions needed to build the latest version of pandoc and XMonad.

This is not necessarily bad. I like to develop in Arch because it usually comes with future versions that will eventually make it into the platform (I have the GHC version in mind, mostly). However, if you want to distribute your code widely, you'd better be sure it builds properly with the current Haskell platform.

Arch tends to follow the latest ghc and cabal-install, not the Platform.