After seeing some demand for packaged Haskell for Mac for the IHaskell project and by word of mouth at BayHac I decided to take a stab at it. Here you go:
Why?
It can be a bit cumbersome to use the GHC builds from haskell.org on Mac, because they don't come "fully baked", you have to ./configure --prefix=/some/path to get them installed and the installation is not relocatable, if you move it the scripts will break and the package.conf.d files will go stale. It also doesn't ship with cabal-install, which is cumbersome as well, especially for new users.
How does it work?
In
https://github.com/ghcformacosx/ghc-dot-app I have a Main.hs script which downloads/builds/etc. all of the prerequisites. Then it patches the shell scripts to use a path relative to themselves (after following symlinks) rather than hard-coding an installation location. It also patches the package.conf.d files to use ${pkgroot} relative paths.
Separately, there's an Xcode project that includes all of the metadata to make it look like a real Mac app bundle (icons, Info.plist) and a small Cocoa application that has a button to open the docs and provides instructions for adding it to the PATH in .bashrc. Due to the heuristics in Mac OS X's man, this will automatically make man pages work as well. Using Xcode here also makes it produce a signed build (which I do, since I am already paying for the $99/year license).
-bob