EclipseOn Mac OS X
Eclipse seems to work ok on OS X, but it doesn't seem like there is much testing on OS X. Installing per instructions will get you editing and building pretty quickly.
However, getting syntax highlighting working requires a little work. The "scion-browser" and "buildwrapper" don't seem to install correctly, probably something to do with permissions. You will see errors whenever your try and let Eclipse install and build them, or just ignore the fact they are missing.
Build them from the command line with cabal.
cabal install scion-browser
cabal install buildwrapper
The executables you need will likely be in a hidden directory "Library/Haskell/bin" of your home directory. i.e. if your user id is "foo" then the path will be "/Users/foo/Library/Haskell/bin". That folder may not show in finder or spotlight.
In Eclipse, select preferences. Select Haskell in the menu and drill down to helper executables. Select the appropriately named executables for Build Wrapper and Browser Executable (for scion-browser). These are links in the bin directory, but eclipse will resolve them fine.
That should at least get syntax highlighting working.
You won't find this anywhere else on the web. If you do, replace this page with a suitable link.
These instructions are applicable to EclipseFp 2.2.2 and Lion, and probably apply to other versions of OS X and EclipseFP.