Stylish-haskell is a new code formatter for Haskell, and people have asked for it to be integrated in EclipseFP. This will be part of the 2.3.1 release.
You install stylish-haskell and set its path as with any helper executable (if it's your path you don't even need to specify the path in EclipseFP). Then there's a preference page to set preferences for all projects in your workspace, and a property page on each project if you want project-specific settings:
Then it's only a matter of right-clicking on the source, and choose Source-> Format. Or of course Ctrl+Shift+F which is the standard formatting shortcut in Eclipse. EclipseFP handles writing the YAML configuration file for you!
Don't know yet when EclipseFP 2.3.1 will be released, pretty soon I think!