FYI, in case it wasn't clear, the system-filepath package provides such an abstract FilePath type, which I imagine is why some people prefer to use it even though it means adding some extra conversions when using the System.IO API (conversions which are fragile because we keep changing the meaning of the String version of FilePath, sigh).
Users of system-filepath should use system-fileio and have fewer needs for System.IO, although system-fileio uses System.IO APIs. Internally, system-filepath has conversion rules that can vary from one ghc version to the next: http://hackage.haskell.org/packages/archive/system-filepath/0.4.6/doc/html/s...
system-filepath looks slightly odd to be because I would have expected the representation internally to be platform-specific (e.g. either ByteString or Text for Unix or Windows respectively), but it just uses String.
I am kind of suprised to see an internal representation using strings also. I imagine that is because paths are usually used with system-fileio which needs strings for the existing Haskell APIs. system-fileio calls encodeString, which is OS specific, and that the internal code base is easier to maintain with 1 type. The encode/decode functions do return a platform specific type. Greg Weber