Warning: I'm not interested in a path parsing/combining library, so my criticisms are perhaps unrelated to your goals.
One thing that I'd be interested in seeing for any Path class would be a simple instance for FilePath (or String, if you want to imagine FilePath will be changed). Not everyone will want the overhead of a massively heavyweight Path datatype.
I'm not convinced that this is massively heavyweight, but your criticism is heard. Perhaps we should realize that there are two separate things going on here. One is the ability to pass around path names as black boxes to the various IO routines without examining the path. The other is the ability to examine and manipulate the path. So perhaps we want something like this:
class PathStorage p where fromBytes :: Int -> Ptr () -> IO p withBytes :: p -> (Ptr () -> IO a) -> IO a
class (Show p,PathStorage p) => Path p where path manipulation routines.... path parsing routines... etc...
Then we could have things like:
instance PathStorage (Ptr ()) where fromBytes _ ptr = return ptr withBytes p f = f p
or
instance PathStorage (ForeignPtr ()) where fromBytes _ ptr = newForeignPtr finalizerFree ptr withBytes p f = withForeignPtr p f
or
instance (Storable p) => PathStorage [p] fromBytes n ptr = peekArray n (castPtr ptr) withBytes p f = withArray p f
as well as the full ADT implementations.
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