[Haskell-cafe] File path programme

[robert dockins]

> 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.