[Haskell-cafe] File path programme

[Malcolm Wallace]

> Would someone like to take up the reigns on the design for the new
> library?

I don't feel I know enough about the corners of DOS, Windows, and
MacOS Classic filesystems to take up the challenge.  But here's a quick
sketch of the kind of thing I had in mind, based on Ben's suggestion:

> having an ADT for paths would not break any code, if what that
> means is that we provide
> 
>     parsePath :: String -> Maybe Path
>     showPath  :: Path -> Maybe String
> 
> and various (new) functions operating on the Path type.

So let's define a new algebraic datatype something like:

    data Path   = Unix  { absolute   :: Bool
                        , dirnames   :: [String]
                        , basename   :: String
                        , extensions :: [String]
                        }
                | DOS   { absolute   :: Bool
                        , drive      :: Maybe Char
                        , dirnames   :: [String]
                        , basename   :: String  -- max 8 Chars
                        , extension  :: String  -- max 3 Chars
                        }
        --      | Win32 ...
        --      | MacOSClassic  ....
        --      | VMS | CPM | Amiga | ARM ....

The implementation of
    parseFilePath :: Prelude.FilePath -> Maybe System.Path
is probably quite involved, but the Show instance should be more
straightforward, something like:

    instance Show FilePath where
      show (Unix abs dirs name exts) =
        (if abs then ('/':) else id)
        (concat (intersperse "/" (dirs++[basename])) ++
         concat (intersperse "." exts))
      show (DOS abs drive dirs name ext) =
        (maybe "" (\c->[c,':']) drive) ++
        (if abs then ('\\':) else id)
        (concat (intersperse "\\" (dirs++[basename])) ++"."++ext)

The signatures of various of the current manipulation functions would
need to change:

 -  splitFileName :: FilePath -> (String, String)
 +  splitFileName ::   Path   -> (Path,   String)

 -  splitFileExt :: FilePath -> (String, String)
 +  splitFileExt ::   Path   -> (Path,   String)

 -  joinFileName :: String -> String -> FilePath
 +  joinFileName ::  Path  -> String ->   Path

 -  joinPaths :: FilePath -> FilePath -> FilePath
 +  joinPaths ::   Path   ->   Path   -> Maybe Path

... and so on.

Regards,
    Malcolm