Type-safe access to filepath manipulations
Construction of type-safe paths
You can use the construction functions as follows:
f :: Path.RelFile f = relDir "tmp" </> relFile "someFile" <.> "ext"
f :: Path.RelFile f = dirPath "tmp" </> filePath "someFile" <.> "ext"
f :: Path.RelFile f = path "tmp" </> path "someFile" <.> "ext"
or just ...
f :: Path.RelFile f = relFile "tmp/someFile.ext"
The first and the last implementations force the most specific types and thus should be prefered.
Overloaded string literals are no longer supported, since this extension is intended for alternative text storage types. It would also decrease the type safety if you could omit the path type and let the compiler guess its type.
You will typically want to import as follows:
import qualified System.Path.Directory as Dir import qualified System.Path.IO as PathIO import qualified System.Path as Path import System.Path ((</>))
System.Path.Generic provides all functions with the OS as type parameter.
System.Path.Windows offers only path constructors and destructors fixed to the corresponding operating system.
System.Path exports either
System.Path.Windows depending on the host system and additionally the manipulation functions from
System.Path.Generic. This mix should be appropriate for the average use and should free the user from writing type annotations.
How to choose proper type arguments
ar and the
fd type parameters have quite different meaning. The types
Rel refer to a property of the path, whereas the type
Dir refers to a property of a disk object. You can decide whether a path is absolute or relative by just watching (the beginning of) the path string. In contrast to that, you have to access the disk in order to check the existence and type of a disk object. Even more, the disk object might change at any time, e.g. the user might delete a file and create a directory of the same name, or the disk object might not exist, and the purpose of the path is to create an according file or directory. That's why even if you have a path of type
FilePath ar, every function accessing the file must check that the refered object exists and is a file. Conversely, there is not much sense in checking the disk object type and then chosing the path accordingly. Instead, you must choose the path type according to what type of disk object your application needs. The reality check must be performed and is performed by the standard functions for every access to the object. If a disk object is not of the type required by the path type then this is a runtime exception that must be handled at runtime but it is not a programming error.
Sometimes you have to change the type of a path as an intermediate step to construct a path for an object of different type. E.g. you may convert the path "pkg" from
FilePath because in the next step you like to extend it to "pkg.tar.gz". This is valid use of the
Path type. E.g. the function
dropExtensions reduces the
FilePath "pkg.tar.gz" to the new
FilePath "pkg" although no-one expects that there is or will be a file with name "pkg". Thus, if a function has a
FilePath parameter then there is no warranty that it accesses the according file and does not touch related disk objects. It may well be that the function derives other file and directory names from the path and accesses them. That is, a
DirPath parameter is mainly for documentation purposes but it cannot prevent you seriously from any damage.
How to cope with user input
You may get a path from the user, e.g. as command-line argument. It might be either absolute or relative and it might refer to an actual file or directory or to something yet non-existing. In most cases it will not be important whether the path is absolute or relative, thus you should choose the
AbsRel type parameter. If somewhere in the program an
Abs path is needed then you can assert that the path is actually absolutized somewhere e.g. by
dynamicMakeAbsolute. If you prevent usage of
genericMakeAbsolute then you avoid to absolutize a path that is already absolutized.
The choice of the
fd type parameter follows a different reasoning: Often you have a clear idea of whether the user must pass a file or directory path. The rule is: Just give the path the type you expect but do not perform any checking (unless you want to warn the user earlier about imminent danger). The disk object type must be checked for every access to the object, anyway, so there is no point in checking it immediately. With your choice of the
fd parameter you just document its intended use.
It might be that the path is only a base name used to construct other directory and file names. E.g. for an Audacity project named
music you have to create the directory
music_data and the file
music.aup. In this case we recommend to give
music the type
FilePath. This type warrants that there is at least one final path component in contrast to a directory path that might be empty. You can easily convert a file path to a directory path using
Path.dirFromFile. The reverse conversion is partial.
ar type parameter
In your application you will often know whether your path denotes a file or directory, thus you will use the
Path.Dir type synonym. In contrast to that, you will often not know whether the path is relative or absolute and often it does not even matter. You can express this either by using the monomorphic type
Path.AbsRelFile or by using the parameterized type
Path.File ar with constraint
PathClass.AbsRel ar. We recommend to use the first variant for command-line option parsing and the second one for the application functions. When you parse options it is the time where you know that you do not know whether the path is absolute or relative. That is, you could neither parse it as
Path.Abs nor as
Path.Rel. Thus a type variable makes no sense. You should always parse to type
Path.AbsRel. Conversely, your application would work equally well with
Path.AbsRel. Thus, using an
ar type variable is the way to go. You must make sure to use different
ar-like variables for independent paths. By equality of
ar-like type variables you can also document dependencies of paths for the programmers.
Command-line argument parsing with
For parsing of path options with the
optparse-applicative package you might use the following helper functions:
module Option where import qualified System.Path.PartClass as PathC import qualified System.Path as Path import qualified Options.Applicative as OP path :: (PathC.FileDir fd) => OP.ReadM (Path.AbsRel fd) path = OP.eitherReader Path.parse pathArgument :: PathC.FileDir fd => OP.Mod OP.ArgumentFields (Path.AbsRel fd) -> OP.Parser (Path.AbsRel fd) pathArgument = OP.argument path pathOption :: PathC.FileDir fd => OP.Mod OP.OptionFields (Path.AbsRel fd) -> OP.Parser (Path.AbsRel fd) pathOption = OP.option path
It performs minimal checking of path strings as part of option parsing. E.g. it will report a user error if the user passes "abc/" as a file path.
File system links
This package does not explicitly handle file system links. We treat a file path containing links like any other file path. The same holds for directory paths. A link is handled like any other path component.
Drive-relative paths on MS Windows
We use the
Rel type for paths that can be relative to any directory. We use the
Abs type for all other paths, i.e. for paths with explicit locations or with restrictions on the set of locations. Windows has a notion of drives and maintains a current directory for every drive. E.g. the path
"c:text.txt" refers to the current directory of drive
C. Since it cannot be freely combined with other directories we treat this path like an absolute path. This is consistent with the behaviour of the
filepath package. E.g.
filepath evaluates all of the expressions
"\\abs" </> "c:driverel",
"c:\\abs" </> "c:driverel",
"d:\\abs" </> "c:driverel" to
"c:driverel". In our package you would have to use
genericMakeAbsolute but we recommend to avoid its use.
Path format strings
You might allow a user to pass a format string such as
"page%06d.png" to your program. How to integrate this into the type-safe path handling? We recommend not to use the
Path for the format string but instead to define a custom type:
newtype Format ar fd = Format String printf :: (PathC.AbsRel ar) => Format ar fd -> Int -> Either String (Path ar fd) printf (Format fmt) n = Path.parse (Pf.printf fmt n)
This makes sure that you will access a path on disk only after formatting the path string.
Currently we select the Posix module on Mac systems. On the one hand this choice is right since Mac uses slashes for path component separation. On the other hand it is wrong since Mac ignores case when comparing filepaths.