Construction of type-safe paths

You can use the construction functions as follows:

f :: Path.RelFile
f = relDir "tmp" </> relFile "someFile" <.> "ext"

or …

f :: Path.RelFile
f = dirPath "tmp" </> filePath "someFile" <.> "ext"

or …

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.

Modules

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.Posix and System.Path.Windows offers only path constructors and destructors fixed to the corresponding operating system. System.Path exports either System.Path.Posix or 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

The ar and the fd type parameters have quite different meaning. The types Abs and Rel refer to a property of the path, whereas the type File and 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 an 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 an 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 DirPath to 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 FilePath or 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 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.

AbsRel vs. ar type parameter

In your application you will often know whether your path denotes a file or directory, thus you will use the Path.File or 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.Abs, Path.Rel and 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 optparse-applicative

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.

Known problems

Mac

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.