A type-safe abstraction for platform-independent file system paths.
bower install purescript-pathy
Applications often have to refer to file system paths in a platform-independent way.
Many path libraries provide a single abstraction to deal with file system paths. This allows easy composition of different kinds of paths, but comes at the expense of the following distinctions:
- The distinction between relative and absolute paths.
- The distinction between paths denoting file resources and paths denoting directories.
Pathy also uses a single abstraction for file system paths, called
Path, but uses phantom types to keep track of the above distinctions.
This approach lets you write code that performs type-safe composition of relative, absolute, file, and directory paths, and makes sure you never use paths in an unsafe fashion. Bogus and insecure operations simply aren't allowed by the type system!
Many paths come from user-input or configuration data. Pathy can parse such string paths and allow you to safely resolve them to expected types.
Building path literals is easy using the following components:
rootDir– The root directory of an absolute path.
currentDir– The current directory (AKA the "working directory"), useful for building relative paths.
file– A file (in the current directory).
dir– A directory (in the current directory).
(</>)– Adds a relative path to the end of a (relative or absolute) path.
(<.>)– Sets the extension of a file path.
(<..>)– Ascends one level in a directory, then descends into the specified relative path.
All path segments (
dir) names are required to be non-empty. This is enforced by
Name being constructed from a
NonEmptyString. At compile time, we can have provably non-empty strings by using
Symbols and a bit of type class trickery:
dirFoo :: Name Dir dirFoo = dir (SProxy :: SProxy "foo")
Here we're using a symbol proxy (
SProxy) and then typing it to explicitly carry the name that we want to use for our path at runtime. There is also a
file' variation on the function that accepts normal
Name values, so if you are not constructing a path at compile-time, you'd be using these instead.
Some example compile-time path constructions:
path1 = rootDir </> dir (SProxy :: SProxy "foo") </> dir (SProxy :: SProxy "bar") </> file (SProxy :: SProxy "baz.boo") path2 = currentDir </> dir (SProxy :: SProxy "foo")
Thanks to the phantom type parameters, Pathy doesn't let you create path combinations that don't make sense. The following examples will be rejected at compile time:
rootDir </> rootDir currentDir </> rootDir file (SProxy :: SProxy "foo") </> file (SProxy :: SProxy "bar") file (SProxy :: SProxy "foo") </> dir (SProxy :: SProxy "bar")
Path a b type has two type parameters:
a– This may be
Rel, indicating whether the path is absolute or relative.
b– This may be
File, indicating whether the path is a file or directory.
You should try to make the
Path functions that you write as generic as possible. If you have a function that only cares if a path refers to a file, then you can write it like this:
myFunction :: forall a. Path a File -> ... myFunction p = ...
By universally quantifying over the type parameters you don't care about, you ensure your code will work with the most paths possible (you also are documenting the expectations of your function to other developers who read your code).
To parse a string into a
Path, you can use the
parsePath function, which expects you to handle four cases:
Path Rel File
Path Abs File
Path Rel Dir
Path Abs Dir
If you need a specific case, you can use helper functions such as
parseRelFile, which return a
parsePath function also expects a
Parser argument so that different path formats can be parsed into the common
Pathy makes it easy to create relative paths, even paths that ascend into parent directories of relative paths. With this power comes danger: if you parse a user string, the user may be able to escape any arbitrary directory.
Pathy solves this security problem by disallowing conversion from a
Path to a
String until the
Path has been sandboxed.
To sandbox a path, you just call
sandbox and provide the sandbox directory, as well as the path to sandbox:
sandbox (rootDir </> dir (SProxy :: SProxy "foo")) -- sandbox root (rootDir </> dir (SProxy :: SProxy "foo") </> dir (SProxy :: SProxy "bar")) -- path to sandbox
This returns a
Maybe, which is
Nothing if the tainted path escapes the sandbox.
After you have sandboxed a foreign path, you may call
printPath on it, which will print the path absolutely.
There is also the option to
unsafePrintPath. This is labelled as being unsafe as it may be depending on how it is used - for example, if a path was sandboxed against some path other than the current working directory, but then used when launching a command in the current working directory, it may still refer to a location that it should not have access to.
There are many other functions available to you for renaming files, renaming directories, getting parent directories, etc.
Documentation for all functions and types is published on Pursuit.