Package

purescript-pathy

Repository
slamdata/purescript-pathy
License
Apache-2.0
Uploaded by
garyb
Published on
2018-06-28

Latest release Build status Dependency status

A type-safe abstraction for platform-independent file system paths.

Installation

bower install purescript-pathy
import Pathy

Introduction

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.

Paths Literals

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 (file / 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 dir' and 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")

The Path Type

The Path a b type has two type parameters:

  • a – This may be Abs or Rel, indicating whether the path is absolute or relative.
  • b – This may be Dir or 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).

Parse Paths from Strings

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 Maybe.

The parsePath function also expects a Parser argument so that different path formats can be parsed into the common Path type.

Sandboxing

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.

Renaming, Transforming, Etc.

There are many other functions available to you for renaming files, renaming directories, getting parent directories, etc.

These are all documented on Pursuit, and you can find test usages for most of them.

Documentation for all functions and types is published on Pursuit.