Support AbstractFilePath

[hasufell]

Motivation

The next filepath release will add support for a new API, which fixes subtle encoding issues and improves cross platform code and memory residence.

You can read about the state of the newly added API here: haskellfoundation/tech-proposals#35

Release candidate with haddock is here: https://hackage.haskell.org/package/filepath-2.0.0.3/candidate

Demonstration about unsoundness of base with exotic encodings and how the new API fixes them is here: https://gist.github.com/hasufell/c600d318bdbe010a7841cc351c835f92

Migrations

Migrations can happen once the filepath, unix and Win32 packages are updated and in sync. A migration would usually involve using the new types from System.AbstractFilePath and the new API variants from unix/Win32.

When writing low-level cross platform code manually (shouldn't generally be necessary), the usual strategy is this:

#if defined(mingw32_HOST_OS) || defined(__MINGW32__)
import qualified System.AbstractFilePath.Data.ByteString.Short.Word16 as SBS
import qualified System.AbstractFilePath.Windows as PFP
#else
import qualified System.AbstractFilePath.Data.ByteString.Short as SBS
import qualified System.AbstractFilePath.Posix as PFP
#endif

crossPlatformFunction :: AbstractFilePath -> IO ()
#if defined(mingw32_HOST_OS) || defined(__MINGW32__)
crossPlatformFunction (OsString pfp@(WS ba)) = do
    -- use filepath functions for windows specific operating system strings
    let ext = PFP.takeExtension pfp
    -- operate directly on the underlying bytestring (which is a wide character bytestring, so uses Word16)
    let foo = SBS.takeWhile
    ...
#else
crossPlatformFunction (OsString pfp@(PS ba)) = do
    -- use filepath functions for posix specific operating system strings
    let ext = PFP.takeExtension pfp
    -- operate directly on the underlying bytestring (which is just Word8 bytestring)
    let foo = SBS.takeWhile
    ...
#endif

Platform specific code can be written using PosixFilePath/WindowsFilePath types.

If you have further questions, please let me know. I'm going to write a blog post outlining the affairs and more in-depth intro and migration strategies close after the release. This is a heads up.

---

Related PR: #136

[Rufflewind]

The bulk of the work has been merged (#136).

There are still some outsanding TODOs, so leaving this issue open for the time being.

[Rufflewind]

Does not look like the TODO will be resolved any time soon, so moving that into #182.

The rest of this issue is done so marking as closed.