Deprecating system-filepath and system-fileio

May 14, 2015

GravatarMichael Snoyman

I posted this information on Google+, but it's worth advertising this a bit wider. The tl;dr is: system-filepath and system-fileio are deprecated, please migrate to filepath and directory, respectively.

The backstory here is that system-filepath came into existence at a time when there were bugs in GHC's handling of character encodings in file paths. system-filepath fixed those bugs, and also provided some nice type safety to prevent accidentally treating a path as a String. However, the internal representation needed to make that work was pretty complicated, and resulted in some weird corner case bugs.

Since GHC 7.4 and up, the original character encoding issues have been resolved. That left a few options: continue to maintain system-filepath for additional type safety, or deprecate. John Millikin, the author of the package, decided on the latter back in December. Since we were using it extensively at FP Complete via other libraries, we decided to take over maintenance. However, this week we decided that, in fact, John was right in the first place.

I've already migrated most of my libraries away from system-filepath (though doing so quickly was a mistake, sorry everyone). One nice benefit of all this is there's no longer a need to convert between different FilePath representations all over the place. I still believe overall that type FilePath = String is a mistake and a distinct datatype would be better, but there's much to be said for consistency.

Some quick pointers for those looking to convert:

  • You can drop basically all usages of encodeString and decodeString
  • If you're using basic-prelude or classy-prelude, you should get some deprecation warnings around functions like fpToString
  • Most functions have a direct translation, e.g. createTree becomes createDirectoryIfMissing True (yes, the system-filepath and system-fileio names often times feel nicer...)

And for those looking for more type safety: all is not lost. Chris Done has been working on a new package which is aimed at providing additional type safety around absolute/relative and files/directories. It's not yet complete, but is already seeing some interesting work and preventing bugs at some projects we've been working on (and which will be announced soon).

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