On Fri, Jun 26, 2015 at 9:08 AM, Herbert Valerio Riedel <hvr@gnu.org> wrote:

We (see From: & CC: headers) propose, plain and simple, to turn the
currently defined type-synonym

type FilePath = String

into an abstract/opaque data type instead.

Why/How/When?

I've had success with a slightly different "How":

Phase 1: Replace FilePath with a type class, with instances for the old FilePath (i.e. String) and the new implementation.

Phase 2: Wait until a suitable amount of hackage builds without the string instance.

Phase 3: Deprecate the String instance - move it to an old-filepath package.

Phase 4: Replace the type class with the new implementation

This way the new implementation is available immediately, packages can begin converting at once, benefits can be assessed.