What's the benefit? To benefit from the nice features that both Play and http4s offer, without drastic nearly-impossible migrations. Play can still be the outside, while you use http4s for the nice fully pure FP.
See http4s/http4s#1819 for more background.
libraryDependencies += "de.lolhens" %% "http4s-play" % "0.0.2"
Run the tests:
$ sbt play-route/test
$ sbt example/run
This project uses the Apache 2.0 License. See the file called LICENSE.